An approach to geometrybased dynamic location referencing
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Abstract
Introduction
An important requirement for knowledge infrastructures in smart cities is the continuous updating of locationbased information. Protocols for dynamic location referencing like e.g. OpenLR or AGORAC tackle the problem of accurately matching locations between dissimilar digital maps. They are mapagnostic and aim at limiting the amount of descriptive data to reduce bandwidth. However, there are applications for which the weaker requirement of mapindependence is completely adequate, and for some there are even no restrictions in bandwidth (e.g. in the ECfunded project ROSATTE, and in the DLR projects MobiLind and KeepMoving), and with relaxed constraints it is possible to learn from methods in similar areas like road network matching and map conflation, in order to achieve a more accurate solution. Following this path, this paper presents a mapindependent approach developed in the ongoing DLR project I.MoVe, which can be combined with a bandwidthefficient dynamic location referencing method like e.g. OpenLR to target applications with bandwidth restrictions.
Methods
The proposed new approach works lineoriented and is guided by a measure of geometric dissimilarity. It is a topdown approach, recursively splitting up the source route into parts, thereby following a divideandconquer strategy to reduce the problem until it can be solved trivially.
Results
It is currently capable of mapping closed line locations (i.e. circular routes, representing either the boundaries of areas or the tours of e.g. delivery trucks) from a TeleAtlas map to a NAVTEQ map onthefly with a success rate of 97.5% (OpenLR: only 82.5%), and also capable of mapping short line locations (i.e. linear routes) onthefly between the same maps, with a success rate of 99.7% (OpenLR: 91.9%
Conclusion
In conclusion, the new approach to match linear or circular routes between two dissimilar maps is highly accurate and mapindependent, but access to both involved maps is required. The approach can also be combined with a bandwidthefficient dynamic location referencing method like e.g. OpenLR to obtain a compact format before the descriptive data is transmitted.
Keywords
Location based services Digital road maps Dynamic location referencing OpenLR Road network matching1 Introduction
Municipal leaders, politicians, civic planners and other key stakeholders in cities are now looking to enable the vision of a “knowledge infrastructure”, i.a. by deploying location based services. Location based services can help in better governance, planning and functioning of smart cities. An important requirement is the continuous updating of locationbased information for transport users.
With many questions arising during this mission, there is a strong interest in protocols for dynamic location referencing like e.g. OpenLR and AGORAC: they are addressing the problem of accurately matching locations between dissimilar digital maps. They are needed since “conventional georeferencing methods using coordinates or a predefined set of identifiers have structural limitations; they may fail to match the same location in different maps due to discrepancies with respect to level of detail, spatial and temporal accuracy, and semantic dissimilarities” [18]. For example, location references based on road names and house numbers could fail due to misspellings and notation differences, see e.g. [27].
Location references solely based on geographical coordinates (e.g., WGS84 coordinates) also have severe problems. Often there are significant offsets between different maps or topological differences due to varying accuracy of the digitalisation from analogue map data, different manufacturing methods, and changes of the road network over time [27]. Therefore, a geographical coordinate located on a road in one map might be situated offroad in another. Moreover, there may be roads which only exist in one of two different maps (this holds even for different releases of maps from the same vendor).
For example, an intersection, modelled as a simple intersection in one map, might be represented by a complex intersection in another map. An additional problem is raised from differences in the representation of roundabouts: since some vendors use patterns to represent them, a roundabout might be exactly circular in a first map, whereas in a second one the roundabout was digitalised in its real world geometry which can be elliptically [27]. Finally, irrespective of the aforementioned deviations between maps, a location reference solely based on a geographical coordinate may not be unique. For instance, take the case of referencing locations on two motorways crossing without allowing access to each other. At the place of the crossing, the same coordinate would stand for two separate locations on different motorways [27]. In all these cases a geographical coordinate is not sufficient for a reliable location reference.
 (a)
they are mapindependent, i.e. they do not require maps from a specific vendor
 (b)
neither the encoder nor the decoder knows about both used maps, but rather the encoder only accesses the source map, and the decoder only accesses the target map.
 (1)
hit rate
 (2)
data size per location reference (message size)
The AGORAC method is reported to have achieved the industry goal of an acceptably small message size and a sufficiently high hit rate [42, 43].
TomTom has provided test results for OpenLR, e.g. using a source map from TeleAtlas and a target map from NAVTEQ: success rates of 93% were achieved both for TMC paths and nonTMC paths [29]. Here, “success rate” is the percentage obtained by only counting the correctly decoded locations, i.e. those where encoder and decoder location were equal, and dividing this number by the number of all locations that could be decoded (for a discussion of the difference to the definition of “hit rate”, see Section 4.1).
In contrast to other approaches to dynamic location referencing like AGORAC which have licensing fees meaning extra cost, e.g. [37], OpenLR is a royaltyfree, open standard under a creative commons license (CC BYND 3.0 [5]), and therefore fosters interoperability and promotes free choice between different vendors and technology solutions [12].
Summarised, current methods for dynamic location methods either imply the extra cost of licensing fees, or do not fully meet the aforementioned industry goal yet, or suffer from both these drawbacks. The starting point for the present paper then is the observation that for a number of applications a mapindependent (cf. part (1) of the previous definition of “mapagnostic”) dynamic location referencing method is completely adequate. That is, an omniscient matching centre with access to both maps can be used. Moreover, even part (1) of the aforementioned industry goal, i.e. a message size below 50 bytes on average, is not relevant, if bandwidth is not restricted: e.g. in the aforementioned ECfunded project ROSATTE [25], static data is sent from a public authority to a map provider, and in two DLR projects, MobiLind and the ongoing project I.MoVe, routebased data from a traffic information provider is transformed from one wellknown map to another, and often this transformation takes place on the same server. For such applications, it is possible to learn from methods in similar areas like road network matching (RNM) and map conflation, in order to achieve a more accurate solution.
A first approach is to obtain a permanent static mapping by use of RNM: since an omniscient matching center has access to both maps, a RNM algorithm (e.g. [23, 44]) can be used to establish such a mapping from the source map to the target map. It provides M : N mappings (M ≥ 1, N ≥ 0) of every road segment in the source map, represented by a graph edge, to zero or more road segments in the target map. This mapping can be used to map a queried route in the source map to a corresponding route in the target map on the fly by “glueing together” the individual matches for the edges of the route.
 (i)
using a less accurate algorithm not tailored towards the mapping of routes
 (ii)
high maintenance cost because of the necessary preprocessing, and longer time for a transition to a new pair of maps;
 (iii)
extra memory requirement for the static mapping, higher initialization time since for a good performance the mapping must be reloaded into main memory every time the server is restarted;
 (iv)
high implementation effort, due to the lack of available open source implementations.
Regarding point (ii), notice that e.g. [23] reports a run time of around two hours to match 100,000 road segments on a standard IPC. The time for an additional interactive editing of the results may even take several days.
This paper presents an approach called “Geometry InterMapMatching Extension” (GIMME) which follows this path but aims to overcome the aforementioned disadvantages. It has been developed in the ongoing DLR project I.MoVe. It assumes that the requirement of a mapagnostic method (in the sense of the previous definition) can be dropped, but still provides a mapindependent approach. It is tailored towards the onthefly matching of short routes, with respect to both execution speed and accuracy. GIMME is a dynamic method, i.e. it does not use a static mapping. Therefore no preprocessing is required. Consequently, setting up or resetting a matching center for a new pair of maps can be done much faster, and requires less memory. GIMME can be expected to have a better success rate than the RNMbased approach (see Section 4.2). On the other hand, the error detection rate of GIMME can be lower than for the RNMbased approach. However, the first quality measure, i.e. the success rate, is a much more important measure in practice (see also Section 4.2.4). If the error detection rate is of concern, GIMME can also serve as a temporary replacement for the RNMbased approach during its preprocessing step when resetting to new maps.
Neither GIMME nor the RNMbased approach is targeting minimal bandwidth. Nonetheless, both approaches can be combined with a bandwidthefficient dynamic location referencing method like e.g. OpenLR to address applications with bandwidth restrictions (see Section 3.5). GIMME is currently capable of mapping closed line locations (i.e. circular routes, representing either the boundaries of areas or the tours of e.g. delivery trucks) from a TeleAtlas map to a NAVTEQ map onthefly with a success rate of 97.5% (OpenLR: only 82.5%), and also capable of mapping short line locations (i.e. linear routes) onthefly between the same maps, with a success rate of 99.7% (OpenLR: 91.9%). When mapping longer linear routes consisting of 20 road segments between the two maps, GIMME showed a success rate of 98.0%, whereas the estimated success rate of the RNMbased approach with DSO (“ RNM_{DSO}”) is only 92.3%, a difference of 5.7%. For routes with 25 segments, GIMME has a success rate of 93.8%, whereas the estimated success rate for RNM_{DSO} is only 90.5%.
The paper is structured as follows: in Section 2, necessary background from location referencing, road network matching, map conflation, and a geometrybased approach to matching of single edges between maps is given. The latter approach is extended to a new algorithm matching complete routes in Section 3. Next, in Section 4, the accuracy of the new algorithm is compared to that of OpenLR and of RNM_{DSO}. Finally, the work is concluded in Section 5.
2 Related work and background
To keep the paper selfcontained, necessary background and related work is briefly reviewed in this section.
2.1 Types and use cases of locations

point locations,

line (or linear) locations, and

area locations.
Point locations are zerodimensional elements in a map that specify a geometric location. An example for a service involving point locations is the search for nearby pointofinterest (POI) objects. Other examples are e.g. locating petrol stations and speed cameras [32].
Line (or linear) locations are onedimensional elements in a map describing roads or a list of connected roads, and are used during e.g. route guidance: recommended routes are calculated based on the current traffic situation. That way, route guidance is adjusted to incoming information such as traffic jams. In this, dynamic traffic information is incorporated into route calculation, and the respective additional new information is locationrelated (e.g., information about a congested stretch of road). Every such locationbased information must be transferred to some processing unit, often a server at a centre, performing the route calculation. In turn, the calculated recommended routes then serve to redirect traffic flows in order to relieve overloaded roads, and clearly they are locationbased information themselves. Recommended routes are displayed on websites or in mobile applications, and often a reliable way to transfer them from a sender, e.g. the centre, to the respective receivers, e.g. the PNDs, is also needed for them.
In the OpenLR standard, area locations are twodimensional parts of the surface of the earth which are bounded by a closed curve. An area location may cover parts of the road network but does not necessarily need to. Examples for area locations not covering the network are areas describing woodland, a sea or an agrarian country. Other more typical use cases for area locations are: a WiFi hotspot with its signal range, weather information, weather reports about e.g. average rainfall for every cell of a grid, low emission zones, areas affected by weather or environmental conditions (bad weather, smog), flood areas, areas that are congested (due to any cause, e.g. traffic overload, public event, or disaster), administrative areas, pedestrian areas, large crowds of people, areas that are blocked for traffic, and areas subject to city toll [32].
According to the OpenLR White Paper v1.5 [32], “OpenLR can handle locations which are bound to the road network but also locations which can be everywhere on earth (not bound to the road network)”. If they are not bound to the road network, a location is represented by sequences of WGS84 coordinates (sometimes plus additional parameters describing the extent of the location). In OpenLR, the only area locations that are bound to the road network are closed line locations. They reference the area defined by a closed path (i.e. a circuit) in the road network [32]. The boundary always consists of road segments. Besides areas, a closed line location may also describe the course of a tour, e.g. a tour of freight vehicles starting and ending at a depot, or even the course of a marathon.
In AGORAC, area locations are represented either in terms of concatenated line locations representing the area’s outline (i.e., explicitly composed) or by means of a set of locations of any type contained within the represented area (i.e., implicitly composed) [18].
2.2 Dynamic location referencing
This section gives a brief overview of previous work on dynamic location referencing. Related work includes the ILOC concept (“Intersection LOCation” [9]), developed in the EVIDENCE project (“Extensive Validation of IDENtification Concepts in Europe” [24], 1997–1998), extended ILOC [2], SPOT (“SPacial Object Tags” [7]), ROSA (“Reconstruction of Objects on a Second mAp” [6]), the Routing Point or Pivot Point approach of Siemens VDO [16], the Extended Geometry or GOODLANE approach of Bosch [15], the AGORA location referencing method, developed in the AGORA project (“Implementation of Global lOcation Referencing Approach” [8, 19], 2000–2002), with its successor AGORAC [18, 42, 43] developed in the mobile.info project ([4], 2004–2007), MEILIN (“MEthod for Identifying Locations In road Networks” [41]), and OpenLR (hinting at “Open Location Referencing”)[32].
An ILOC (Intersection LOCation) is a junction where two or more intersecting roads with different road descriptors (e.g., different street names) meet. In the ILOC approach, the location of the ILOC is directly referenced by a WGS84 coordinate, and then the direct reference is extended with additional information like three road descriptors of connecting roads. This helps to avoid ambiguity [9]. The ILOC concept was implemented and tested in the EVIDENCE project, and hit rates of about 80% were achieved [24, 43].
The extended ILOC approach uses the ILOC codes of the location’s bounding intersections for determining the expansion of the location [27], and at the same time is based on the geometry of a location and descriptive elements [43]. Of note is that a variant of extended ILOC has been developed into TPEGLoc, part 6 of the TPEG Generation 1 standard, which had been issued as ISO standard ISO/TS 182346 [8, 20, 22, 33].
ROSA, developed around the time of the AGORA project, is in essence a variant of Extended ILOC, and maximum hit rates of about 85% were reported [6, 43]. According to [43], “[...] the Routing Point approach [is based] on connectivity and route calculation, and the Extended Geometry approach on the matching of characteristic geometry”. According to [27], “the main idea of these approaches [i.e., extended ILOC, Pivot Point, and GOODLANE] was to select special points of the road network and supply them with certain attributes to make them easily identifiable”. For a detailed description of these methods, see [8].
The SPOT approach of Bosch generates three components of a location reference: position, object type, and descriptor. After transmission of the reference, a “search window” is generated around the position in the decoder map. Then, all objects of the requested object type are requested within the search window, the given descriptor is verified with the descriptors of the collected objects, and finally that object in the search window is selected which matches the descriptor [7].
In the AGORA project an attempt was made to combine three dynamic location reference methods, namely the aforementioned extension of the ILOC approach, the Pivot Point approach and the GOODLANE approach. The resulting hit rate was good enough to meet the stated industry goal in terms of a 95% hit rate [17, 27], but the message size was unacceptably high [42, 43]. Therefore in the successor AGORAC (the “C” stands for “compact”, akin to a reduced message size, and also hints at “ALERTC”, the official name for the TMC standard [43]), complexity was reduced by focussing on two approaches only, the ILOC extension and the Pivot Point approach. The AGORAC method is reported to have achieved the industry goal of an acceptably small message size and a sufficiently high hit rate [42, 43]. It is of note that the method is in the process of being accepted as ISO standard ISO/DIS 175723 [21], which was successfully approved as a Draft International Standard by ISO and will be issued as an International Standard pending final approval [18, 37].
Rather than aiming at reduction of message size, MEILIN puts more emphasis on a better identification of the surrounding road network of the location in order to facilitate identifying the correct lanes. As the key idea, “[...] one special point is chosen which is either in or close to the location to be referenced. Rooted at this point, a tree is built up in the surrounding road network following certain rules. The vertices of the tree are points on connected roads which have a certain driving distance from the root” [27]. After transmission, the coordinate is placed into the decoder map. Within a certain search area around that coordinate potential root points are chosen on the map, and “the same algorithm as on the encoder side is used to build up a tree rooted at each of the chosen points” [27]. After that, the resulting trees are matched with the transmitted information. Since the tree can grow large if a longer location needs to be encoded, splitting the locations and using multiple smaller trees might be necessary [27].
OpenLR is an open standard for dynamic location referencing, i.e. for encoding, transmitting, and decoding location references in digital maps [31]. It was launched by TomTom International B.V. in September 2009 and developed for the use case of transferring traffic information from a centre to invehicle systems, builtin or used as an addon (PND, Smart Phone). OpenLR is published as an opensource framework, including a reference implementation of the proposed standard (in contrast, no reference implementation is publicly available for any of the aforementioned approaches). Everyone is invited to contribute to this open source software project and to enhance the existing solution.
The standard addresses the problem of accurately matching locations between dissimilar digital maps, enabling a reliable data exchange for many different types of location information, and crossreferencing through maps of different vendors, versions, or different cartographic standards [31]. OpenLR has been certified by the Travellers Information Services Association (TISA) [35] as an industry standard (TPEGOLR [34]) which is part of the UMLbased TPEG structure generally known as TPEG Generation 2 (TPEG2). Moreover, the OpenLR extension for DATEX II [11] makes it possible to use DATEX II [10] with an extended location referencing model containing OpenLR. OpenLR has a growing number of users [30]. Users include the Swedish Transport Administration who is using OpenLR in its DATEX II services, GEWI who develops TIC, a commercial offtheshelf software platform used to process data for information services, TrafficNet who is South Africa’s largest supplier of Traffic and Travel News to the Broadcast and Media Industry, and other leading companies in the field of vehicle tracking, trafficrelated data warehouses, and geographic information systems (GIS). OpenLR has also been used in project ROSATTE (“ROad Safety ATTributes exchange infrastructure in Europe” [25], 2008–2010): a first benefit was that no licensing fees had to be paid, and a second, that the effort needed to implement encoders and decoders was significantly reduced because a framework of Java components is provided [28]. The results were positive: for example, OpenLR location references created from two different maps describing a spatial extend of a speed limit (update), could be decoded on a third independent map with average location matching rates of over 90% [28, 36].
The main idea of OpenLR for locations which are bound to the road network (such as line locations or closed line locations, describing an area bounded by a closed path, i.e. a circuit of road segments) is covering the location completely with a concatenation of (several) shortestpaths. Each shortestpath is specified by information about its start line and its end line, given as socalled location reference points (LRPs): an LRP describes an object in a digital map which consists of coordinates and additional information about a line in the map, and every single LRP specifies a position or a line in a digital map. The resulting set of LRPs uniquely identifies the location. The path described by such a location reference (the “location reference path”) may be longer than the original location and offsets trim this path down to the size of the location path [32].
2.3 Road network matching
This section briefly outlines background from the three mostcited works in road network matching. The reviewed methods are usually used in the context of combining two distinct maps into one new map (map conflation). Moreover, a previous geometrybased approach to match single edges between two maps, “Geometry Matching”, is briefly reviewed. The results of this method are the starting point for the proposed approach GIMME. The first and the third method, i.e. “Iterative Closest Points” and the “DelimitedStrokesOriented Approach”, have mainly been included to complement and complete the understanding of the second method, “Buffer Growing”. Buffer growing, a lineoriented bottom up method, is the approach most similar to GIMME, which is a recursive topdown method (see Section 3).
2.3.1 Iterative closest point
Iterative Closest Point (ICP) was formulated in [1] as a generic algorithm to match two clouds of points by minimizing the difference between them.
Subsequently, it has been adopted in the field of road network matching [38]: first all nodes of the two networks are extracted and then correspondences of nodes between the two resulting datasets are established by combining initial thresholds for dissimilarity measures based on the Euclidean distance, the number of incident edges (i.e. the valence), and the angle differences between emanating edges. Next, if line segments are enclosed between two nodes of one dataset, they are matched to those enclosed by the corresponding nodes. The rationale is that if two roads from different datasets have corresponding start and end nodes, then there is a high likelihood that the roads themselves are corresponding counterparts [44]. The algorithm recomputes the initial thresholds based on data of the first computed node correspondences, and then iterates the approach with stepwise relaxed constraints. This works well when matching whole roads since this only requires establishing the node correspondences between intersections. However, finding the respective correspondences for the remaining start and end nodes in two digital road maps from different vendors is more difficult. The reason is that the geometry of road segments (e.g., length and the number of segments per road) varies strongly among different maps. In contrast to the lineoriented BG algorithm in Section 2.3.2, ICP is designed for the larger data sets involved when matching large portions of two road maps and therefore seems to be more appropriate for map conflation than for matching individual line or area locations.
2.3.2 Buffer growing
Buffer Growing (BG) [39] is an efficient algorithm for the general task of line matching [44]. BG accounts for the aforementioned problem of possible significant differences between corresponding line segments with respect to their start and end points and their lengths (cf. Section 2.3.1) by introducing the concept of a growing buffer. At start, 1:N, N ≥ 1 correspondences between line segments are considered: a spatial buffer around the one source segment is defined, and for a valid correspondence all N target segments must be completely confined to this buffer. If no fitting target segment can be found in the current buffer, it is stepwise expanded until a respective number of target segments can be found [40]. This process also facilitates computing M:1 or M:N, M > 1 correspondences (i.e., matching of complete routes): for this purpose, new logical integrities are built from previously found correspondences, which are then subject to new applications of BG in subsequent steps.
After the Buffer Growing process, a list of potential candidates for the matching reference is computed. The list may be ambiguous and typically contains a large number of matching candidates. By computing the geometric and topologic similarity between each matched pair, the best matching candidate can be confirmed as the final solution [45].
2.3.3 Delimitedstrokeoriented approach
The basic idea of the DelimitedStrokeOriented (DSO) approach is to exploit contextual information as much as possible. For this purpose, a kind of preprocessing step identifies potential “[…] fundamental elements at more abstracted levels”, i.e. a “series of conjoint road objects [chained together and acting] as the fundamental element in the matching process” [44]. For this purpose, the Delimited Strokes are progressively constructed at three different levels: on the first level, a Delimited Stroke represents a series of connected segments which have “good continuity” to each other (which means that one segment follows the other in almost the same direction), and are delimited by “efficient terminating nodes” (which are either prominent crossings with at least 4 incident nodes or deadends), whereas on the second level it is an arbitrary series (i.e., sharp turns are now allowed) which is delimited by arbitrary crossings or deadends. Only on the third level single road segments (edges) are considered.
Experimental results showed that the outlined contextual approach increases the accuracy of road network matching significantly [44]. Of course, this algorithm is designed for matching a whole map to another (i.e., for map conflation), and requires a significant amount of preprocessing. It is left to show that there can also be benefits for the onthefly matching of single line or area locations between dissimilar maps (i.e., for the more focused problem addressed in this paper).
2.3.4 Geometry matching
In [26], digital road networks have been benchmarked with respect to their fitness for route finding and traffic simulation. For this purpose, a geometrybased approach was used to match single edges (not complete routes) in a source map to one or more edges in a target map (thereby establishing 1:N but not M:N, M > 1 relationships). In the following, the basic ideas of this algorithm called Geometry Matching (GM) are given briefly.
 (i)
average distance
 (ii)
angular difference, and
 (iii)
length of the mutual projection of the candidate and the source edge,
Firstly, the candidate list is reinitialised with those outgoing edges of the current best edge, for which the dissimilarity measure is good (i.e. low) enough, and ordered with respect to the measure. Then, a new best edge is calculated and appended to the result list, and so on until an empty candidate list is encountered. Secondly, an analogous process starts with the incoming edges of the current best edge until termination. In the end, the result list is either empty if no candidates could be found, or is a sequence of one or more connected edges forming a stretch of road in the target map corresponding to the source edge.
Summarised, the GM method resembles the idea of the BG algorithm (see Section 2.3.2), although no building of new logical integrities from previously found correspondences takes place here, and consequently no M:N, M > 1 correspondences, i.e. no matches for complete routes are constructed. It is worth mentioning that GM uses both geometric and topological attributes of the edges. The use of a threshold for the average distance between source and candidate edges is an interesting novelty (see Section 4.1 for the details), since previous methods were using measures on top of the Hausdorff or the Fréchet distance.
3 Geometry InterMapMatching Extension (GIMME)
This section introduces the proposed new approach called “Geometry InterMapMatching Extension” (GIMME). It is divided into five subsections: Section 3.1 presents the algorithm, and Section 3.2 concludes the presentation of the algorithm with further remarks. GIMME uses GM to calculate the list of potential candidates, and therefore, the presentation of GIMME is concluded by a brief review of GM’s thresholds for exclusion of a candidate in Section 3.3. Then, Section 3.4 demonstrates the presented algorithm by a worked example. Next, Section 3.5 discusses how GIMME can be combined with a bandwidthefficient dynamic location referencing method like e.g. OpenLR to target applications with bandwidth restrictions.
3.1 Algorithm
In this section, first the idea of the algorithm is derived from key observations made for the problem at hand. Next, pseudocode is given for the algorithm GIMME. Regarding the variable names in the pseudo code, capital Latin letters are used for sets (e.g. for a set of solutions, i.e. a set of edge lists), whereas small Latin letters are used for set elements, associative arrays, lists, and for natural numbers.
Similar to the BG algorithm (see Section 2.3.2), it works lineoriented and is guided by a measure of geometric dissimilarity. Another analogy is that (partial) solutions are only considered when they are topologically connected. However, in contrast to the BG algorithm, which is expanding and combining partial solutions bottom up, GIMME is a topdown approach, recursively splitting up the source route into parts, thereby following a divideandconquer strategy to reduce the problem until it can be solved trivially.
Divideandconquer algorithms are known to make efficient use of memory caches: once a subproblem is small enough, it and all its subproblems can be solved within the cache, without accessing the slower main memory [14].
 (i)
Firstly, in the context of this paper, source and target routes must be directed paths, i.e. finite sequences of edges connecting a sequence of vertices where the first and the last vertex may coincide. It is of note that cycles have not been excluded here, and that the edges must represent road segments which are connected in such a way, that a vehicle can drive on them in the driving direction, that is, from the start of the first segment to the end of the last segment.
 (ii)
Secondly, there must be a nonempty candidate list for every source edge: if this is not the case, then there is at least one source edge which cannot be matched to any edge in the target map. Consequently, also the whole route cannot be matched. Notably, partial matches are not of interest here, complying with the fact that the intended use case is dynamic location referencing (see the introduction to Section 4).
 (iii)
There are M:N relations between source and target edges. This means that for a single source edge 1:n relations must be established, where N ≥ n ≥ 0. For an example of the case n = 0, consider e.g. a 3:2 relation between source and target edges: if all source edges are mapped to at least one target edge, then there must be one destination edge occurring twice in the resulting target route. This means that this target route can not be homologous. Notice that the fact that a source edge might not become mapped to any target edge does not contradict (ii): also in this case eligible candidates for the respective edge do exist, but all these candidates are “consumed” for other (i.e. incident) source edges.
 (iv)
Of note is that the order of a candidate list has nothing to do with that of the edge sequence of a directed path. This is because the candidates are ordered with respect to dissimilarity to the source edge. Therefore, GIMME considers the set of candidates in a candidate list, i.e. the order of the elements in the list is not used (cf. Algorithm 2, line 9). To build the final target route by combining partial sequences drawn from the pool of candidates for every individual source edge, GIMME considers all permutations of subsets of a candidate list, including the empty set because of the case n = 0 in (iii). Since this seems to introduce a very complex step to the algorithm, the following observation is exploited to ensure efficiency of the approach:
 (v)
Since the final route must be a directed path, it is not necessary to consider partial sequences for which it is already known that this property does not hold.
 (vi)Point (v) gives rise to the following recursive formulation:
 1.
The final target route is constructed by trying to connect partial routes (i.e., partial solutions) for the head and tail of the list of source edges. Whenever a current partial solution for the head can be connected topologically to a partial solution for the tail (cf. Algorithm 2, line 32), then we have a match for the whole source route, and this solution is stored as a potential solution.
 2.
More precisely, the partial solutions for the head are computed by subsequently considering all permutations of subsets of its candidate list (cf. Algorithm 2, line 13 and line 18). To cover the case n = 0 in (iii), GIMME also considers the empty subset as a possible (partial) solution for the head, but only if the current head solution can also be used as the prefix of a solution for the tail. A test for this is performed in Algorithm 2, line 28. If it succeeds, GIMME stores the corresponding solution for the tail (i.e. without a preceding match for the head) as a possible solution (cf. Algorithm. 2, line 29).
 3.
Only connected permutations (i.e. directed paths) are considered (cf. (v) and Algorithm 2, line 14). The partial solutions for the tail are computed by invoking the outlined algorithm recursively on the tail. Every recursive call returns a list of all (partial) solutions found (where a solution respects (v) by construction), and the calling code then checks each returned solution for the tail for topological connectedness with the current solution for the head (cf. Algorithm 2, line 32). The boundary case of the recursion is reached when the tail has shrunk to an empty list (for which an empty list of solutions is returned, cf. Algorithm 2, line 3).
 4.
Finally, from the list of solutions for the whole route returned at the topmost calling level, the best solution must be chosen. Currently, the best solution is defined as the largest admissible solution (i.e. one that is a longest list) for line locations (cf. Algorithm 1, line 11), and as the first found cyclic path for closed line locations (cf. Algorithm 1, line 13). A largest admissible solution is preferable because such a solution is more likely to cover the whole source route (i.e. to not miss any part of it). For an explanation of the term admissible, see (vii). In the (rare) case that there is more than one largest admissible solution, currently one is simply picked randomly.
 1.
 (vii)
A solution is admissible, if and only if (a) it respects (ii), i.e. for every source edge at least one corresponding candidate edge is used in it (but not necessarily for that respective source edge, see the case n = 0 in (iii)), and (b) the length of the solution does neither exceed 120% nor fall below 80% of the length of the source route matched by this respective solution.
3.2 Further remarks
This section gives further implementation details and discusses the requirements regarding digital road maps.
In Algorithm 1, line 3 the result of the GM algorithm (cf. Section 2.3.4) is checked for completeness (cf. (ii)), i.e. gimmeMatch returns a null value if there is a source edge for which GM could not return a candidate list. Notice that candidate lists returned by GM are never empty, but rather the entry for the respective source edge is missing in the returned associative array.
3.3 Dissimilarity threshold
While GM [26] computes a dissimilarity measure and then orders the candidates in the candidate list with respect to this measure, GIMME currently does not use this order (see Section 3.1). Nonetheless, GIMME uses GM to calculate the list of potential candidates, and GM’s thresholds for exclusion of a candidate from the candidate list are briefly reviewed in the following.
 a)
Maximum average distance a: 20 m
 b)
Maximum angle α: \(40^{\circ }\ \widehat {=}\ 0.698\ \text {rad}\)
 c)
Minimum mutual projection length: 3 m
 d)
Maximum for D: 30
 e)
Maximum for D ^{′}: 40
3.4 A realworld worked example
We denote the two road segments (or edges in the underlying graph, respectively) by a and b (see the upper TeleAtlas part in Fig. 2). The driving direction is from a to b, and b is an outgoing edge of a. In the following, it is described how GIMME matches the short route (a,b) to two road segments (edges) in the NAVTEQ map, denoted x and y. The driving direction here is from x to y, and y is an outgoing edge of x. Candidate edge x has a mutual projection length above the threshold of 3 m with source edge a (the remaining dissimilarity measures given in Section 3.3 do not exceed the respective maximum values, respectively), but not with source edge b. In contrast, candidate edge y has a mutual projection length above the threshold with both source edges a and b (and also the remaining dissimilarity measures do not exceed the respective maximum values, respectively).
Therefore, besides the longest (and therefore best) target route (x,y), also a route consisting of only the candidate edge y (i.e. the route (y)) will be considered as a possible solution during the matching process of GIMME.
GIMME is invoked by a call to gimmeMatch with parameters e = (a,b) (the list of edges), and t = LINE (the type of the location, i.e. a linear route). After a call to the GM algorithm with argument e, map m is assigned to an associative array: m = {a : (x,y),b : (y)}. Here, x, y denote the candidate edges identified by the GM algorithm (see the lower NAVTEQ part in Fig. 2). NAVTEQedges x and y are connected in driving direction, i.e. y is an outgoing edge of x. Map m maps edge a to the list of candidates (x,y), and edge b to (y), that is, to a singleton with only one candidate (NAVTEQedge y). No details of operation of the GM algorithm are given here (see Section 2.3.4 and [26] for more details about GM). In line 6 of Algorithm 1 (gimmeMatch), Algorithm 2 (solutions) is called with parameters e and m.
This is a recursive algorithm, and the shallowest calling level has been invoked. This calling level will be referred to as level 0. During the run of the algorithm, level 0 will recursively call solutions, thereby invoking a deeper calling level, which will be referred to as level 1, and so on.
In level 0, solutions for head and tail of e, the passed source route, are considered subsequently. The code starts by addressing the head: for this purpose, M is assigned to the set of candidates for the first element of e = (a,b), i.e. for a. That is, according to map m, M is assigned to {x,y}. Next, the power set of M is computed in line 12 of Algorithm 2, and assigned to P, i.e. P = {∅,{x},{y},{x,y}}.
Now S loops over all subsets of P, starting by S = ∅. Here, nothing has to be done (see lines 15  17). Next, S = {x}, and via a call to connectedPermutations, D is assigned to the set of all directed paths which are constructable with S, that is, D = {(x)}.
Now d loops over all directed paths in D. In other words, d loops over all possible solutions for the head of the route e, constituted by candidates of the currently considered subset S. There is only one iteration with d = (x). The tail of list e = (a,b) passed to level 0 is assigned to t, i.e. t = (b). To address the tail, solutions is called recursively on the tail in line 22, that is, t is passed as the formal parameter e = (b), the same map m is passed, and level 1 is invoked.
In level 1, M is assigned to {y} since the first element of e = (b) is b and since m maps b to (y) . Next, the power set P is computed as P = {∅,{y}}, and again S loops over all subsets of P, starting by S = ∅. Again, nothing has to be done here. In the next iteration, S = {y}, and D is calculated as D = {(y)} by a call to connectedPermutations.
Now there is again only one iteration for loop variable d with d = (y). The tail of list e = (b) is the empty list, and therefore no recursion on the tail takes place (see line 20). Therefore, the current solution for the head of the level 1 route (i.e. d = (y)) is added in line 24 to the tentative set of solutions X (which had initially been initialized as the empty set), i.e. X = {(y)}. Another side effect is that lines 26 to 34 are not executed: since no solutions have been found for the tail of the level 1 route, there is no need to check for possible connections between the current head solution (d = (y)) and tail solutions. Consequently, no more solutions are found on this level, and level 1 returns X = {(y)} as the set of solutions found for e = (b). Notice that before returning to level 0 this solution is inserted into the cache with the key (b) (see line 37).
Execution returns to level 0, back to the iteration with S = {x}, and to the (one) iteration of the inner loop in line 18 with d = (x). Lines 26 to 34 first check whether the current head solution is a prefix of a tail solution (which is not the case for head solution d = (x) and tail solutions T = {(y)}), and then call subroutine leftExtensions in line 32 (with parameters d = (x) and T = {(y)}) which subsequently attempts to topologically connect the current solution for the head to solutions for the tail. Since edge y is an outgoing edge of edge x in driving direction (see Fig. 2), this works for d = (x) and the tail solution (y), and hence the new solution (x,y) is added to the tentative set of solutions X. Since X had been initialized as the empty set, it is X = {(x,y)}.
In level 0, execution continues to loop over all subsets of P, i.e S = {y}, followed by the assignments D = {(y)}, d = (y), and again the recursive call on the tail of e = (a,b), i.e. on t = (b). In line 6, level 1 of solutions immediately returns X = {(y)} since this solution had been inserted into the cache with the key (b) in a previous call on level 1 (see above).
Back at level 0, the returned solution is assigned to T, i.e. T = {(y)}. Since T is not empty, the algorithm now checks whether the current head solution is a prefix of a tail solution in line 28. This is the case since d = (y) is a prefix of (y) ∋{(y)} = T. Therefore, (y) is added to X = {(x,y)}, and we have X = {(x,y),(y)}. The subsequent call to leftExtensions in line 32 on the arguments d = (y) and T = {(y)} yields the empty set since there is no topological connection between the last edge of the current head solution, i.e. between y, and a tail solution in T = {(y)} (see Fig. 2). Thus, at the end of the iteration with D = {(y)} and d = (y), X remains unchanged, i.e. X = {(x,y),(y)}.
The next considered subset of P is S = {x,y}, and thus we have the assignments D = {(x,y)} and d = (x,y), respectively, followed by the recursive call on the tail of e = (a,b), i.e. on t = (b). Like before, level 1 of solutions immediately returns X = {(y)} from the cache.
Back at level 0, the returned solution is assigned to T, i.e. T = {(y)}. Firstly, d = (x,y) is not a prefix of a solution in T = {(y)}. Secondly, there is no topological connection between the last edge of the current head solution, i.e. between y, and a tail solution in T = {(y)} (see Fig. 2). Consequently, X remains unchanged, and we have X = {(x,y),(y)} as the final set of solutions for e = (a,b) on level 0. This set is returned to Algorithm 1 (gimmeMatch), and assigned to S.
Since S is nonempty, and since the type of location is T = LINE, largestAdmissibleSolution is called with parameters e = (a,b), the same map m as before, and with S = {(x,y),(y)}. The largest admissible solution is (x,y), and this solution is returned as the final match for (a,b), together with a positive offset of 27 meters and a negative offset of 18 meters. The offsets had been calculated before by GM for each candidate edge, and are passed through additional attributes in m (which is a technical detail that, for brevity, has not been addressed in the instructive code of Algorithm 1).
The initial source route and the tails on which Algorithm 2 has been invoked by a relevant call, are marked with left arrows pointing to them.
It is of note that always n relevant recursive calls of Algorithm 2 are executed if a source route is matched successfully (i.e. this is not a property specific to only the demonstrated example). This is because for n > 0 the initial call on e = (e _{1},…,e _{ n }) always calls the algorithm recursively on (e _{2},…,e _{ n }) for every candidate solution for (e _{1}), and the first such call already induces a chain of n − 1 subsequent recursive calls on increasingly short tails (i.e. calls on (e _{2},…,e _{ n }),…,(e _{ n }). Since e is assumed to be successfully matched, every such call must return a nonempty list of solutions, and therefore every such list of solutions is inserted into the result cache with the respective key. In particular, the list of solutions for the subproblem (e _{2},…,e _{ n }) is cached with this subproblem as the key. Therefore, any further recursive call of the initial call on (e _{2},…,e _{ n }) is not relevant since the solution is immediately returned from the cache.
This property of always having n relevant recursive calls for a successful match will also play an important role in the discussion of the amortised average run time in Section 4.2.3.
3.5 Combining GIMME with a bandwidthefficient method
Notice that GIMME needs access to both the source and the target map. This requirement is met whenever an omniscient matching centre with access to both maps can be used. If location references need to be transmitted from a sender to a receiver, the algorithm can compute the matches at the receiving side, using both maps. The input of gimmeMatch is essentially a list of edges. In the case of a matching centre, there is usually a common main memory for sender and receiver instances, and then this list can be represented as e.g. a list of instance variables pointing to edge objects in main memory (then assuming that the maps have been loaded into an objectoriented structure of vertex and edge objects, see e.g. [13]).
In the case that sender and receiver are not running on the same machine, a list of edge IDs for the respective source map format can be used. Usually, transmission of this ID list will require (much) more bandwidth than e.g. transmitting the small set of intermediate LRPs used by an OpenLR reference to uniquely identify the location (where these LRPs represent a concatenation of shortestpaths covering it, see Section 2.2). Indeed this is prohibitive to a direct usage of GIMME if bandwidth is restricted for one or more transmission lines between sender and receiver. Fortunately, it is possible to obtain a compact format for the descriptive data before transmission. For this purpose, GIMME is combined with a bandwidthefficient dynamic location referencing method like e.g. OpenLR. More precisely:
It is wellknown that dynamic location referencing protocols usually work perfectly (i.e. with an accuracy of 100%) if source and target map coincide (e.g. this is the case for OpenLR). This can be exploited for avoiding the transmission of the whole list of edge IDs over a bandwidthrestricted line: instead, e.g. a compact OpenLR reference is created in the source map at the sender side, and transmitted over the respective line. The receiver side holds the same source map, and so the OpenLR reference can be correctly decoded in it. Thereby the list of edges of the encoded route is recreated, represented as e.g. a list of instance variables pointing to edge objects. This list can now be passed to GIMME (e.g., as an instance variable via the internal main memory of the receiving server) without having to respect any further bandwidth requirements, and usually without any decrease in accuracy.
A certain drawback is the increased run time of the decoder, since every location needs to be decoded two times, first with e.g. OpenLR and second with GIMME. Notice that the sender side runs e.g. an OpenLR encoder, and the receiver side runs e.g. an OpenLR decoder and GIMME. Moreover, the receiver side running GIMME needs access to (an identical copy of) the source map, and to the destination map.
4 Evaluation

short line locations (linear routes) consisting of up to 5 road segments

closed line locations (i.e., tours or circular routes) consisting of up to 69 road segments
Run time measurements give a comparison of run times for GIMME and OpenLR, and demonstrate the linear relation between route length and run time of GIMME. This linear relation then is subject to a more formal discussion of the amortised average run time in Section 4.2.3. Finally, the section discusses the results of previous experiments with RNM algorithms, and compares them to the obtained results for GIMME.
The outlined experimental setup complies with the use of GIMME for dynamic location referencing (DLR), which has been the main use case in several projects of German Aerospace Center: in the projects MobiLind, KeepMoving, and I.MoVe, traffic information like e.g. the information about congested stretches of road, or the spatial extent of a whole congestion area needed to be processed as incoming OpenLRreferences like e.g. OpenLR line locations or closed line locations (please recall that closed line locations are area locations which can be used to describe twodimensional parts of the surface of the earth, see Section 2.1). During the projects, these references had been encoded in a TeleAtlas map, but the used process chain required to decode them in a NAVTEQ map.
Moreover, the design of the setup is such that it is very similar to the setups in previous works in the context of DLR. This has the advantage of making our results comparable to previously reported results.
Notice that protocols for DLR usually encode the whole stretch of road or the whole area in question in one single location reference, which they transmit in one piece from sender to receiver. That means, a location is not split into e.g. multiple individual edges, and then distributed across a respective number of location references. This would mean unnecessary overhead, contradicting the aim of bandwidth reduction: e.g. OpenLR achieves a compact format by encoding linear or circular routes as sequences of intermediate location reference points (LRPs, see Section 2.2). Their number is usually much smaller than the number of edges. Moreover, every message has a header, and thus sending multiple messages would mean increasing the overhead.
In the scenario of an omniscient matching centre with access to both the source and the target map, there are no restrictions for the bandwidth. Nonetheless it is still advantageous to match complete locations instead of matching single edges: matching complete (linear or circular) routes with GIMME (cf. Section 3.1) instead of matching individual edges can yield more accurate results. This is because the algorithm keeps multiple matching candidates per edge, and since a best combination of candidates for all route edges is constructed. That way, GIMME is capable of correcting a wrong choice subsequently, until a best admissible (i.a. directed) path in the target map has been found (for more details, see Section 4.2.4).
It is of note that, in the context of this paper, a successful match always means a complete match, i.e. partial matches are not of interest and are considered as a failed match. This complies with the usual approach in DLR: matching failures, i.e. unmatched location references can easily be suppressed or replaced at the application level, e.g. a trafficmonitoring application can always drop an unmatched reference to a congested stretch of road, or perhaps replace it by a less accurate, more general textual warning. In contrast, basing traffic information on a partial match holds the risk of providing wrong information to the user during e.g. route guidance or navigation. This is the main reason why DLRprotocols like e.g. OpenLR usually discard partial matches and simply report them as a decoding error.
4.1 Evaluation measures
Notice that the hit rate, success rate, and the error detection rate correspond to the wellknown evaluation measures “accuracy”, “precision”, and “negative predictive value” for a confusion matrix in e.g. Machine Learning, respectively.
The hit rate is combining the two aspects of good performance of a dynamic location referencing method, namely correct location decoding and truthful detection of absence of a homologous counterpart in the target map. This however introduces a subtle dependency on the particular maps, together with the tested locations, e.g. on the ratio of true positives to true negatives in a particular constellation. Giving both success and error detection rates separately instead has also the advantage of providing more detailed information about the two different aspects of quality.
4.2 Experimental results
This section describes the results of experiments with GIMME and OpenLR, and also gives a comparison to previous RNM approaches. In detail, the section is divided into subsections as follows: Firstly, Section 4.2.1 gives the results of experiments that have been conducted with OpenLR, with a hybrid of OpenLR and GIMME, and with GIMME standalone, applied to randomly generated line and closed line locations (in contrast to AGORAC, a reference implementation of OpenLR is publicly available [31]). Secondly, Section 4.2.2 gives a comparison of run times for GIMME and OpenLR, and demonstrates the linear relation between route length and run time of GIMME. This linear relation then is subject to a more formal discussion in Section 4.2.3, which considers the amortised average run time. Next, Section 4.2.4 discusses the results of previous experiments with RNM algorithms, and compares them to the obtained results for GIMME. Finally, Section 4.2.5 gives depicted examples for successful matches of line and closed line locations as resulting from GIMME, and also discusses a situation where GIMME fails to find the correct match, and how to remedy it.
4.2.1 Quantitative comparison of GIMME and OpenLR with respect to evaluation measures
In previous experiments described in the literature, the results of applying AGORAC and OpenLR to line locations have been assessed [17, 29, 43]: In [29], i.a. 1,000 randomly selected line locations in The Netherlands have been encoded in a TeleAtlas map, and then decoded in a NAVTEQ map. In [43], i.a. 881 randomly selected line locations in The Netherlands have been processed in the opposite direction, i.e. encoded in a NAVTEQ map, and then decoded in a TeleAtlas map. These locations each consisted of a set of one to five connected edges forming a path in the road network.
In order to arrive at comparable results, an experimental setup similar to the setups in the aforementioned previous works seemed preferable. OpenLR has the advantage of a publicly available reference implementation, and therefore it was decided to compare GIMME with OpenLR. To be in line with the approach in [29], encoding was done in a TeleAtlas map, and decoding in a NAVTEQ map. Firstly, 1,000 short routes in a TeleAtlas map of Potsdam, Germany (TA 2012.06), consisting of 1 to 5 road segments like in [43] have been created randomly. They comprised of 2.84 edges on average, and have been created without any further structural restriction. Consequently, the generated routes were allowed to intersect themselves, and also to be circular as a special case of a linear route. Nonetheless, the vast majority of generated routes were noncircular and did not intersect themselves. All generated routes have a start node and an end node, and are lists of edges representing road segments which are connected in such a way, that a vehicle can drive on them in the driving direction, that is, from start to end.
Secondly, 1,000 closed line locations have been generated randomly in the same TeleAtlas map for Potsdam. Essentially, they are linear routes where start and end node coincide. In effect, closed lines are circular, and form a special case of linear routes. Again, closed lines are lists of edges representing road segments, and a vehicle can drive on them in the driving direction from start to end. Different from a noncircular route, a vehicle can also continue to drive past the end, thereby starting a next “round”, and so on. They have been created such that a location boundary does not intersect itself, and such that the locations are as small as possible: that way, the experiments could be conducted under conditions comparable to those for the aforementioned experiments (e.g., [29, 43]). More precisely, the generated closed line locations comprised of 18.55 edges on average, the number of connected edges forming the respective closed paths in the road network ranged from 2 to 69. Notice that in order to generate such a large sample of 1,000 randomly generated closed line locations, more than the maximum of five connected edges as in the evaluation of [43] had to be allowed for: this is simply because such a large number of distinct short closed lines does not exist in the road network of Potsdam, Germany.
All of the 1,000 generated random line locations and all of the 1,000 closed line locations have been encoded successfully. Three experiments have been conducted for each of the two location sets.
For the second experiment, GIMME has been applied to all routes of the first set which OpenLR had not been able to decode in the target map (i.e., a hybrid approach of first using OpenLR and then GIMME has been applied). For this purpose, GIMME was given full access to both the source and target map. In contrast, OpenLR as a mapagnostic dynamic location referencing protocol has only access to the source map at the sender side, and to the target map on the receiver side. This is sufficient for OpenLR since OpenLR only transfers intermediate location referencing points as mapindependent WGS84 positions, in order to limit the amount of descriptive data (cf. Section 2.2).
As a result, the hybrid approach decoded 149 more routes, and all of them correctly (which has been validated by manual inspection). This is an increase in correctly decoded routes of 31.2%. As a consequence, the success and the error detection rate of the hybrid approach was 94.2% and 65.0%, respectively.
In the third experiment, GIMME was used as a standalone algorithm to find matches in the NAVTEQ map for the same set of 1,000 routes in the TeleAtlas map. This resulted in a success rate of 99.7%, and in an error detection rate of 69.0% (as determined by manual inspection). Compared to the previous run with OpenLR, 100 or 21.0% more routes could be decoded correctly by GIMME. When comparing to the previous run with the hybrid approach, the hybrid yielded 49 more correctly decoded routes than GIMME standalone.
To understand this, first note the following: although GIMME has a better error detection rate than OpenLR, it is still prone to false negatives, and even more so is OpenLR. However, since GIMME and OpenLR are very different methods, this does not necessarily mean that GIMME and OpenLR will falsely report negatives for the same locations. But rather GIMME might fail to decode a location which OpenLR decodes correctly, and (probably more frequently) it might be vice versa. And since the hybrid approach will report a negative (i.e., a failure to decode a location) only if both OpenLR and GIMME failed to decode the location, the frequency of successful (and correct) decodings will be higher than for GIMME standalone. This seems to be an advantage of the hybrid approach. However, both the success rate and the error detection rate lie between those for OpenLR and for GIMME. This may be due to the fact that the measured numbers n _{tp} and n _{tn} are influenced by the lower accuracy of OpenLR (cf. Eqs. 2 and 3).
Success and error detection rates for line locations
Method  success rate [%]  error detection rate [%] 

OpenLR standalone  91.9  55.9 
Hybrid OpenLR/GIMME  94.2  65.0 
GIMME standalone  99.7  69.0 
Success and error detection rates for closed line locations
Method  success rate [%]  error detection rate [%] 

OpenLR standalone  82.5  18.4 
Hybrid OpenLR/GIMME  87.2  28.0 
GIMME standalone  97.5  21.2 
To summarise: The results of the three methods are of different accuracy (in terms of success and error detection rate). The results of GIMME standalone have the highest accuracy, followed by the accuracy of the results of the hybrid approach, and the lowest accuracy has been achieved with OpenLR. Of note is that the hybrid approach yields the highest absolute number of correctly decoded locations. However, the use of OpenLR as part of it also introduces a higher number of false positives than GIMME standalone, which is the reason why the success rate of the hybrid approach is lower than for GIMME, though (cf. Eq. 2). The increase in accuracy as obtained by GIMME seems to be significantly larger for longer routes than for short routes. In other words, whereas OpenLR shows a clear degradation of accuracy with increasing length of the routes, there is only a small such degradation for GIMME.
4.2.2 Run time measurements
Run times of the various methods for line and closed line locations
Run times for 1, 000…  

line locations [s]  closed line locations [s]  
excl. I/O  incl. I/O  excl. I/O  incl. I/O  
OpenLR standalone  82  174  185  445 
Hybrid OpenLR/GIMME w/ cache  114  206  399  659 
GIMME standalone w/ cache  82  174  361  621 
GIMME standalone w/o cache  89  181  30,692  30,952 
In the following, the given percentages all refer to the run times excluding the time spent for loading the location files from hard disk. While there is only a small benefit of the result caching (see e.g. Algorithm 2, line 6) when decoding only short line locations (only 3.9%), the gain in performance is large when decoding the longer closed line locations: GIMME without result caching required about 8.5 hours to decode the set of closed line locations, whereas it only took 361 seconds (not much more than 6 minutes) with the result cache, i.e. a speedup of more than 85X. The hybrid approach has longer run times than OpenLR standalone (39.0% longer for line locations, and 115.7% longer for closed line locations). Since the hybrid approach additionally calls GIMME on those locations which OpenLR failed to decode, this is an expected result. For the short line locations, the run time of standalone GIMME (with caching) equals that of OpenLR (without using the cache, it was 8.5% longer). On the other hand, for the longer closed line locations, the run time of GIMME (with caching) was 95.1% larger than that of OpenLR. Hence, at least for longer routes, the significant increase in accuracy comes at the price of a longer run time.
The reason for this behaviour is that the probability of a positive match for a route is decreasing with the number of segments in it: the longer a considered source route is, the more unlikely it is that a homologous counterpart exists in the target map. In effect, the measured run time is that for a decreasing number of positives, plus the run time for the remaining number of negatives. The run time is dominated by the effort for the positives since on average, Algorithm 2 needs less time to identify a negative than a positive: for a negative, e.g. recursive calls on route tails (cf. Algorithm 2, line 22) tend to return less solutions, or even no solutions at all, saving effort in lines 26  34 of the algorithm. Moreover, for many of the nonexisting routes, GIMME can identify a negative quickly in line 3 of Algorithm 1, that is, using the fact that the GM algorithm already identifies nonexistence of a match (for single edges) by a quick proximity search. The contribution of these routes to the run time is negligible.
The standardised run time is linearly increasing with the number of segments. The orange curve depicted in Fig. 9 shows a linear fit of the run time, using the function y ≈ 21.62 ⋅ x + 5.06.
As can be seen in Figs. 9 and 10, the relation of the (standardised) run time of GIMME to the route lengths is linear, regardless whether it is measured with or without the run times for calls of the GM algorithm.
In order to shed more light on this empirical result, the next section discusses the amortised average run time of Algorithm 2 (solutions) in a more formal way. For this purpose, i.a. the relation of two quantities to the route length must be known. These quantities are being processed by Algorithm 2. The first is M, the number of candidates of a source edge, and the second is T, the number of returned solutions for the tail.
To obtain the average number of returned solutions for the tail for one of the experiments with a fixed route length, an observed total must be divided by the number of locations, 1,000, and also by the respective route length n. The factor \(\frac {1}{n}\) accounts for the fact that for every successfully matched source route exactly n relevant recursive calls of Algorithm 2 (solutions) have been executed (for more details, see the discussion of Fig. 3 in the worked example in Section 3.4, and see Section 4.2.3). The mean \(\overline {T}_{\text {standardised}}\) of the six average numbers for the respective fixed route lengths 1,5,…,25 is \(\overline {T}_{\text {standardised}} \approx 3.26 \pm \sigma \) with a small standard deviation σ ≈ 0.14. This means that, empirically and on average, only slightly more than 3 solutions have been returned for each recursive call on a tail, and that this rate does not increase with the number of segments in the source route.
The empirical results for C and for \(\overline {T}_{\text {standardised}}\), respectively, will be subject to further discussion in the next section.
4.2.3 Amortised average run time
This section discusses the amortised average run time of GIMME in more formal terms, using the empirical results for C and \(\overline {T}_{\text {standardised}}\) of the previous section, Section 4.2.2. The following more theoretical results have been confirmed by the experimental results of the previous section, and shed more light by explaining them.
Algorithm 1 first calls geometryMatches. This routine applies the GM algorithm ([26], see also Section 2.3.4) to all n edges of the source route e and constructs the respective candidate lists. Since GM does the work for every of the n source edges separately, i.e. each time working on an input of constant size, a linear (worstcase and average) run time should be expected. Further, also the percentage of the run time for GM on the total run time of GIMME is of interest. Both has been measured during experiments given in Section 4.2.2, confirming the theoretical expectations of a linear relation between the runtime for geometryMatches and the route length.
The second routine called by Algorithm 1 is Algorithm 2 (solutions). This is the actual divideandconquer algorithm in question, and the remainder of the section will focus on analysing its average run time behaviour (see below). Moreover, Algorithm 1 calls largestAdmissibleSolution and findFirstClosedLineSolution: their (worstcase and average) run time is linear in the length of their input, the set of solutions returned by the first recursive call in line 6 of Algorithm 1. By the empirical result from Section 4.2.2 for \(\overline {T}_{\text {standardised}}\) we know that on average, the size of this set does not increase with n, the source route length. Rather the average size is asymptotically bounded by a small constant (slightly above 3), and thus the aforementioned two routines run in amortised constant average time.
In order to settle the run time behaviour of a divideandconquer algorithm like solutions, one needs to solve the underlying recurrence. The terms of this recurrence are describing the computational cost for (i) dividing the input problem and for (ii) combining the partial solutions.
In brief, the cost of dividing the input e of Algorithm 2 (solutions) into head and tail is that of list operations with constant run time. The cost for combining the partial solutions does not directly depend on n, the size of the input source route e. Instead this cost depends on (i) M, the number of candidates, because all sizes of lists and sets, and with it also the number of iterations in the respective loops of the program code, depend on this number; and on (ii) T, the number of solutions returned by a recursive call (the responsible code is found in the lines 26  34 of Algorithm 2).
More precisely: The cache operations in lines 5, 6, and 37 in Algorithm 2 are assumed to have amortised constant average run time. The computation of the power set of the set of candidates in line 12 of Algorithm 2 (solutions) is done with a wellknown recursive standard implementation for this task (no instructive pseudo code is given). It has the expected run time of Θ(2^{M}) = Θ(2^{ C }) where C is the empirical maximum length of a candidate list determined in Section 4.2.2. Further, GIMME has to consider a maximum of \({\sum }_{k=0}^{C} \frac {C!}{k!}\) permutations of subsets of a candidate list (see e.g. [3]). Both stated terms may appear large, but with our result C = 3 (see previous section) it is 2^{ C } = 8 and \({\sum }_{k=0}^{C} \frac {C!}{k!}=16\).
Hence the total number of considered directed paths d ∈ D (line 18 in Algorithm 2) for all subsets S ∈ P (line 13 in Algorithm 2) is bounded by 16.
Algorithm 2 uses Algorithm 3 (connectedPermutations) to construct the set of connected permutations for a set of edges. Subsequently, Algorithm 3 is called on each subset of the respective candidate set. It generates the connected permutations using the following recurrent relation:
Let 𝜖 be a set of edges. A connected permutation of 𝜖, say π, is either (e) if 𝜖 = {e} (i.e. 𝜖 is singleton), or there must be e ∈ 𝜖 such that there is a connected permutation of 𝜖 ∖{e}, say ρ, such that the permutation resulting from prepending e to ρ equals π.
The (worstcase and average) run time for the call of Algorithm 3 in line 14 is constant since the length of its input is constantly bounded, as are the inputs of Algorithm 4 (leftExtensions) called within Algorithm 3 (line 15): S is bounded by C = 3, a singleton with only one edge is passed to Algorithm 4 together with D, and D is bounded by (C − 1)! = 2! = 4.
In more detail: the run time of Algorithm 4 is in O(e⋅D) where e is the first and D is the second parameter of leftExtensions. This is because in the worst case, e elements in e are prepended to a copy of d ∈ D in D iterations of the loop (prepending a single element is done in constant time). Notice that the run time of creating the shallow copy of the list d in line 6 of Algorithm 4 is constant (i.e. essentially only an object maintaining the list is “cloned”, holding a constant number of maintenance and reference fields, pointing to e.g. first and last list element, respectively). Regarding Algorithm 3, it is straightforward to see that the number of recursive calls to Algorithm 3 for a subset of maximal size 3 is bounded by 10. Since there are at most 8 subsets, the total number of calls to Algorithm 3 is bounded by 8 ⋅ 10 = 80 during the execution of one call of Algorithm 2 (solutions).
Then, in lines 26  34 of Algorithm 2, GIMME checks every combination of a solution for the head for topological connectedness with a returned solution for the tail (the number of which typically remains small since only topologically connected paths are returned). The related work is done (i) in a loop for all solutions s ∈ T (lines 27  31 in Algorithm 2), (ii) in the call to Algorithm 4 (leftExtensions) in line 32 in Algorithm 2), and (iii) in line 33 in Algorithm 2 which adds newly found solutions (i.e. lists of edges) as new elements to X, the tentative list of solutions (a list of edge lists).
Regarding (i), the run time of this loop is in O(T) since in the worst case, during each of the T iterations of the loop, one more element (a solution s ∈ T) is added to set X, an operation of constant run time.
Regarding (ii), by a previous consideration we already have that the run time of Algorithm 4 is in O(e⋅D) where e is the first and D is the second parameter of leftExtensions (see above). Therefore, the run time for (ii) is in O(d⋅T) = O(T) since d is a directed path constructed from at most C = 3 candidates for the head of the source route.
Regarding (iii), the operation of adding L elements to the set X runs in O(T) since L≤T elements are added in constant run time each.
By the empirical result from Section 4.2.2 for \(\overline {T}_{\text {standardised}}\) we know that the average of T is asymptotically bounded by a small constant, and thus the operations of (i), (ii), and (iii) all run in amortised constant average time.
 (a)
constant (worst case and average) time cost for dividing the problem, and
 (b)
amortised constant average time cost for combining the partial solutions.
 (i)
a worstcase and average time cost in Θ(n) for geometryMatches using GM, and
 (ii)
an amortised average time cost in Θ(n) for solutions, and
 (iii)
amortised constant average time costs for either largestAdmissibleSolution or firstClosedLineSolution.
These more formal results are confirmed by the linear run time behaviour observed during the experiments described in Section 4.2.2. Note that for a failed match, the run time can only be smaller, and therefore in this case GIMME has a worstcase run time in O(n).
4.2.4 Qualitative comparison of GIMME and the RNMbased approach
A majority of the recent road network matching (RNM) approaches are based on Buffer Growing (BG), cf. Section 2.3.2, Iterative Closest Point (ICP), cf. Section 2.3.1, or the combination and evolution of them [44]. The two most recent ones are the DelimitedStrokeOriented (DSO) algorithm [44] (see also Section 2.3.3), and NetMatcher [23].
Unfortunately, open source reference implementations are not available for the aforementioned RNM approaches. DSO as well as NetMatcher are quite complex algorithms, which makes a reimplementation very costly. Consequently, reference matching results allowing for a quantitative comparison to GIMME for the same pair of maps are very expensive to obtain.
Thus, this section gives a comparison of GIMME and the RNMbased approach on more qualitative terms. It is based on previously reported results and on the results of Section 4.2.1.
For a description of DSO, see Section 2.3.3. Like ICP, NetMatcher matches vertices first, and subsequently matches edges that are connected to matched vertices. Netmatcher assumes maps from the same vendor with different levels of detail, which is a strong limitation. Therefore, a qualitative comparison in terms of evaluation measures will focus on comparing GIMME to the RNMbased approach with DSO (RNM_{DSO}), since DSO (like GIMME) has no such restriction. On the other hand, run times for DSO are reported only for small map sections around 6,000 edges in Munich [44]. In contrast, the run times reported for NetMatcher were measured for road maps of a more practical size around 100,000 edges. Therefore, in Section 1 we have referred to the run times reported for NetMatcher.
In [44], evaluation measures are reported for the following experiment with DSO: sections of maps for Munich, Germany from TeleAtlas and NAVTEQ describing the road network in the downtown area have been matched. From all experiments described in [44], this is the one most similar to the first experiment described in Section 4.2.1: in both experiments, a TeleAtlas map for a road network in a German city is matched to the respective NAVTEQ map. On the other hand, neither the same pair of maps nor the same city or comparable map sizes have been used. Hence, the reported results only allow for a qualitative comparison.
During the aforementioned experiment, the success rate of DSO has been reported as 99.6% (termed ”matching correctness” in [44]). This rate reflects the probability of matching an edge of the road network successfully (i.e., correctly) with DSO. As before, we refer to RNM_{DSO} as the RNM method establishing the required static mapping with DSO. The approach RNM_{DSO} matches a linear route in the source map consisting of m edges to the target map by matching all of its edges individually to the target map, using the established static mapping, and by “glueing together” the corresponding edges or lists of edges to the final target route. The probability of a successful match can be estimated to 0,996^{ m } since all individual edge matches must be successful for a successful match of the route. Notice that GIMME does more than merely glueing together matched edges: instead it keeps multiple matching candidates per edge, and then a “best” combination of candidates for all route edges is constructed. This helps to match e.g. long parallel roads with a short average distance to each other (a more detailed explanation follows below).
Moreover, for the aforementioned experiment with DSO, the following numbers of ”proper nonmatches” (i.e., true negative matches), and of false negative matches have been reported in [44]: n _{tn} = 521,n _{fn} = 122. The respective error detection rate is \(\frac {521}{521+122} \approx 0.810=81.0\%\). Consequently, for RNM_{DSO} the probability of truthfully identifying a route as not having a homologous counterpart in the target map should be at least 81.0%: this is because a source route has no counterpart in the target map if at least one of its edges has no counterpart(s) in the target map, and because the probability of correctly classifying a first such edge is approximately 81.0%. Theoretically, if this first edge has been misclassified, i.e. if it rather has a match in the target map, the route could still be correctly identified as having no counterpart, if other edges without counterpart(s) exist in it. For some pairs of maps, this effect could cause an increase of the error detection rate.
Success rates (q _{success}) and error detection rates (\(q_{\text {error\_detection}}\)) for line locations with various fixed numbers of segments
Number of segments  GIMME  RNM_{DSO} (estimated)  

q _{success} [%]  \(q_{\text {error\_detection}}\) [%]  q _{success} [%]  \(q_{\text {error\_detection}}\) [%]  
1  99.7  69.0  99.6  81.0 
5  99.7  69.0  98.0  81.0 
10  99.6  69.0  96.1  81.0 
15  99.0  69.0  94.2  81.0 
20  98.0  69.0  92.3  81.0 
25  93.8  69.0  90.5  81.0 
For example, let us consider two parallel roads with a short average distance to each other, existing in both the source and in the target map. Let us assume that there is a significant offset between the two maps, and that one of the parallel roads needs to be matched. Because of the offset, the source road might on average be farther away from the homologous road than from the “wrong” second road, i.e. the one that is running parallel to it in close vicinity. Consequently, a road network matcher just matching edgebyedge might make many wrong decisions here. In particular, the static mapping of an RNMbased approach stores only one matching candidate per edge, and consequently the approach is not capable of correcting such incorrect choices later. In contrast, GIMME stores every reasonable candidate edge and is therefore able to discard wrong decisions later. Provided that the wrong road deviates from the correct one at some point of the course, GIMME will detect it, discard the wrong decisions and finally construct the route from only the correct matches.
Notice that the error detection rate of GIMME did not increase with the number of segments per route. This means that, at least for the considered pair of maps, there is no increase in the error detection rate due to multiple unmatchable edges. Consequently, no such increase would be observed for the RNMbased approach either, and therefore the respective error detection rates for RNM_{DSO} in Table 4 are all estimated as 81.0%. GIMME showed a lower detection rate of 69.0%. Nonetheless it is noteworthy that also DSO’s error detection rate is significantly lower than it’s success rate like it was the case for both GIMME and OpenLR (see Section 4.2.1).
In practice, a good success rate is much more important than a good error detection rate, since the matching errors cause much more problems than the unmatched objects. This holds for both considered application domains, i.e. for RNM as well as for dynamic location referencing (DLR). For RNM, this holds because, “[…] the unmatched objects can be very easily marked as doubtful, whereas the process to detect the errors is time consuming and labour intensive, because erroneous matches need to be analysed one by one” [44]. For DLR, unmatched location references can easily be suppressed or replaced at the application level, e.g. a trafficmonitoring application can always drop an unmatched reference to a congested stretch of road, or perhaps replace it by a less accurate, more general textual warning. On the contrary, actually delivering a piece of wrong information about a jam (because of an unnoticed matching error) can irritate the users.
To summarise: For the limited scope of the experiments described in [44] and in Section 4.2.1, the RNMbased approach with DSO (” RNM_{DSO}”) can be compared to GIMME in qualitative terms as follows: GIMME is expected to have a higher success rate than RNM_{DSO}. For all considered algorithms (RNM_{DSO}, GIMME, and OpenLR), the error detection rate is significantly lower than the success rate. The error detection rate of GIMME is expected to be lower than that of RNM_{DSO}. It is of note that the success rate is the more important measure for both application domains, i.e. for both RNM and DLR.
4.2.5 Example matches
This section gives depicted examples for successful matches of circular and linear routes as resulting from GIMME, and also discusses a situation where GIMME fails to find the correct match, and how to remedy it.
Consequently, GIMME does not find a match in this case though the found candidate edge is obviously homologous to the source edge. This could be remedied by an increase of the respective threshold, but another possible remedy is to add an optional “snap mode” to GIMME. This mode is enabled at the application level (i.e., by an application making use of GIMME), and it is assumed that the application always passes source routes starting and ending at a graph node of degree greater than 2 (that is, starting at an intersection rather than somewhere between two intersections). Candidate routes are then either trimmed or expanded by “snapping” start and end position of the route to the nearest graph node of degree greater than 2 in the target map. The rationale is that intersections are usually prominent enough to occur in both maps. Therefore, the snapping strategy should identify the correct start and end position of the candidate route in most cases. It goes without saying that GIMME with “snap mode” succeeds to find the correct match in the described example.
It is of note that the experiments reported in Section 4.2 have been conducted without applying the outlined “snap mode”, and that situations similar to the described one did not occur frequently.
For the experiments conducted in Section 4.2.4, cases like that had been excluded and replaced by new randomly generated instances until no problematic locations were contained in the sample anymore.
5 Conclusion
A new approach to match linear or circular routes between two dissimilar maps has been presented. It is highly accurate and mapindependent, but access to both involved maps is required. This requirement is met whenever an omniscient matching centre with access to both the source and the target map can be used. If location references need to be transmitted from a sender to a receiver, the algorithm computes the matches at the receiving side, using both maps. In such a setting, the approach can also be combined with a bandwidthefficient dynamic location referencing method like e.g. OpenLR to obtain a compact format before the descriptive data is transmitted. Notice that in this case run time will increase due to the overhead of calling the additional bandwidthefficient method for every location.
The new approach exploits the fact that information from both maps is available. It achieves high accuracy by comparing geometrical route descriptions derived from both maps. It advances on the path of previous methods in the area of road network matching and map conflation like e.g. Geometry Matching and Buffer Growing. In an experimental evaluation, the new approach has been compared to TomTom’s OpenLR, and the results clearly demonstrate the increased accuracy of the resulting matches: it is currently capable of mapping closed line locations (i.e. circular routes) from a TeleAtlas map to a NAVTEQ map onthefly with a success rate of 97.5% (OpenLR: only 82.5%), and also capable of mapping short line locations (i.e. linear routes) onthefly between the same maps, with a success rate of 99.7% (OpenLR: 91.9%).
Notes
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