A three-step strategy for generalization of 3D building models based on CityGML specifications
- 1.6k Downloads
3D building models of the world exhibit multi-scale properties. Different level-of-details (LoDs) are important for different applications. Therefore, generation of multi-scale representation of 3D building models to fulfill the demands of these applications is a generalization problem. In order to generalize 3D buildings, different pieces of information need to be preserved or removed to restrict the amount of data represented on a certain LoD. In this work, a three-step strategy based on simplification, aggregation and reconstruction of generalized buildings represented in City Geography Markup Language (CityGML) is proposed. The minimum length of edges (threshold value) for removal of amount of data is restricted to generalization specifications of CityGML characterized by differing accuracies and minimal dimensions of objects for each LoD. The main part of this paper is simplification of ground plans. For this purpose, a new approach is proposed to restrict number of edges, curves, and corners of ground plan of 3D building model on a certain LoD. Algorithms for simplification with the aim to derive LoD1 from exterior shells of buildings at LoD3 are implemented and tested on a number of buildings of Putrajaya city. The experiments showed that length of edge as threshold value is directly proportional to the size of generalized models.
Keywords3D Simplification Aggregation CityGML City models
City Geography Markup Language (CityGML) as Open Geospatial Consortium (OGC) standard is one of the modelling tool used to model and represent components of buildings such as outer shells, openings (windows, doors), outer building installations, interiors (chair, table, decorations, etc.) at different level-of-details (LoD1–LoD4) (Gröger et al. 2007). Furthermore, different LoDs of building models are important for different applications for efficient visualization and analysis purposes. But, direct rendering of these models can increase rendering time due to detailed data associated with buildings Mao et al. (2011). Therefore, generation of multi-scale representation of 3D buildings to fulfill the demands of these applications is a generalization problem. For this purpose, insignificant or detailed pieces of data need to be removed to restrict the amount of data on a certain LoD.
Several methods are applied to restrict the amount of data on a certain LoD by removing complicated edges, points, corners, etc. of 3D building models. In some cases, removal of transitional vertices is made as basis of generalization or larger edges are trimmed or averaged and a shortcut based on displacement is created to replace larger lines with generalized lines. In order to simplify components of 3D building models, generalization strategy is tailored to define which edges or vertices need to be removed or replaced by generalized edges.
Ground plans of 3D building models consist of a combination of features like points, lines and curves. Simplification methods such as point-reduction (Douglas and Peucker 1973), edge move (Buchin et al. 2011) and removal/adjustment of shortest edge (Fan and Meng 2009, 2012; Mao et al. 2011); Sester and Brenner (2004) and (Baig and Rahman 2013) can be applied to simplify ground plans. Edge move is a local operator to preserve the orientation of the edge but can always be performed on a non-convex polygon while preserving area, orientation and topology (Commandeur 2012). Additionally, in edge move, the basis of generalization needs to be defined to determine which edges need to be removed/adjusted. Similarly, adjustment/removal of shortest edges cannot be applied on curves as curves cannot be stored in their original shape in geo-data structures similar to line or polygons. Therefore, independent methods are necessary to simplify specific parts of ground plan.
In this paper, a framework based on three-step strategy is proposed. These steps include simplification, aggregation and reconstruction operations. Size and shape of generalized buildings are restricted to CityGML generalization specifications. The main part of this research is simplification of ground plans. For this purpose, a new simplification approach is proposed to restrict number of lines, curves and corners of ground plan of 3D building model on a certain LoD. In case of curved shapes, vertices of edges are taken into account for simplification instead of edges while non-curved shapes are simplified based on removal of shortest edge approach. Curved parts of ground plans are transformed down to the level of points and certain transitional points are removed to generate generalized edges while edges are considered for non-curved shapes.
With the aim to complete generalization, simplified structures of ground plans of walls and roofs need to be adjusted accordingly. For this purpose, height of the bounding box or length of highest wall at LoD1 is considered.
Algorithms for generalization with the aim to derive LoD1 are implemented and tested on a number of buildings of Malaysian city of Putrajaya.
Generalization specifications of CityGML are discussed in Sect. 2 followed by a framework and workflow based on three-step strategy for generalization of 3D building models in Sect. 3. Implementation and result are described in Sect. 4. Finally, the concluding remarks are presented in Sect. 5.
CityGML generalization specifications
CityGML specifications for generalization of LoDs (adopted from (Gröger et al. 2007)
CityGML generalization specifications
Objects blocks as generalized features
>6 × 6 m/3 m
Objects as generalized features
>4 × 4 m/2 m
Objects as real features
>2 × 2 m/1 m
Constructive elements and openings are represented
Generalization framework and workflow
The fundamental structure of the proposed 3D generalization framework is shown in Fig. 2. It consists of following four operations: extraction of exteriors from CityGML dataset; projection of exteriors as ground plans; generalization of ground plans and re-projection as generalized LoD. Generalization operation is the main part of this study which operates on ground plans.
CityGML schemas use the groundsurface type to define the ground plan of a building. On the other hand, it can be derived from the exterior shells of buildings modelled in CityGML by projecting the wall on the ground and connecting the footprint into a closed polygon (Fan and Meng 2012). However, this method is based on the exterior shell generated by their algorithm which is quite complex and time consuming (Mao et al. 2011).
Generalized structures are converted to LoD1 and exported with the aim to visualize using LandXplorer CityGML Viewer 2009.
Simplification of 2D footprints
Generalization based on simplification of footprints is one of the efficient methods to modify or eliminate insignificant details from 3D buildings. Footprints of 3D building models may contain corners, offsets and intrusions/extrusions. Therefore, generalization strategy is adopted based on certain characteristic of neighbourhood and a threshold value to reduce transitional points, selection and replacement of corners, edges, offsets, etc. However, the fundamental criteria to remove/adjust potential points, edges and polygons are still subject to the research. Besides selecting points (Douglas and Peucker 1973) and edges (Sester and Brenner 2004) by a threshold value, angles in Fan and Meng (2012) and areas in (Bose et al. 2006) are made as a basis of simplification operation.
In this study, to avoid removal/addition of important parts of buildings, geometry-based rules based on CityGML generalization specification (see Table 1) are imposed. Additionally, in certain cases similar to (Fan et al. 2009), an important object, which lies on the edge need not to be removed. If more than one smaller parts make a pattern consecutively and which are smaller than threshold, all of them needs to be taken into consideration collectively similar to (Baig and Rahman 2013). Features should be treated together if the removal or the modification can affect the neighbouring feature or if there is an overlapping between the two smaller features.
Case 1: shortest edge
Case 2: curved shapes
Aggregation of ground plans
Reconstruction of generalized models
Implementation and results
This research focused on derivation of LoD1 from outlier of 3D buildings modeled at LoD3 in CityGML. In order to achieve this objective, generalization of 3D buildings is implemented in a MATLAB (version Matlab 7.6.0, MathWorks) operated on a Core ™ i7 CPU @ 2.40 GHz, 4.00 GB RAM and Microsoft Windows 2007. A number of 3D building models of Putrajaya city of Malaysia are used to test the proposed algorithms.
Implementation process is divided into following four operations: (1) projection, (2) simplification, (3) aggregation of ground plans and (4) reconstruction of generalized LoD. Decomposed 3D building outliers down to the level of vertexes is used for further processing. Vertices of 2D faces are used to calculate length to remove smaller edges or corners of curved shapes. Nodes of edges are moved, adjusted and merged using programming routines of MathWorks for simplification purposes. The output of simplification process was stored into arrays and converted into XML-based CityGML format. Additionally, for better visualization, generalized models were visualized in LandXplorer CityGML Viewer.
A comparison of threshold values (m), data reduction (%) and computing time (sec)
Data reduction (%)
Computing time (s)
The result of all proposed simplification and aggregation procedures presented in this paper, to derive LoD1 of building models of Putrajaya city of Malaysia are visualized in LandXplorer CityGML Viewer V. 2009. Buildings located closer to viewing point of a user seem less generalized as compared to those locating far from user in street view.
The aim of this study was to develop a strategy for data reduction of 3D city models from higher level- of-details to lower level-of-details. We investigated and implemented generalization operations (e.g. simplification, aggregation and reconstruction) to obtain this objective.
Data file of original buildings modeled in CityGML cannot be loaded in Matlab directly. Therefore, only nodes of exteriors of buildings from these source files were extracted and loaded for further processing.
A new method for simplification of curves along with intrusion and extrusion is included in this paper. Differential changes and ratios are important but are not taken into account. A part of this paper is an extension of our previous paper (Baig and Rahman 2013) (e.g. Case 1).
Based on vertices, the edges were generated to be used for implementation of Case 1 and aggregation purposes. These methods were implemented and tested with the aim to reduce complexity of ground plans of 3D buildings.
Secondly, all buildings were generalized into LoD1 with footprints or ground plans. Convex envelope technique was applied to aggregate building footprints.
Further research on generalization and adjustment of distinctive roof structures with walls of buildings at LoD2 would be an addition in future.
We would like to express our deepest acknowledgement to Universiti Teknologi Malaysia (UTM) for providing research grant Vote No. Q.J130000.7127.04J81. Our sincere appreciations to Research Management Centre (RMC) of UTM and Ministry of Higher Education (MOHE), Malaysia for enabling us to carry out this research project.
- Anders, K.-H. (2005). Level of detail generation of 3D building groups by aggregation and typification. Paper presented at the XXII International Cartographic Conference, A Coruña, Spain.Google Scholar
- Buchin, K., Meulemans, W., & Speckmann, B. (2011). A new method for subdivision simplification with applications to urban-area generalization. Paper presented at the Proceedings of the 19th ACM SIGSPATIAL International Conference on Advances in Geographic Information Systems, Chicago, Illinois.Google Scholar
- Bundy, L., Jones, B., & Furse, E. (1995). Holistic generalisation of large-scale cartographic data. In J. C. Müller, J. P. Lagrange, & R. Weibel (Eds.), GIS and Generalisation, Gisdata 1 (pp. 106–119). London: Taylor & Francis.Google Scholar
- Commandeur, T. (2012). Footprint decomposition combined with point cloud segmentation for producing valid 3D models. The Netherlands: Masters, Delft University of Technology, Delft.Google Scholar
- Coors, V. (2003). Graphical Abstraction and Progressive Transmission in Internet-based 3DGeoinformationsystems. Doctorate, Technischen Universität Darmstadt, Darmstädter. (D17).Google Scholar
- Douglas, D. H., & Peucker, T. K. (1973). Algorithms for the reduction of the number of points required to represent a digitized line or ITS caricature Cartographica. The International Journal for Geographic Information and Geovisualization, 10(2), 112–122. doi: 10.3138/fm57-6770-u75u-7727.CrossRefGoogle Scholar
- Fan, H. C., & Meng, L. Q. (2009). Automatic derivation of different levels of detail for 3D buildings modelled by CityGML. Paper presented at the 24th International Cartography Conference, Santiago, Chile.Google Scholar
- Fan, H. C., Meng, L. Q., & Jahnke, M. (2009). Generalization of 3D Buildings Modelled by CityGML. Advances in Giscience, 387-405. doi: 10.1007/978-3-642-00318-9_20.
- Glander, T., & Döllner, J. (2007). Cell-based generalization of 3D building groups with outlier management. In 15th annual ACM international symposium on advances in GIS (pp. 364–367). Seattle: Washington.Google Scholar
- Gröger, G., Kolbe, T. H., & Czerwinski, A. (2007). Candidate OpenGIS® CityGML Implementation Specification (City Geography Markup Language) 2007-06-17. 0.4.0, from http://www.opengeospatial.org/legal/.
- Sester, M., & Brenner, C. (2004). Continuous generalization for visualization on small mobile devices. Paper presented at the Developments in Spatial Data Handling—11th International Symposium on Spatial Data Handling, Berlin, Heidelberg.Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.