Can recreational echosounder-chartplotter systems be used to perform accurate nearshore bathymetric surveys?
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This work intends to determine if low-cost surveying techniques based on recreational echosounders can be used to perform nearshore bathymetry for analysing evolution of coastal sectors. For that purpose, two hydrographic surveying techniques were compared, i.e. (1) a real-time kinematic differential global positioning system (RTK-DGPS) synchronised with a single beam echosounder with real-time tidal elevation correction and (2) a low-cost recreational echosounder-chartplotter system using Global Navigation Satellite Systems (GNSS) with real-time European Geostationary Navigation Overlay Service (EGNOS) augmentation services and depth values post-processed using measured sea level. Two bathymetric data sets were obtained, one by each method, for the same area and survey lines at an ebb tidal delta (Tavira Inlet, Ria Formosa Portugal). Vertical differences were determined assuming no morphological variations between surveys. Results showed that depth elevation differences between bathymetric surfaces were of 0.10 ± 0.16 m, slightly higher but within the same order of the error attributable to the used interpolator (0.00 ± 0.11 m, triangular surface fitting). The differences between surveys performed with two different equipment sets and using different methodologies for correcting water elevations are very small both quantitative and qualitatively. Those differences can be diminished by improving the tidal level correction and uncertainties associated to different tidal slopes throughout the survey area. Pitch/roll corrections performed with low-cost GPS receivers would be also a valuable addition to the accuracy and precision of the method. It is then concluded that navigation with EGNOS augmentation services and sounding devices ten times cheaper than combined RTK-DGPS with single beam echosounders allow to measure and monitor accurately the nearshore bathymetry.
KeywordsBathymetry Soundings EGNOS positioning Digital terrain model Ebb tidal delta
This paper is a contribution to the project EROS – Erosion of Rocky Shores – differences in protection promoted by sandy beaches and shore platforms (FCT–PTDC/CTE-GIX/111230/2009) and project MOSES - Moving Sands - Equilibrium State of Crenulated Coasts (PTDC/GEO-GEO/3981/2012) funded by the Portuguese Foundation for Science and Technology. André Pacheco was supported Portuguese Foundation for Science and Technology grant number SFRH/BPD/76110/2011. The authors would like to acknowledge the contribution of João Salvé-Rainha for logistical support during data acquisition. We also greatly acknowledge the contribution of the reviewers that greatly contributed to improve early versions of this work, particularly to Edwin Danson.
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