Limitations in Extraction of Survey Data from Real-Time Kinematic GPS ADCP Systems

Abstract

A total of 595 real-time kinematic global positioning system (RTK-GPS) water surface elevations (WSE) were measured using georectified and nongeorectified survey systems along a 21-km-long river reach to evaluate vertical coordinate accuracy of a nongeorectified RTK-GPS acoustic Doppler current profiler (ADCP) system. A georectified survey of a series of quality control benchmarks produced a vertical root-mean-square error (RMSE) of 0.033 m, which was considered globally accurate for river survey purposes. The WSE measured simultaneously at the same locations comparing the nongeorectified to the georectified system resulted in a RMSE=0.741 m. Evaluation of intersetup precision of the nongeorectified WSE confirmed relative accuracy to each initialized arbitrary vertical datum but were found not to be globally accurate. Only 7% of the nongeorectified base station setups resulted in RMSE less than the combined manufacturer’s tolerances when compared to the georectified system at the same spatial locations. When vertical datum corrections were applied to the nongeorectified system from the globally accurate RTK-DGPS system, 80% of the base station setups demonstrated RMSE within the combined manufacturer’s tolerances. Datum corrections varied greatly, ranging from −2.43 to +1.80 m. This study demonstrates that when using nongeorectified RTK-GPS ADCP systems for purposes other than discharge measurement, such as in the extraction of bathymetry data for use in hydraulic assessments and models, great care must be exercised when verifying WSEs