University GrenoblenAlpes, Laboratory ISTerre, UMR CNRS-UGA-USMB-IRD-UGE 5275, Grenoble, France

We assess the accuracy and the precision of the TanDEM-X digital elevation model (DEM) of the western Gulf of Corinth, Greece. We use for that a dense set of accurate ground coordinates obtained by kinematic GNSS observations. Between 2001 and 2019, 148 surveys were made, at 1 s sampling rate, along highways, roads and tracks, with a total traveled distance of ~25,000 km. The data are processed with the on-line Canadian Spatial Reference System precise point positioning software. From the output files, we select 885,252 coordinates from epochs with theoretical uncertainty below 0.1 m in horizontal and 0.2 m in vertical. Using specific calibration surveys we estimate the mean vertical accuracy of the GNSS coordinates at 0.2 m. Resampling the DEM by a factor of ten allows to compare it with the GNSS in pixels of metric size, thus smaller than the width of the roads, even the small trails. The best fit is obtained by shifting the DEM by 0.47 ± 0.03 m upward, 0.10 ± 0.1 m westward, and 0.36 ± 0.1 m southward. Those values are twenty times below the nominal resolution of the DEM. Once the shift is corrected, the root mean square deviation between TanDEM-X DEM and GNSS elevations is 1.125 m. In forest and urban areas, the shift between the DEM and the GNSS increases by ~0.5 m. The metric accuracy of the TanDEM-X DEM paves the way for new applications for long-term deformation monitoring of this area.