Satellite-derived bathymetry of Arabian Gulf ports under leakage-free validation: a multi-site benchmark of empirical, machine-learning and deep-learning models against 285,000 multibeam soundings, with an outlook for UAE waters
Satellite-derived bathymetry (SDB) is often proposed as a low-cost complement to multibeam surveys of ports, but its accuracy is seldom measured on an area the model has never seen. We test SDB in the turbid, shallow and tidal waters of the Abu Dhabi coast against 285,619 multibeam soundings from twelve surveys at six port areas, reduced to Lowest Astronomical Tide. The Stumpf ratio, the Lyzenga transform, a monotonic gradient-boosted tree model (LightGBM) and a U-Net ensemble were trained on 72 Sentinel-2 images and tested in transfer to an unseen site and with local control (about 20% of a site's soundings, spatial blocks beyond a 225 m buffer). The validation design changed the result more than the model: the same LightGBM model gave 0.57 m RMSE under a random pixel split, 1.30 m under spatial blocks and 1.66 m at an unseen site. In transfer the Stumpf ratio (1.45 m) was not beaten by LightGBM, the U-Net or Lyzenga. With local control a hybrid of the tree output and the band ratio reached 1.11 m over 0-11 m, 67% of pixels within the IHO S-44 Order 2 tolerance and a conformal 95% interval with 95% observed coverage; in clear water it reached 0.29-0.53 m. The seabed could not be read beyond 11 m, and summer images were five to six times more turbid than winter ones. ICESat-2 ATL24 agreed with the surveys to 0.25 m after a depth-dependent offset was removed. A 60 m reconnaissance mosaic of 17,200 km2 of the UAE coast, calibrated cell by cell on ICESat-2 without any vessel survey, reached 1.42 m RMSE on held-out blocks over half of the product. Sentinel-2 SDB with about a hundred control depths per image is a usable reconnaissance and change-monitoring tool in these waters to about 10 m; it does not meet S-44 Order 1 without local control, and accuracies from random splits should not be taken as operational accuracy.