Corrected GEBCO 2025 and BEDMAP3

[Alan Wallcraft]

See https://data.hycom.org/pub/awall/datasets/topo/ for:

-rw-r--r--. 1 wallcraf 0375G018   59888902 Nov  8 15:48 bedmap3_ocean.nc
-rw-r--r--. 1 wallcraf 0375G018  711296315 Nov  8 15:48 bedmap3_sunk.nc
-rw-r--r--. 1 wallcraf 0375G018 2673604070 Nov  8 19:01 GEBCO_2025I2021.nc
-rw-r--r--. 1 wallcraf 0375G018   69531529 Nov  8 19:00 GEBCO_2025I2021_TID.nc
-rw-r--r--. 1 wallcraf 0375G018       1249 Nov 11 16:07 README.topo.txt
-rw-r--r--. 1 wallcraf 0375G018    5795084 Nov  8 22:25 Wallcraft_GEBCO25_coastal.pdf

GEBCO 2025 is the first satellite gravity based bathymetry to include SWOT radar altimeter (Science, 2024). However, it has 55,839 false islands in shallow water. The GEBCO_2025I2021 dataset provided here is a modification of GEBCO_2025 that removes its false land and false sea and re-interpolates near false land. It also re-interpolates near depths from navigation charts. It is designed for creating the bathymetry for ocean models and in particular for HYCOM. The design of GEBCO_2025I2021 is covered in the presentation: Wallcraft_GEBCO25_coastal.pdf. 

Antarctic floating ice shelves are important for tidal modeling. HYCOM treats them either as land or by "sinking" the ice onto the bottom so that the ice cavity is the correct thickness but at the surface rather than under ice. GEBCO can be used directly when treating floating ice shelves as land, but the best data set to use is BEDMAP3 which extends from the south pole to 60S. The bedmap3_ocean.nc and bedmap3_sunk.nc datasets are combinations of BEDMAP3 fields with floating ice shelves as land and sunk onto the bottom respectively. They can be used with the new HYCOM-tools program bathy_bedmap3 to produce a HYCOM bathymetry, with any reagion north of 60S set to data void.

See HYCOM-examples/GOMb0.08 /topo/ for how yo use a GEBCO 15-second dataset to create a HYCOM bathymetry.