GEBCO (Gridded Bathymetric Chart of the Oceans)
GEBCO provides global bathymetry data sets for the world's oceans. However, it also includes land data, largely derived from the SRTM30 data set. The data is available at two grid spacings: 30 arcseconds (GEBCO_08 Grid) and 1 arcminute (GEBCO One Minute Grid).
How to Import
For the GMT/NetCDF format, choose Import DEM → GMT/NetCDF Raster from the File menu and then select the file, which will usually have the filename extension "*.grd".
See comments for Data Source.
Main Page: http://www.gebco.net
Download Page at BODC (UK): https://www.bodc.ac.uk/data/online_delivery/gebco/
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