128x64 `T42 Gaussian' Ocean Depth and Land Elevation

We document here a 128x64 (i.e. `T42 Gaussian') ocean depth and land elevation data set which can be utilized as a land-sea mask where ELEVATION < 0 represents oceans and seas, and ELEVATION 0 represents land. Note that `T42 Gaussian' is simply a nominal designation in this case and does not imply spectral truncation of any kind. Sea ice is not represented in this data set.

The 128x64 version was derived from a 5' x 5' NGDC (National Geophysical Data Center) ocean depth and land elevation data set made available by the Data Support Section (DSS) at NCAR. See TerrainBase, Global 5-Minute Ocean Depth and Land Elevation, from NGDC which provides useful background and commentary. Bilinear interpolation with binary search / lookup was used to perform the transformation from a 5-minute 4320 x 2161 grid to a 128 x 64 `T42 Gaussian' grid.

Note that the extremes of the Earth's actual topography are not well represented in the range of elevations shown in the color bar below. This is an artifact of sampling at 128x64 resolution which bypasses, for example, many of the maxima such as Everest (+8850 m) in the Himalayas, and minima such as the the Challenger Deep (-11034 m) in the Mariana Trench in the western Pacific Ocean. On the other hand, coastal demarcation, and broadscale features such as the Tibetan Plateau, the Rocky Mountains, and abyssal plains, are reasonably well represented ¹.

Incidentally, using the 5' x 5' NGDC ocean depth and land elevation data set, we have computed a total ocean volume of 1.3314 x 1018 m3 with an estimated error of ±0.0001 x 1018 m3. (The total ocean volume reported here includes Hudson Bay and the Mediterranean, Red, Black, and Caspian seas.)

¹ Areas where the 128x64 coastal demarcation deviates noticeably from the actual coastline include portions of the Red Sea and Persian Gulf, Java and Irian Jaya in Indonesia, the Korean Peninsula, Chiapas and the Yucatan Peninsula in Mexico, the Panamanian Isthmus in Central America, and the Florida Peninsula in the United States. These predominantly are regions with very limited longitudinal resolution on a 128x64 grid.

Our ocean depth and land elevation data set is archived on a 128x64 (nominally `T42 Gaussian') grid with longitudes and latitudes identical to those in the netCDF and Fortran direct access budget product files. See T42b_ELEVATION.nc and T42b_ELEVATION.Fda in ftp://ftp.cgd.ucar.edu/pub/CAS/NEWBUDGETS.

An ncdump -h T42b_ELEVATION.nc gives

netcdf T42b_ELEVATION {
        lon = 128 ;
        lat = 64 ;
        float lon(lon) ;
                lon:long_name = "longitude" ;
                lon:short_name = "lon" ;
                lon:units = "degrees_east" ;
        float lat(lat) ;
                lat:long_name = "latitude" ;
                lat:short_name = "lat" ;
                lat:units = "degrees_north" ;
        float ELEVATION(lat, lon) ;
                ELEVATION:long_name = "Ocean Depth and Land Elevation" ;
                ELEVATION:short_name = "ELEVATION" ;
                ELEVATION:units = "meters" ;
                ELEVATION:_FillValue = 1.e+36f ;

// global attributes:
                :DATA_DESC = "TerrainBase global 5-minute ocean depth and land elevation from NOAA/NGDC, interpolated to T42 resolution           " ;
                :DATA_SOURCE = "Derived from ds759.2 of the Data Support Section, Scientific Computing Division, National Center for Atmospheric Research" ;

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