The GTOPO-30 filter imports uniform lattices from several two-dimensional elevation grid file formats.
File Description
GTOPO30 is a global digital elevation model (DEM) resulting from a collaborative effort led by the staff at the U.S. Geological Survey's EROS Data Center in Sioux Falls, South Dakota. Elevations in GTOPO30 are regularly spaced at 30-arc seconds (approximately 1 kilometer). GTOPO30 was developed to meet the needs of the geospatial data user community for regional and continental scale topographic data.
GTOPO30 is a global data set covering the full extent of latitude from 90 degrees south to 90 degrees north, and the full extent of longitude from 180 degrees west to 180 degrees east. The horizontal grid spacing is 30-arc seconds (0.008333333333333 degrees), resulting in a DEM having dimensions of 21,600 rows and 43,200 columns. The horizontal coordinate system is decimal degrees of latitude and longitude referenced to WGS84. The vertical units represent elevation in meters above mean sea level. The elevation values range from -407 to 8,752 meters. In the DEM, ocean areas have been masked as "no data" and have been assigned a value of -9999. Low lying coastal areas have an elevation of at least 1 meter, so in the event that a user reassigns the ocean value from -9999 to 0 the land boundary portrayal will be maintained. Due to the nature of the raster structure of the DEM, small islands in the ocean less than approximately 1 square kilometer will not be represented.
File Format
To facilitate electronic distribution, GTOPO30 has been divided into 33 smaller pieces, or tiles. The area from 60 degrees south latitude to 90 degrees north latitude and from 180 degrees west longitude to 180 degrees east longitude is covered by 27 tiles, with each tile covering 50 degrees of latitude and 40 degrees of longitude. Antarctica (90 degrees south latitude to 60 degrees south latitude and 180 degrees west longitude to 180 degrees east longitude) is covered by 6 tiles, with each tile covering 30 degrees of latitude and 60 degrees of longitude. The tiles names refer to the longitude and latitude of the upper-left (northwest) corner of the tile. For example, the coordinates of the upper-left corner of tile E020N40 are 20 degrees east longitude and 40 degrees north latitude. There is one additional tile that covers all of Antarctica with data in a polar stereographic projection.
File Name Extensions
GTOPO-30 .HDR, .STX, .DEM
A GTopo30 data set consists of multiple files. The following files must exist in the same directory:
.HDR contains header information
.STX the statistics file, used to obtain zmin, zmax
.DEM contains the actual grid nodes
Format(s) Supported for Import
2D uniform lattice; 8-, 16-, 32-bit integer, float, double
Import Method
Choose the File | Import command.
Remarks
GTopo30 files are extremely large. The files can be read, though approximately 220 MB of RAM is needed for this operation due to the grid size. You must have the associated .DEM, .HDR, and .STX files to use GTopo30 files. These files are necessary for computing the X, Y, and Z limits. Some GTopo30 files contain a "no data" value of -9999 in the file. The no data value is set to zero elevation.
See Also