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The dataset of the GTOPO30 can also be mapped into spherical co-ordinates by this import converter to create a spherical model of the globe as illustrated by this example (rendered with the Okino's NuGraf software):
DEM datasets typically contain 60000 or more quadrilateral polygons, or 1200000 triangles (for a 258x258 resolution sample; the maximum DEM dataset size if 2050x2050 which would result in 4.2 million quadrilaterals or 8.4 million triangles). This is an enormous number of polygons for most 3d rendering programs so this DEM converter incorporates two unique options to overcome this problem:
In 1997 the United States Geological Survey (USGS) created a new, all-encompassing file format called SDTS which can hold many variations of the USGS data set archives. A SDTS dataset normally consists of 18 .ddf files which make up the spatial data transfer, rather than a single .dem file.
In 2004 the USGS office created the NED file format as described here: http://ned.usgs.gov.
GTOPO-30 is a new twist on the DEM data format. Data in this format can be obtained from http://edcwww.cr.usgs.gov/landdaac/gtopo30/gtopo30.html.
DEM elevation data spacing varies from 30 meters for 7.5-minute DEMs to 3 arc seconds for 1:250,000 scale maps. All DEM data are similar in logical data structure and are ordered from south to north in profiles that are ordered from west to east.