Terrain Data Files
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Terrain Data Files used by the Terrain object are binary or ASCII files that contain terrain or elevation data that can be imported into FreeFlyer using any of the following GDAL (Geospatial Data Abstraction Library) formats.
FreeFlyer can load any of the terrain data sets shown in the table above using the Terrain Index Generator. These terrain data sets are commonly referred to as Digital Elevation Models (DEM). DEM's are arrays of regularly spaced elevation values referenced horizontally either to a Universal Transverse Mercator (UTM) projection or to a geographic coordinate system to generate Terrain Data Sets. The GTOPO30, SRTM, and NED terrain data sets are the most commonly available that are valid with the FreeFlyer Terrain object.
Note: The following information is provided to familiarize you with some publicly available terrain data sets. There are many more data sets in existence, whether publicly available, or owned by specific organizations, or classified with restricted access. FreeFlyer can process terrain data in any of the file formats listed above, regardless of the origin of the data set.
Global 30-Arc-Second Elevation (GTOPO30)
GTOPO30 is a global digital elevation model (DEM) with a horizontal grid spacing of 30 arc seconds (approximately 1 kilometer) that was derived from several raster and vector sources of topographic information. 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 (EGM96). GTOPO30 DEM data is stored on the USGS Earth Explorer website in two file formats:
To download any of the above file formats, access Earth Explorer and select your area of interest. Then under the data sets tab, select "Digital Elevation → GTOPO30" and download the desired file format. For help in obtaining GTOPO30 data sets review the USGS Earth Explorer Help guide to get started.
Shuttle Radar Topography Mission (SRTM)
SRTM is a near-global digital elevation models that covers land from 56°S to 60°N to generate a high-resolution digital topographic database of Earth. The horizontal datum is referenced to WGS84. The vertical datum represents elevation in meters using the Earth Gravitational Model 1996 (EGM96) as the reference. SRTM data are distributed in two levels: SRTM1 (for the U.S. and its territories and possessions) with data sampled at one arc-second intervals in latitude and longitude, and SRTM3 (for the world) sampled at three arc-seconds. Three arc-second data are generated using three by three averaging of the one arc-second samples. SRTM DEM data is stored on the USGS Earth Explorer website in three file formats:
To download any of the above file formats, access Earth Explorer and select your area of interest. Then under the data sets tab, select "Digital Elevation → SRTM → SRTM 1-ArcSecond Global" and download the desired file format. For help in obtaining SRTM data sets review the USGS Earth Explorer Help guide to get started.
National Elevation Data (NED)
The National Elevation Dataset is the primary elevation data product produced and distributed by the USGS. The NED provides seamless raster elevation data of the conterminous United States, Alaska, Hawaii, and the island territories. The NED is derived from diverse source data sets that are processed to a specification with a consistent resolution, coordinate system, elevation units, and horizontal and vertical datums. The data is distributed in geographic coordinates in units of decimal degrees, and in conformance with the North American Datum of 1983 (NAD 83). All elevation values are in meters and, over the conterminous United States, are referenced to the North American Vertical Datum of 1988 (NAVD 88). The vertical reference will vary in other areas. NED data is stored on the USGS National Map website, typically, in three file formats: