|USGS Digital Elevation Model
Both USGS 7.5 minute and 1 degree ASCII DEM files are supported for reading by the GDB library. USGS DEM files contain one 16-bit channel containing elevation data, but no auxiliary data.
Supported USGS DEM projection codes are "METER", "UTM", "GEOGRAPHIC", and "ALBERS". By definition USGS DEM files are supposed to be based on the NAD27 (E0) datum, however, PCI can also detect some that were based on the NAD83, WGS72, WGS84, Old Hawaii and Puerto Rico datum.
USGS DEM elevations may be meters, feet, or perhaps even other units. Upon import the DEM channel will be marked with metadata indicating an ELEVATION_UNITS of either FEET or METRE. This information can be inspected by a user, and a simple model can be used to convert between feet and metre units or vice-versa.
Areas of missing data (void areas) in USGS DEM files are set to a background value of -32767. The NO_DATA_VALUE metadata is attached to the channel indicating the background value (-32767).
The DEMWRIT function may be used to write USGS DEM files. The input PCIDSK file must be georeferenced using UTM coordinates with a zone number. Export support, however, is not available through GDB.
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