|USGS ASCII DEM files.|
Standard format for the distribution of terrain elevation data for the United States.
United States Geological Survey (USGS) standard.
DEM is an acronym for Digital Elevation Model.
Typically distributed as .zip or .gz compressed file.
The .dem file extension is often omitted.
Common variants are 1-degree and 7.5-minute-degree DEMs.
1-degree DEMs cover 1-by-1-degree blocks at a 3-arc-second grid spacing and 1201-by-1201 raster size.
The 1-degree USGS DEM format is occasionally referred to as 1:250,000 scale DEM.
7.5-minute DEMs cover 7.5-by-7.5-minute blocks at a 30-meter grid spacing and correspond to the USGS 1:24,000 and 1:25,000 scale topographic quadrangle map series.
The raster coordinates used for 7.5-minute DEM files are referenced in the Universal Transverse Mercator (UTM) projection, resulting in irregular raster dimensions.
Elevations are given in meters relative to the National Geodetic Vertical Datum of 1929 (NGVD) for the continental United States and relative to the local mean sea level in Hawaii.
|"Elements"||list of elements and options available in this file|
|"Rules"||full list of rules for each element and option|
|"Options"||list of rules for options, properties and settings|
|"Data"||array of terrain elevation values|
|"Graphics"||DEM file rendered as ReliefPlot|
|"Dimensions"||size of the elevation data array|
|"ElevationRange"||range of elevation coordinates, given in meters|
|"ElevationResolution"||resolution of elevation values, given in meters|
|"SpatialRange"||range of geographic coordinates, given in decimal degrees|
|"SpatialResolution"||grid spacing in arc seconds|
|BoxRatios||Automatic||effective 3D bounding box ratios|
|ColorFunction||"Topographic"||how to determine the color of surfaces|
|DataRange||Automatic||the range of latitude and longitude values to assume for the data|
|DataReversed||False||whether to reverse the order of rows|
|ImageSize||Automatic||the overall size of the image|
|LightingAngle||Automatic||the effective angle from which simulated illumination is taken to come|
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