Didger can import and export USGS Digital Elevation Model .DEM files.
The .DEM is a grid file and can be used to create grid based maps such as contour maps, 3D surface maps, etc.
The USGS DEM files have been replaced with the USGS SDTS files. Didger still reads this older file type .DEM files containing arc-second XY coordinates are automatically converted to latitude/longitude decimal degrees.
The USGS produced two DEM file types: the 7.5 minute DEM data and the 1:250,000-scale DEM data.
The 7.5 minute DEMs correspond to the standard 1:24,000 scale quadrangles. Elevation points are provided along north-south profile lines, and use a 30 meter spacing between adjacent data points and adjacent profile lines. Point locations are referenced to the UTM (Universal Transverse Mercator) coordinate system. The 7.5 minute quadrangles are usually not oriented exactly north-south. The DEM polygon contained within a 7.5 minute quadrangle consists of profiles that do not always have the same number of data points because of the variable angle between the UTM coordinate system and true north. This results in the potential for a regular, stair-stepped arrangement of the data points along the edges of the map which can translate to blanked grid nodes along the edges of grid files produced in Didger. This can lead to discontinuous contour lines along the edges of contour maps, or blanked edges on surface plots.
The 1:250,000 DEMs cover a 1 degree by 1 degree block representing one-half of a 1 degree by 2 degree 1:250,000-scale map. Point locations are based on the latitude/longitude coordinate system. Elevation points are provided along north-south profile lines, and use a 3 arc-second spacing between each data point and between profile lines. Three arc seconds correspond to approximately 90 meters in the north-south direction and variable spacing in the east-west direction due to convergence of meridians as latitude increases (approximately 90 meters at the equator and approximately 60 meters at 50 degrees latitude).
Import Options Dialog
No import options dialog is displayed.
Export Options
Didger does not currently export this file format.
Remarks
The USGS header may contain some fields that Didger does not support.
The elevation data contained in USGS DEM files are regularly spaced, but the direction of the grid lines may not exactly coincide with the grid edges. This is due to the variable angle between the UTM coordinate system (used in some DEM files) and true north. This results in the potential for a regular, stair-stepped arrangement of the grid nodes along the edges of the map which can translate to blanked grid nodes along the edges of maps produced in Didger. If the DEM file contains three-second arc coordinates, the coordinates are automatically converted to latitude/longitude coordinates in Didger.
See Also