Grid Files

Didger can import and save grid files. Didger can import Surfer .GRD, USGS DEM .DEM, GTopo30 .HDR, and DTED .DT* grid files. DEM files containing arc-second XY coordinates are automatically converted to latitude/longitude decimal degrees. Didger saves Surfer .GRD files.

GRD

Files with the .GRD extension are Surfer grid files. Didger can import grid files in the Surfer 7, GS ASCII, or GS Binary formats. Didger saves grid files in the Surfer 7 format.

DEM

The USGS DEM files have been replaced with the USGS SDTS files. Didger still reads this older file type. 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. This can lead to discontinuous contour lines along the edges of contour maps.
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).

GTopo30

GTopo30 files are extremely large. Didger can read the files, though approximately 220 MB of RAM is needed for this operation due to the grid size. To reduce the file size, you can use Grid | Extract in Surfer. You must have the associated .DEM, .HDR, and .STX files to use GTopo30 files in Didger. These files are necessary for computing the X, Y, and Z limits. Some GTopo30 files contain a "no data" value of -9999 in the file. The no data value is set to zero elevation in Didger.

DTED

Digital Terrain Elevation Data (DTED) are grids originally produced by the United States National Imaging and Mapping Agency ( NIMA). The files contain latitude/longitude coordinates with the Z units in meters. There are multiple "levels" of DTED files. Level 0 files .DT0 are 1-degree square blocks with 30 arc-second spacing, similar to GTOPO 30 files. Level 1 files .DT1 use three arc-second spacing. Level 2 files .DT2 use one arc-second spacing.