Each 7.5-Minute DEM quad covers an approximate area of 7 x 9 miles.
Digital Elevation Models (DEMs) are terrain elevations located at regularly spaced horizontal intervals. They are available in the following scales: 10 and 30 meter, 7.5', and 3 arc second / 60'. These files cover the same area as a standard USGS quadrangle.
What is the difference between a 1 Degree DEM and a 7.5 Minute DEM?
The 1 Degree X 2 Degree DEMs are sampled every 3 arc seconds (~ 280 ft). They cover 2 Degrees of Longitude and 1 Degree of Latitude, or approximately 3,700 square miles. They cover a larger but less detailed area than the 7.5' DEMs.
The 7.5 minute DEMs are sampled every 30 or every 10 meters. They cover 7.5 minutes of Latitude and 7.5 minutes of Longitude, or about 60 square miles. They cover a smaller area but have greater accuracy than the 1 Degree DEMs.
Which Software packages support DEMs?
DEMs can be imported into ArcView with Spatial Analyst, IDRISI, ENVI, ERDAS Imagine, MicroDEM, 3DEM, AutoCAD (both DXF Mesh and Point files), MapInfo with Vertical Mapper, Global Mapper and most software that supports ASCII files. Please feel free to download some of our DEM samples to see which format works for you.
The SDTS (Spatial Data Transfer Standard) formated DEMs have been corrected for a geo-positional error. The SDTS files contain the vector, raster and point profiles necessary for creating a USGS standard DEM file. They are available here for free download with a limit of ten files.
From the SDTS data, we are able to create several formats of the DEM data, including DEM, ASCII point, ASCII Grid, DXF Mesh, DXF Point, Vertical Mapper GRD, MapInfo TAB, ASC files (compatible with ArcView Spatial Analyst) and others. The formatted files import into your software package already reprojected and formatted exactly to your specifications. There is no limit to the number of formatted DEMs you may order.
The default projection for these data is UTM -- majority being in NAD 27. Custom reprojections are available.