DEM (Digital Elevation Model) files

What It Is:

• a DEM file is a simple, regularly spaced grid of elevation points
• the most common kind of DEM files come from the USGS (U.S. Geological Survey)
• a fact sheet provides a quick introduction to DEM
• the DEM standard is described in the USGS DEM Standards documents
  • specifically, the file format is in Part 2, DEM data specification (PDF file)
  • there is public domain source code for reading DEM files, as well as many freely available tools
• types of USGS DEM:
  • 1-Degree DEM 3x3-arc-second (~90m) data spacing: 1201*1201 data points; poor quality
  •  30 minute DEM (2-arc-second)
    • distributed as four units of 15-minute DEM data covering full 30- by 30-minute areas
    • this format is less commonly used than the others
  • 7.5-minute DEM
    • ground spacing: 30 meters ("1 arc-second") or 10 meters ("1/3 arc-second")
    • the boundaries are on a lat-lon grid, but the datapoints are in UTM, resulting in irregular sizes, e.g. 370 x 465 data points
    • these 10m DEMs are the best, most accurate data available from the USGS
• non-USGS DEMs
  • there have some other formats over the years which use the ".DEM" file extension but are a different, incompatible format
  • for example, there was a popular landscape rendering program called "VistaPro" which defined it's own, different DEM format, so you may occasionally encounter a "VistaPro DEM"
  • There is a format called "Japanese DEM" with the file extension ".mem"
  • i've encountered a non-standard DEM, very similar to USGS DEM but with the following differences:
    • header is 893 bytes, instead of the standard 864 or 1024
    • uses UTM with 100m spacing
• SDTS DEMs
  • DEM data which has been converted to the SDTS format is referred to as "SDTS DEMs"

Where to Get It:

• DEM in SDTS format
  • are freely available online from the USGS
  • when it's not online, it can be very inexpensively purchased from the USGS
  • some high-res DEMs for individual states are available free online - see the Locations Page

Utilities

• DEM2DXF (290k) is a small Windows program for people in the CAD world who are stuck using DXF

USGS DEM - Technical Details:

• Classification Levels
  • Root-mean-square error (RMSE) is a measure of how closely a data set matches the actual world
  • Level-1 DEMs
    • reserved for 7.5-minute DEMs created by scanning NAPP photography
    • vertical RMSE of 7 meters is the desired accuracy, 15 meters is the maximum permitted.
  • Level-2 DEMs
    • have been processed or smoothed for consistency and edited to remove identifiable systematic errors, derived from hypsographic and hydrographic data digitizing
    • RMSE of one-half contour interval is the maximum permitted.
  • Level-3 DEMs
    • are derived from DLG data from both hypsography (contours, spot elevations) and hydrography (lakes, shorelines, drainage)
    • RMSE of one-third of the contour interval is the maximum permitted.
  • sometimes you will encounter both level 1 and 2 DEMs for the same area; the higher level number is much better data and should be used
• File format
  • type A record: header information, one per file
  • type B record: made up of data from one-dimensional arrays called profiles, one per line of elevation data
  • type C record: contains statistics on the accuracy of the data in the file
  • Logical record size of 1,024 bytes
    • more than one record is usually required to store a single record type B
• Coordinate System Datum
  • DEM files use one of the following:
    

    Datum		Ellipsoid
    1=North American Datum 1927 (NAD 27)		CLARKE_1866
    2=World Geodetic System 1972 (WGS 72)		WGS_72_ELLIPSOID
    3=WGS 84		WGS_84_ELLIPSOID
    4=NAD 83		GRS_1980
    5=Old Hawai‘i Datum		CLARKE_1866
    6=Puerto Rico Datum		CLARKE_1866

  • it's important to keep these in mind if you are converting projections, or trying to match data from different sources