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
    • 1-Degree DEM 3×3-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 DEMUTM-style 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:

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

Source

LEAVE A REPLY

Please enter your comment!
Please enter your name here

Comment moderation is enabled. Your comment may take some time to appear.