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ENVI

DEM Extraction Module

DEM Extraction Module

The DEM Extraction Module requires a separate license.

A digital elevation model (DEM) is a regularly spaced raster grid of elevation values of a surface terrain. You can use DEMs to produce maps such as contour maps, orthophoto maps, and perspective maps. You can also use DEMS for route planning in the construction of highways and railways. In remote sensing, DEMs are used in mapping, orthorectification, and land classification.

The DEM Extraction Module enables you to extract elevation data from scanned or digital aerial photographs, or from an along track or an across track pushbroom satellite acquisition, such as those from the ALOS PRISM, ASTER, Cartosat-1, FORMOSAT-2, GeoEye-1, IKONOS, KOMPSAT-2, OrbView-3, QuickBird, RapidEye, SPOT, WorldView-1, WorldView-2, or Ziyuan-3A satellites. Along track stereo images are acquired on the same orbital pass by a satellite which usually has more than one sensor looking at the Earth from different angles. Across track stereo images are those taken by the same sensor on multiple orbits.

The DEM Extraction Module also supports Digital Point Positioning Data Base (DPPDB) data.

The DEM extraction process requires a stereo pair of images containing rational polynomial coefficients (RPC) positioning from aerial photography or pushbroom sensors. RPCs are used to generate tie points and to calculate the stereo image pair relationship. DEM extraction does not currently support replacement sensor model (RSM) positioning.

The DEM Extraction Module is comprised of DEM Extraction Wizard and three DEM tools: DEM Editing Tool, Stereo Pair 3D Measurement Tool, and Epipolar 3D Cursor Tool.

DEM extraction is a multi-step, decision-making process that involves setting numerous parameters. You can run the steps individually from the Toolbox, or from within the DEM Extraction Wizard. The Wizard guides you through nine steps. It presents you with objective parameters, such as minimum/maximum elevation of the area of interest, as well as other strategy parameters that depend upon the terrain relief, cultural content, image quality, shadowing, and the desired speed of operation.

As with the DEM Extraction Wizard, you can extract a relative or absolute DEM from a stereo pair of images using the Stereo Pair 3D Measurement Tool and Epipolar 3D Cursor Tool.

  • The Stereo Pair 3D Measurement Tool allows you to select a common point from two stereo images and calculate an elevation value for that point (see The Stereo Pair 3D Measurement Tool for information about this tool).
  • The Epipolar 3D Cursor tool allows you to perform 3D measurements in a 3D stereo viewing environment based on an existing epipolar stereo pair of images. You can view an anaglyph of epipolar images and adjust the apparent height of the cursor to extract elevation data (see The Epipolar 3D Cursor Tool for information about this tool).

Create a DEM


There are three steps in creating a DEM that are crucial to generating acceptable results: epipolar image creation, image matching, and DEM geocoding.

  • Epipolar Image Creation: Creating epipolar images is an essential processing step in DEM extraction. Epipolar geometry describes the geometrical constraint between two frame images of a stereo pair. It represents the fact that a ground point and the two optical centers lie on the same plane. This means that for a given point in one image, its conjugate point in the other image must lie on a known line in the second image. By creating epipolar images, the search space for finding corresponding image points in automatic image matching is reduced. For information about building epipolar images for the use with DEM Extraction Wizard, see Generate Epipolar Images.
  • Image Matching: Image matching finds the conjugate points on both the left and right images which correspond to the same ground feature. The output of the image matching procedure is called a parallax image, in which the x-coordinate difference (along epipolar lines) between the left and right image is stored and is used to build the DEM. Thus, the quality of image matching largely determines the quality of the output DEM.
  • DEM Geocoding: DEM geocoding reprojects the DEM from the epipolar projection to an output map projection and units that you specify. This step involves filling failed pixels and resampling to a pixel spacing that you specify. When you provide ground control points (GCPs), you can compute the absolute orientation of the computed terrain model in this step.

Output DEM Types


An output DEM can either be relative or absolute, depending on the imagery and its associated information.

  • A relative DEM is one with possible differences in position, scale, and rotation from the geodetic coordinates on the ground (horizontal reference system) and the mean sea level (vertical reference system). If the satellite imagery or aerial photography does not have GCP information associated with it, the DEM Extraction Wizard can derive a relative DEM result.
  • An absolute DEM is one that is generated with GCPs. The DEM result is absolute in the sense that the horizontal and vertical reference systems are tied to geodetic coordinates. The DEM Extraction Wizard enables you to obtain either a relative DEM or an absolute DEM. Once you have extracted a DEM, you can edit the results to remove any processing artifacts.

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