Select Input Files for DEM Extraction

The DEM Extraction Wizard requires a stereo pair of images containing rational polynomial coefficient (RPC) information from an along-track or across-track satellite acquisition, or from scanned or digital aerial photography. 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.

  1. From the menu bar, select File > Open As > sensor type.

    2. Or, select File > Open to open a left and right image of a stereo pair from scanned or digital aerial photography.

    For ASTER Level 1A data, both the left and right images (bands 3N and 3B) are contained within one file. Select File > Open from the menu bar to open an ASTER data file.

    You can use spatially subsetted images as long as you used ENVI’s Select Spatial Subset dialog to subset them. Do not further edit the headers of the spatially subsetted files, particularly the XSTART value, YSTART value, or RPC information.

  2. From the Toolbox, select Terrain > DEM Extraction > DEM Extraction Wizard: New. Step 1 of the DEM Extraction Wizard opens.
  3. Click Select Stereo Images to pick the left and right images for DEM extraction. ENVI computes RPCs and populates the Scene Elevation in Meters values.

    4. For ASTER data, select Band 3N as the left image and Band 3B as the right image. Also refer to Retain RPC Information from ASTER, SPOT, and FORMOSAT-2 Data.

    Note: If a data file contains an image obtained from a nadir-viewing direction while the other image is obtained from an off-nadir viewing angle, it is recommended that you use the nadir-viewing image as the left image. The basis of this recommendation is that the left image is used as the base image during image matching, and it contains less geometric distortion.

    Also, the base-to-height (B/H) ratio of the two images should be close to 1, but not 0. This ensures the viewing angles provide enough parallax information for ENVI to effectively compute elevations.

    You can use spatially subsetted images as long as you used ENVI to subset them. Do not further edit the headers of the spatially subsetted files, particularly the xstart, ystart, or RPC information.

  4. The Minimum and Maximum Elevation values (in meters) are automatically set from the RPCs; however, you can refine these values if you have a better estimate. Values are required in these fields to continue.
  5. Click Next to continue to the next step of the wizard.