Creating a geocoded DEM

If necessary, you can, as an option, set up automatic DEM extraction to geocode and merge the epipolar DEMs.

Important: You need only specify the options under Geocoded DEM if you want to create a geocoded DEM based on the generated epipolar DEMs. If you want to edit the DEM before it is geocoded, do not select the Create geocoded DEM check box.

To create a geocoded DEM

  1. Under Geocoded DEM, select the Create geocoded DEM check box.

    The Automatic DEM Extraction window refreshes to display additional options under Geocoded DEM.

  2. To permanently remove the epipolar DEM from disk after generating the geocoded DEM, select the Delete epipolar DEMs after use check box.

    Removing the epipolar DEM after use also saves disk space.

  3. In the Epipolar DEM clipping box, type a value or values to represent the amount of data to clip from the epipolar DEMs before merging and geocoding, and then click percent or pixels, as applicable.

    That is, the value or values you enter indicate the amount of the actual image data to clip, either as a number of pixels or, if specified, as a percentage of the image size. You can enter up to two values. The first defines the number of pixels in the horizontal (left-right) direction. The second defines the number of lines in the vertical (up-down) direction. For example, entering a value of 5, 10, clips five pixels from the left and right edges, and 10 pixels from the top and bottom edges.

    When you enter a single value, clipping applies to each edge of the image; for example, when you enter a value of 5, five pixels are clipped from all four edges.

    The default is zero, meaning no clipping occurs. The maximum percentage value is 49.

  4. In the Output file name box, type the path and folder name for the geocoded DEM file in PCIDSK format (.pix) or, to select a file to which to add to an existing geocoded DEM file (.pix), click Browse.
    Note: If you select an existing DEM, the bounds and resolution will be set based on the selected file. If you want to modify the bounds, resolution, or both, you must save the file with a new name. When you select an existing DEM, the resulting image is written automatically to channel 1 in that file.
  5. To set the resolution for the output file, in the Resolution boxes for X and Y. The extents of the data will be determined using the specified resolution.
  6. To determine the extents of the DEM, beside DEM bounds, do one of the following:
    • To use the extents of all the images in the Stereo pairs table as the extents for the DEM, click All images.
    • To use only the extents of the images selected in the Select column of the Stereo pairs table, click Selected images, and then, if necessary, you can enter new values in the Upper left and Lower right boxes for X and Y, respectively.
    • To use a custom extents of the images, click Custom, and then, if necessary, you can enter new values in the Upper left and Lower right boxes for X and Y, respectively.
  7. In the Output option list, to determine which pixel value to use when a new geocoded DEM is added to the output file and it overlaps an existing geocoded DEM, do one of the following:
    • To replace the pixel values in the overlap area in the existing geocoded DEM with the pixel values of the geocoded DEM being added to the file, click Use last value.
    • To replace the pixel values in the overlap area with the average pixel values between the existing geocoded DEM and the one being added to the file, click Average.
    • To replace the pixel values in the overlap area with the pixel value that has the highest correlation score between the existing geocoded DEM and the one being added to the file, click Highest score.

      This option is useful only if, under DEM Extraction Options, you selected the Create score channel check box.

    • To apply a combination of Use last value, Average, and Highest score, click Merge.

      Available when SGM (Semi-global matching) is selected for DEM Extraction Options, this option merges a set of geocoded DEMs from a stereo airphoto or satellite project into a single DEM. The merged DEM is typically of higher quality, because each geocoded DEM has a different viewing geometry. Occlusion in one DEM can be filled from another DEM. In addition, the multiple DEM elevations of the same ground pixel can be used to detect blunders. Averaging multiple good elevations helps to increase vertical accuracy.

      Merge also attempts to straighten building edges, if applicable. That is, with Merge selected, you can select the Clean up building edges with filter size check box, and then in the box to the right, type or select the size of the filter to use. The recommended value is 13.

      With Merge selected you may also choose to create an image mosaic by selecting Create image mosaic, then in the text box, type the path and folder name of the image mosaic file or, to select a file interactively, click Browse. This file will be a color-balanced mosaic of all of the original input imagery used in DEM extraction. Available output formats are .pix, .tif, and .jpg.

    • To use cutlines to determine a seam line with minimal elevation change between epipolar pairs, click Blending.

      With this option selected, 10 pixels are blended together using the average of the overlapping input values on either side of the cutline. This option is the default.

  8. Return to extracting a DEM from epipolar pairs to complete the rest of the setup.
Parent topic: Extracting a digital elevation model from epipolar pairs
Previous topic: Additional setup for SGM
Next topic: Setting a start time for extracting the DEM

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