When reviewing tie points, use the Image to Image GCP List to turn individual points on or off, to delete points, and to view RMS errors for individual points.

Tips for selecting good tie points:

  • When selecting seed or tie points, switch the Geographic Link toggle to On to get the crosshairs in the two images to approximately the same location. Once a good feature is identified, switch the Geographic Link toggle to Off, fine tune the crosshair positions, and add the point.
  • When using any object with height above the ground, use the base of the structure rather than the top. Parallax caused by different viewing geometries (even for “simultaneous” panchromatic/multispectral imagery) makes these tie points sub-optimal.
  • Distribute tie points as evenly as possible across the entire scene. Try not to leave large areas without any tie points.
  • The more tie points the better. There is eventually a point of diminishing returns. A good number is 50 tie points, although you can obtain excellent results from as few as 10, depending on the scene.
  • If you used manual tie point selection:
  • Use the Ground Control Points Selection dialog to pick tie points.
  • See Image-to-Image Ground Control Points and Fixing Tie Points for assistance.
  • If you used automatic tie point selection:
  • The tie points automatically display. Tie points are colored indicating good, satisfactory, and poor RMS error values. Look at the RMS Error in the Ground Control Points Selection dialog. This number should be as small as possible. Less than 1 is excellent, but anything less than 3 should produce acceptable results.
  • Enlarge the Image to Image GCP List dialog so you can see the RMS column.
  • From the Image to Image GCP List dialog menu bar, select Options > Order Points by Error to sort the tie points by the highest RMS.