When the viewing angle difference between the images is less than 7 degrees, meaning a base to height (B/H) ratio less than 0.1 to 0.15, you should collect the minimum required ground control points (GCPs) for each image in your project. In this case, you should not use tie points, tie points with elevation, or even stereo GCPs. These points become mathematically useless due to the same-side weak stereo intersection.
If you are co-registering two scenes from a single sensor, such as a panchromatic image and a multispectral image of the same scene, do not use tie points to relate the images. Use the same GCPs on both scenes. Collecting the same GCPs on adjacent images can also improve the math model by improving the B/H ratio.
When the viewing angle difference between the images is between 7 and 30 degrees, meaning a B/H ratio between 0.1 to 0.15 and 0.6, you can use tie points with elevation to reduce the minimum required GCPs for each image.
When the viewing angle difference between the images is more than 30 degrees, meaning a B/H ratio more than 0.6, you can use tie points to extend the ground control. If you use tie points with elevation, you can reduce the minimum required GCPs for each image.
For example, if you have stereo left and right SPOT images, you can collect two GCPs on the left image and two GCPs on the right image. With tie points with or without elevation in the overlap, you can obtain a geometric model.
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