The importance of collecting tie points

Tie points (TP) are features that you can identify clearly in two or more overlapping images and that you can select as reference points. In contrast to GCPs, TPs do not have known ground coordinates; however, you can use them to extend ground control over areas in which you do not have GCPs. Used in rigorous models, such as aerial photography and satellite orbital (high-resolution and low-resolution) math models, TPs identify how the images in your project relate to each other. Essentially, you can use them to compute a transformation to improve the fit between the images.

Projects using the satellite orbital math model generally have fewer images, so you can collect TPs wherever overlap occurs. Because the overlap between satellite images is unpredictable, satellite imagery generally covers a large area containing a lot of ground control.

Using the TPs in calculating the math model ensures the best fit not only for the individual images, but for all of the images united as a whole. Therefore, the images fit the ground coordinate system, and overlapping images fit each other.