Image correlation is a technique by which the conjugate point of a slave image (right) corresponding to the master image (left) will be searched for the maximum correlation coefficient. Image correlation is applied to stereo images for DEM (digital elevation model) generation or multi-date images for automated recognition of ground control points.
As shown in Figure 10.11.1, the master window in the left image is fixed, while the slave window in the right image is moved to search for the maximum image correlation as computed from the following formula.
or
where ai : image data of the master window
bi : image data of the slave window
n : total number of image data
Because the above two correlations show almost no difference, the first correlation is preferred to save computing time.
The size of the window should be selected depending on the image resolution and feature size. For example, 5 x 5 to 9 x 9 windows might be selected for SPOT stereo images, while 9 x 9 to 21 x 21 would be better used for digitized aerial photographs.
When the conjugate points of stereo images are determined, the corresponding digital elevation can be computed using collinearity equations based on photogrammetric theory.
Figure 10.11.2 shows the conjugate points as white dots in a pair of SPOT stereo images, which were automatically recognized by image correlation techniques.
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