Automated generation of DEM is achieved by photogrammetric methods based on stereo aerial photography and satellite stereo imagery.

The principle of aerial photography and satellite stereo imagery is the same, in using the theory of parallax of a stereo pair as show in Figure 3.11.

Parallax is defined as difference between left and right photographs or image coordinates. The higher the elevation is, the bigger the parallax is. If the parallax is constant, equal elevation or contour lines will be produced.

Determination of three dimensional coordinates of terrain points is achieved by searching the corresponding points called conjugate points on the image planes of left and right stereopair. Intersection of two corresponding 3D rays will give the 3D coordinates of terrain points. 3D rays are generated by photogrammetric geometry, called colinearity equation that connects lens of camera (X0, Y0, Z0 image point )(x, y) and terrain point )(X, Y Z) as follows.

Automated generation of DEM is achieved by automatic searching of conjugate points on stereo pair, called image matching or stereo matching.

Image matching is usually executed in two steps.

step 1: Resampling along epipolar lines as shown in Figure 3.12, because conjugate points are located on epipolar lines
           theoretically.
step 2: Image correlation between fixed window of n x n pixels on the left image and moving window of the same size on the
           right image is maximized to determine the conjugate point as shown in Figure 3.13.