The digital terrain model (DTM) can either come from an existing source or it can be developed from the aerial photography. The aerial photos are taken using a stereoscopic camera, with which two pictures of a particular area are simultaneously taken, but from slightly different angles.
he overlapping area of the two resulting photos is called a stereo pair. Using a computer called a stereoplotter, the stereo pair can be viewed as a single image with the appearance of depth or relief. Ground control points are established based on ground surveys or aerial triangulation and are viewed in the stereoplotter in conjunction with the stereo pair. In this setting, the image coordinates of any (x,y,z) point in the stereo pair can be determined and randomly selected and digitized. These points, in conjunction with 3D breaklines and control points, comprise the data points for the DTM. The accuracy of the final digital orthophoto will depend in large part on the accuracy of the DTM.