Disclosed is a topographic measurement system wherein at least one satellite is used to scan the earth surface and send picture frames of a target area captured at different positions to an earth station. The picture frames are combined to produce a number of pairs of frames which constitute a stereoscopic image of the target area. Each frame pair is analyzed according to a number of visual characteristics and evaluated with a set of fitness values representative of the degrees of fitness of the frame pair to topographic measurement of the target area. A total of the fitness values is obtained from each frame pair and compared with the total values of other frame pairs. A frame pair having the highest total value is selected as a best pair. A parallax between the best pair frames is determined to produce first and second sets of line-of-sight vectors for conversion to topographic data.