A system for determining heights of terrain along a line of flight by using an airborne laser altimeter for measuring height variations in the terrain and a combination of accelerometers, gyroscopes, optical distance measuring instrument (DMI) and computer for determining the position of the altimeter. Corner cube reflectors are placed on geodetic markers. The DMI measures distance and direction (with respect to the gyroscopic system) to these reflectors; this information, together with information from accelerometers and gyroscopes, is sent to the computer which determines position from these data. The system was developed for the U. S. Geological Survey by C. S. Draper Laboratories.