Harold E. Malde 1973 <p>A photograph made with a level camera, if taken at a&nbsp;known height above a permanent mark on the ground, can be later&nbsp;repeated with exactness for measurement of changes in terrain. Such a&nbsp;photograph is one of several means for establishing a geologic bench&nbsp;mark and is especially useful for monitoring the subtle qualities of a&nbsp;landscape that are otherwise hard to map and describe, including the&nbsp;effects of man's use. Moreover, the geometry of such a photograph&nbsp;provides the same angular measurements between objects as can be&nbsp;made with a transit. A measurement of distance on a single photograph,&nbsp;however, requires control points. These can be surveyed at any&nbsp;convenient time, not necessarily when the initial photograph is made.&nbsp;Distances can also be determined by simple stereophotography from a&nbsp;base line of suitable length.</p> application/pdf en U.S. Geological Survey Geologic bench marks by terrestrial photography article