what is photogrammetry?

Answer 1

Well, a photogram is a piece of art made by laying objects down upon light sensitive paper and exposing the paper to light. This is typically done in a darkroom, where it is then developed, and dried. The result is a piece of paper that has white shapes where the objects were placed, while exposed areas are black.

Edit:

I just looked up your term, which has nothing to do with the art of photograms. Photogrammetry is the process of making maps or scaled drawings through photography. You can also take measurement through this study.

Answer 2

Photogrammetry is literally 'measurement by photography'. It is the technique used for producing topographic maps from stereo pairs of aerial photographs. The photos are taken by a special camera that takes stereo-pairs of photographs. When these are placed in a plotting machine, the stereo pairs can be aligned so that contour lines can be traced AND transferred to a plotting machine. More generally, by using a stereo-scope (a twin lens viewer), you can see a three dimensional view by combining the two stereo photo images.
Very interesting technique, especially in stereo-geology; but unfortunately I was never able to do it, because I am a geologist with only one eye.