Taking into account parallax, shadows of features found on a photo can be used to determine the heights of buildings as well as the time of day and year. As the name indicates, the Kelsh stereoplotter is of German origin. A stereophotograph was used to make a contour map by projecting contours onto a piece of paper then tracing them on a map. This is still a common method for creating contour maps today. However, even though a computer is involved - human assistance is required in operation of it to place a dot in the visual field on the surface to determine elevation.
Some systems are very sophisticated and are able to automatically place the dot.
Human intervention will be needed when the computer is fooled by a tree or building. An orthoscope is an optical distortion device used to remove distortion from a photograph. When aerial photo is taken it has a central perspective. This means that it is being viewed downward at the central point causing distortion near the edge. To reduce the amount of distortion a planemetric perspective will be achieved.