Parallax is a way of determining distances and heights by measuring the angles of a certain point from different locations. Once you have the angles from two points you can then work out the distance by working out the third angle of a triangle and measuring the sides. Amongst other uses parallax is the method in which the distance to stars is measured.
Parallax is also the title of a recent donation to the Map Room, Parallax without tears, the determination of heights from vertical airphotographs
and published in the war years by the Royal Canadian Engineers. Using a giant as an example, the booklet teaches how to measure heights by the use of aerial photograph, the giants eyes
becoming the camera eye with pictures taken from different angles of the object to be measured.