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

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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

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becoming the camera eye with pictures taken from different angles of the object to be measured.

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