ABSTRACT
The purpose of engineering surveying is to establish the relative positions of points in three dimensions, so a three-dimensional co-ordinate system is needed to record the findings. The one absolute direction which can always be found is ‘up’, using nothing more sophisticated than a plumb-bob. For this reason alone, it makes sense to define one axis of the co-ordinate system in the upwards direction. The other two axes can then be conveniently defined to form an orthogonal set with the first axis, and can be thought of as lying at right angles to each other on the surface of a bowl of water held at the foot of the plumb-bob.
