ABSTRACT
The purpose of engineering surveying is to find the relative positions of points in three dimensions; so a 3D 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 coordinate system in the upward 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.
