On a construction or civil engineering project, the setting out engineers control the dimensional accuracy of the works - so any steel, concrete or earthworks is in the corrrect position (hence gps) and is the correct shape.
To give a car analogy, the engineer creates the "jig" for the construction of the building (or whatever). On the cross rail project in London setting out engineers ensure that the tunnels thread accurately through the existing tube network and all the other stuff that buried beneath the streets.
Eye of a needle stuff right there