In static GPS surveying we use two receivers to measure the 3D vector between two points. This is known as a baseline. If the coordinate of one of the points is known, the coordinate of the other point can be determined by adding the 3D vector to the known coordinate.

Theoretically, GPS baselines can be measured to an accuracy of 4mm in the horizontal and 7mm in height. In practice, things can be worse!

GPS signals arrive at the phase centre of the GPS antenna. Therefore, a baseline measures the vector between two antenna phase centres. To reduce this vector to two points on the ground, the height of the antenna phase centres above each point must be measured and the antennas set up over each point as precisely as possible.