A surface measurement apparatus having at least one sensor and a reference point with pre-determined relative position relative to the sensor measures a distance between the sensor and a point on a surface over which the surface measurement apparatus is configured to travel. A receiver receives data related to a position of the reference point a calculated distance between the reference point and a position measurement station; and a processor calculates an absolute position of the point on the surface based on (a) the measured distance between the at least one sensor and the point, (b) the pre-determined relative position difference between the reference point and the at least one sensor, and (c) the position of the reference point or the calculated distance between the position measurement station and the reference point.