Deflection of the Vertical Survey

A completely independent way to determine geoid slopes is to measure the local deflection of the plumb line from what one would expect if all of the Earth's mass was evenly distributed inside the Earth Ellipsoid. This deviation is known as the deflection of the vertical, and it is typically presented in North (Greek letter chsi) and East (Greek letter eta) components.

At night, a camera is placed on each GSVS bench mark and very precisely leveled to the local plumb line. At a given time and location, the camera knows what stars it "should" see when looking straight up (azimuth). The deviation of the actual star position from the expected location is a measure of the local deflection of the vertical. Note that the GPS derived positions (and time) are inputs to the DoV algorithm, but the slope is determined solely by the camera system. Local deflection angles can approach many 10s of arc-seconds, and can be determined with an accuracy of 0.1 arc-seconds.