Thanks; I glossed right over it. "Also, the apparent rate of a clock depends on the local gravitational potential: comparing two clocks measures the gravitatio

Thanks; I glossed right over it. "Also, the apparent rate of a clock depends on the local gravitational potential: comparing two clocks measures the gravitational redshift between them, and thus yields their height difference. Such measurements provide data points for the geodetic reference surface, the so-called “geoid”. This research approach is pursued jointly by physicists and geodesists in the Collaborative Research Centre 1128 (“geo-Q”) of the German Science Foundation (DFG)."