County Error Followup in Benchmarking Posted November 8, 2005 · Edited November 8, 2005 by TerraVador If you could follow the actual level surface, which would be the surface of a lake at that height with no waves, you would not be path dependent. You may want to browse Geodesy for the Layman Bill, the surface of a lake or ocean is not flat (or spherical). Even in the absence of waves or air pressure, it has peaks and valleys due to gravity anomalies. Refer to the picture below. The curve labeled GEOID represents the surface of the ocean. Imagine you want to determine the elevation of the point at the intersection of the dashed line labeled NORMAL TO ELLIPSOID and the solid line labeled NORMAL TO GEOID. Call this point X. Lets call the base of the letter P in the word ELLIPSOID above the MASS DEFICIENCY point P, and assume it is 100 meters. Lets call the base of the letter O in the word GEOID above the MASS SURPLUS point O. Its elevation is 0(zero) since it is at sea level. What is the measured elevation of point X? From point P, can't you see that X appears to be well above sea level? From point O, X appears to be below sea level. From point X, O appears to be below sea level. A closed loop would not yield a zero net elevation change. It is my semi-educated guess that these gravity variation/change in vertical effects only show up in very precise surveys many miles long. I am not a geodesist, but I believe you are right. I don't know how much error is introduced per mile in a typical survey due to gravity anomalies.