Whats a datum?
The short answer is – A datum is a fixed, three-dimensional surface, an oblate spheroid, that is approximately the size and shape of the Earth. From this surface, Latitude, Longitude and Elevation are computed. (Note – because the Earth has an ellipsoidal cross-section, Latitude and Longitude can’t be measured, they must be calculated.) The long answer is – Basically, a datum is an admission of failure. With satellites, Mankind can now measure the shape of the Earth with fair accuracy. But before this, men measured the Earth with very-long surveys. Everyone knew that the estimates derived from these data were wrong. The proof of the pudding was that no two geodesists ever came up with the same figure. In fact, single geodesists couldn’t come up with the same figure twice in a row (e.g. Clarke 1866, Clarke 1880). A datum is when surveyors all get together and agree to be wrong. They take a spheroid model of the Earth and fix it to a base point. For NAD27, the U.S.G.S. decided that Clark
