Most gravity surveys are conducted to estimate subsurface density contrasts for one application or another. From large-scale crustal studies to relatively small exploration surveys, it is necessary to determine in some way what the normal gravity field should be in order to identify anomalous features. The anomalies then represent deviations to be interpreted in light of the original model. It is a central limitation of potential field methods that this model, sometimes representing a so-called "regional" field, is not unique. In the case of gravity, this model has traditionally involved geometrical approximations. It is generally assumed that variations in station elevations are small compared with the radius of the earth--an obviously excellent approximation, but one needs to be mathematically consistent.