Hi. When reducing the value of measured gravity to produce gravity anomalies, the measured gravity is reduced to it's value on the geoid (conventional interpretation). This is then compared to the value generated by the reference ellipsoid at the ellipsoid surface. I would have thought that measured gravity should be reduced to it's value at the reference ellipsoid rather than the geoid for comparison. But I've read (without explanation!) that the value of reduced gravity at the geoid is exactly comparable to that on the reference ellipsoid. This result is somehow tied up in the consideration of the respective potential fields...but I don't understand how... Any explanations welcome (except those stating that the geoid undulation can be neglected!) Thanks.