Two systems of terrain corrections are used for gravity surveys in the United States. For detailed gravity surveys, the terrain corrections are made with the Hammer charts, which extend out to a radius of about 13.6 miles. The Hammer charts emphasize the effect of local terrain. In large regional surveys it is advisable to use the Hayford-Bowie charts. These charts cover the entire surface of the earth.
Recently it has been found desirable, in specific instances, to incorporate the results of local surveys into larger surveys. In order to avoid repeating the terrain correction of the local surveys it is necessary to be able to convert from one system of terrain corrections to the other.
Nine conversion zones have been completed and are presented in tables.
The essentials of this paper are found in Table II. The relationship of the two sets of zones is shown in Table I.