Gravity control network in the western United States

The gravity control network in the western United States is a critical part of the over-all national gravity control network of pendulum and gravimeter bases, in that it contains the United States part of the North American primary gravity standardization and calibration range. Gravity control prior to 1951 was based on the pendulum gravity station network that had been established by the U. S. Coast and Geodetic Survey [Duerksen, 1949]. This network of gravity bases, consisting of approximately 1200 stations, gave good regional coverage of the country and was believed to have a reliability of about ±1.0 mgal. However, as regional gravimeter coverage was developed throughout the country, it became obvious that the pendulum base network was subject to both systematic and random errors. The systematic errors appeared to be related to (1) time, in that all measurements made during a given field season might be off by a fixed amount, and (2) latitude, presumably because no corrections were made for the effect of changes in magnetic field strength on the period of the Invar pendulum used. The random errors resulted from poor measurements occasioned by local environmental effects or poor radio reception and tares (abrupt changes in pendulum period).