New standards for reducing gravity data: The North American gravity database

The North American gravity database as well as data- bases from Canada, Mexico, and the United States are being revised to improve their coverage, versatility, and accuracy. An important part of this effort is revising pro- cedures for calculating gravity anomalies, taking into ac- count our enhanced computational power, improved ter- rain databases and datums, and increased interest in more accurately defining long-wavelength anomaly components. Users of the databases may note minor differences be- tween previous and revised database values as a result of these procedures. Generally, the differences do not impact the interpretation of local anomalies but do improve re- gional anomaly studies. The most striking revision is the use of the internationally accepted terrestrial ellipsoid for the height datum of gravity stations rather than the conven- tionally used geoid or sea level. Principal facts of gravity observations and anomalies based on both revised and pre- vious procedures together with germane metadata will be available on an interactive Web-based data system as well as from national agencies and data centers. The use of the revised procedures is encouraged for gravity data reduc- tion because of the widespread use of the global position- ing system in gravity fieldwork and the need for increased accuracy and precision of anomalies and consistency with North American and national databases. Anomalies based on the revised standards should be preceded by the adjec- tive "ellipsoidal" to differentiate anomalies calculated us- ing heights with respect to the ellipsoid from those based on conventional elevations referenced to the geoid.