|Alaska1(ak1_iso) Gravity Data
|The gravity station data (72,677 records) were compiled by the U.S. Geological Survey
and the State of Alaska Division of Geological & Geophysical Surveys. This data base
was received in February, 1993. Principal gravity parameters include Free-air anomalies,
isostatic anomalies and Simple Bouguer anomalies (no terrain correction applied).
The "iso" suffix indicates that a crustal thickness of 25 kilometers was used in the
isostatic calculation. The observed gravity values are referenced to the International
Gravity Standardization Net 1971 (IGSN 71). The gravity anomaly computation uses the
Geodetic Reference System 1967 (GRS 67) theoretical gravity formula. Note that the
original USGS Open File Report by D. F. Barnes mentions the use of the 1931 International
Ellipsoid for gravity reduction but this is believed to have subsequently been changed
to the GRS 67 system.