High‐precision absolute gravity observations in the United States

From May 1987 to June 1988 the National Geodetic Survey (NGS) made approximately 50 observations at 30 sites with one of the six absolute gravimeters built by the Joint Institute for Laboratory Astrophysics (JILA) between 1983 and 1986. Of the 10 sites where two to three observations were made, the scatter about the mean site values ranged from under ±1 μGal to ±4.0 μGal. This high degree of repeatability is the result of three factors: the improvements made to the instrument at JILA in 1986, methods of observation and quality control of the data sets, and independent monitoring of and correcting for the environmental effects on gravity. The data correction methods now employed at NGS allow the establishment of high-precision reference gravity stations in the United States and abroad for monitoring the temporal variations of gravity and studying vertical ground motions.