GRAVITY BASES IN NEW YORK AND EASTERN PENNSYLVANIA

During the past several years, a total of 8,000 gravity stations were established in New York State and Pennsylvania. A gravity survey almost always consists of creating base stations consisting of observed gravity values relative to a previously established base. This is very similar to leveling operations to determine elevations with reference to benchmarks. A network of base stations in New York and Pennsylvania were tied to a network of Innes (1958) of the Canadian Geological Survey. The Innes network consisted of 36 bases extending from Ottawa, Canada to Washington, DC.

Most of the bases were set up by looping, which requires two readings at each base. This method gives two differences in gravity between two bases so errors due to misreading the instrument and drift are minimized. The two readings provide segments of drift curves to correct for drift. In some cases, closure errors were determined by returning to the initial base for another measurement. The differences between the gravity readings and the calibration factor of the gravimeter enable one to calculate the observed gravity at a base.

Over 100 bases were established and their locations indicated by drawings. The drawings show the exact location of the reading and the observed gravity at that location. Theses bases could be useful to others who wish to tie their gravity measurements in with previous gravity anomalies base upon the same base network.