The international network of gravity bases that has been used for both datum control and as a means for standardizing gravity values is a patchwork structure representing measurements made by various observers with various sets of pendulums over a considerable period of time. The basic structure is the national gravity reference bases, most of which have been connected directly to Potsdam with pendulums. To this basic structure have been added national series of pendulum sub-bases, as that of the United States Coast and Geodetic Survey, which are referenced to the national base; international series of pendulum measurements such as those established with the Gulf-Wisconsin pendulums, the Cambridge pendulums, the Vening Meinesz submarine pendulums, and the inverted Holweck-Lejay pendulum, which are referenced to various national bases; and within recent years bases established with gravimeters referenced to various pendulum bases. If there were no questions regarding the effect of ex-ternal factors such as temperature, pressure, and magnetic field on the pendulums, changes in pendulum period with time, reliability of the time standard employed, and tares, the existing pendulum base network would effectively guarantee reliable gravity control throughout the world. Unfortunately, this ideal situation does not exist and as a consequence discrepancies in pendulum results amounting to as much as 31 mgal have been noted (Gulatee 1950).
It was this uncertainty in pendulum gravity base values that led to the initiation of the Woods Hole-University of Wisconsin gravimetric program by the writer in 1948, and the adoption in 1953 by international agreement of
Figures & Tables
International Gravity Measurements
“Originally established to test reliability of the world network of firstorder international gravity bases, the program was expanded to include establishment of a series of reliable pendulum gravity control measurements in North America to assure a potential accuracy of 1 mGal or better on any global series of measurements. Studies also expanded and standardized existing gravity surveys of the United States for reliable gravity anomaly maps, and evaluated reliability and gravity standards used for global work.”