Office of Geomatics: Gravity Reference Base Station File History
NOTE: If you are unfamiliar with geodesy and geodetic terms, see Publications on Geodesy and Geophysics or similar introductory sources.
The National Imagery and Mapping Agency's Geospatial Sciences Division (GOG), St. Louis, Missouri, has data on approximately 9,000 gravity reference stations. What is now the Geospatial Sciences Division, has evolved from a number of organizations. Many sources have contributed to Geospatial Sciences's holdings including the agency's own survey units, other government agencies, private companies, and colleges and universities.
Although the use of gravity data by the federal government gained momentum in the 1950's, the roots of this work go back to the turn of the century. In 1906, pendulum instruments were used to measure absolute gravity in Potsdam, Germany. These early measurements led to the first gravity datum -- the Potsdam System. For the next 70 years, relative and absolute instruments would be used to expand this system into a worldwide network of stations tied to Potsdam.
1940's & 50's
In the United States, the United States Coast and Geodetic Survey (the forerunner of the National Geodetic Survey) conducted gravity surveys in the 1940's. In the defense arena, the United States Air Force, Army Map Service, and Navy made extensive use of gravity data in the 1950's for such purposes as inertial guidance and geoid computation. Two offices in operation at the time eventually became NGA's Geospatial Sciences Division. One was a geodetic survey unit with gravity capability which began to operate from West Palm Beach, Florida in the summer of 1959. This unit was part of the Military Airlift Command's Air Photographic and Charting Service. A second organization based in St. Louis, the Aeronautical Charting and Information Center, responded to Air Force requirements and started to collect and process gravity data.
The Military Airlift Command's gravity survey group moved from West Palm Beach to Orlando, Florida in the early 1960's. From Orlando the survey unit supported worldwide mapping efforts which were a major focus at the time. As needs changed, the Survey Squadron moved from Orlando to F. E. Warren Air Force Base in Cheyenne, Wyoming. Cheyenne's location provided a convenient location from which to support the strategic missile wings located in the western United States. While at Cheyenne the Survey Squadron reached its peak staffing level of about 600 personnel.
In the field of geodesy, a lot of change took place in the 1970's. On July 1, 1972 the Aeronautical Charting and Information Center became the Defense Mapping Agency Aerospace Center (DMAAC). The Potsdam System was replaced by the International Gravity Standardization Net 1971 (IGSN 71)*. Around 1976 the Army Map Service quit doing gravity surveys and transferred its remaining personnel to the Geodetic Survey Squadron in Cheyenne.
|*The IGSN 71 was approved and addopted by the International Union of Geodesy and Geophysics in 1971, and replaces the Potsdam Datum as the international gravity standard. The concept of the IGSN 71 differs from that of earlier gravity reference systems in that the daum is determined, not by an adopted value at a single station, but by a least squares adjustment of 1,854 stations with respect to ten absolute gravity values at eight stations in North America, South America, and Europe.|
Gravity survey instruments had been greatly improved by the 1980's. Pendulum absolute meters had given way to more accurate instruments utilizing a falling body. In the 1980's significant numbers of these improved absolute meters were in use around the world.
The 1990's have so far seen a centralization of the federal government's gravity surveying and processing capabilities. The Inter-American Geodetic Survey (IAGS) disbanded in 1991 (IAGS had conducted surveys in Central and South America). Cheyenne's Geodetic Survey Squadron, which was then under the control of the Geodesy and Geophysics Department, moved to St. Louis in 1993. As a result, personnel and processes from these various organizations converged into what was DMA's Geodesy and Geophysics Department. In October 1996, the Defense Mapping Agency merged with other government components to form the National Imagery and Mapping Agency (NGA).
The future promises to bring more change. NGA is now shifting from an emphasis on specific products to populating user-accessible geospatial databases. Instrumentation is constantly improving. Absolute devices are becoming smaller and more portable, and may eventually make gravity surveying as simple as taking a reading anywhere without having to tie to a control station. But for now, a basic network of gravity reference stations, much the same as the original Potsdam System, remains at the core of geodetic work.
Metadata on this subject: Gravity Reference Base Stations
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Document last modified April 29, 2013