A datum is a reference from which measurements are made. Datums are typically used for relative quantities, such as position. For example, latitude and longitude are measured in relative angles from the arbitrarily chosen references of the equator and the meridian running through Greenwich, England. This is arbitrary since a convention of using the South Pole and Paris, France meridian, for example, could easily have been chosen instead. Elevations are vertical distances measured above or below “something.” Usually this something is the geoid (historically, though somewhat incorrectly, called “mean sea level”). In some inland countries or remote islands, however, the reference can be a primary benchmark with some chosen elevation.
An obvious question might be, “Why are there datums for accelerations of gravity data?” After all, gravity is seemingly an absolute quantity: the weight (a force) of an object divided by the object’s mass equals gravity. The need for gravity datums is connected to the instruments used to measure gravitational acceleration and the establishment of reference stations.
Establishing Reference Stations
Historically, the instruments used to make absolute gravity measurements have had poor accuracies. Thus, most instruments used to measure gravity were relative instruments. Relative instruments determine the gravity difference between a station with known gravity (such as one determined using an absolute—or direct—measurement) and a new station. Relative instruments could survey differences relative to a national base station to establish state reference base stations. More reference stations could then be established from these. Of course, with each transfer, the measurement error of the transfer increased.
Both simple pendulum and spring-based gravimeters could make relative measurements reliably, quickly, and accurately. Before the 1960s, the only accurate way to make an absolute measurement was with a reversible pendulum done in laboratory conditions.
Worldwide Gravity Datums
In the late 19th century, the Vienna Gravity System was established by F.R. Helmert based on pendulum measurements in Vienna, Austria. This system had an estimated relative accuracy of ±10 mGals. By 1909, this system was replaced by the Potsdam (East Germany) Gravity System, which had a relative accuracy of ±3 mGals and corrected the Vienna System by -16 mGals.
For more than six decades, all gravity readings were made relative to the Potsdam value. In North America, national base stations were established in Ottawa, Mexico City, and Washington, DC, using the Potsdam value. Washington, DC, eventually had three national base stations: at the Smithsonian Museum, the old U.S. Coast and Geodetic Survey headquarters on New Jersey Avenue, and at the Hoover Commerce Building. Of the five stations in these three cities, only the Smithsonian site survives into 2007. The University of Wisconsin established a base station at the Madison airport for its nationwide and worldwide surveys; this station has also been destroyed.
By the 1960s, it was realized that the Potsdam datum was off by about -14 mGals. However, until a better system could be devised, world experts agreed that it was better to leave all measurements on a single datum.
The International Gravity Standardization Net 1971
In the late 1950s and 1960s, concerted efforts were made to make new worldwide pendulum and spring-based gravimeter ties and to include the new ballistic absolute gravimeter measurements in the adjustment. These data were combined and solved simultaneously and published as the International Gravity Standardization Net 1971 (IGSN-71).
About 1,900 worldwide sites, including about 450 U.S. sites, were in this network. Each site had an estimated standard error of less than ±50 μGals, with a correction of -14.0 mGals at the Potsdam site. However, corrections at other sites varied.
The IGSN-71 remains the official gravity datum worldwide today. A number of countries, including the U.S., have re-observed the portions of IGSN-71 in their jurisdictions. Besides finding some errors, a number of temporal changes due to local geologic changes have been determined. The Potsdam and ISGN-71 datums each have a corresponding formula for the calculation of theoretical gravity (a simple, but fictitious, formula which accounts for the average variation of gravity by latitude and elevation, and thus accounts for over 99 percent of the total variation in gravity worldwide), which are used in the calculations of Bouguer (and other) "gravity anomalies."
Absolute Gravimeter Measurements
Since the 1960s, numerous ballistic, absolute gravimeters have been made. Today, these gravimeters make routine measurements with accuracies of under ±10 μGals. Modern absolute gravimeter measurements loosely confirm the IGSN-71 values, within its error limits.
The absolute gravimeter measurements are technically their own datums, since they measure gravity directly without relative ties to any other stations. Internal design elements and the ways they are combined to make a gravimeter cannot guarantee that the published number is a ‘true’ gravity value any more than old pendulum gravimeters could. The best researchers can do is to compare gravimeters to each other to be sure they are all measuring similarly and thus have a semblance of a being a datum.
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