- Industry: Earth science
- Number of terms: 93452
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Founded in 1941, the American Congress on Surveying and Mapping (ACSM) is an international association representing the interests of professionals in surveying, mapping and communicating spatial data relating to the Earth's surface. Today, ACSM's members include more than 7,000 surveyors, ...
A quantity δg <sub>Hc</sub> added to the value of gravity calculated from a gravity formula (or subtracted from a gravity anomaly) to take into account the attraction of topographic masses condensed into a layer on or below the reference surface. Masses outside the reference surface are condensed into a layer on or inside that surface to prevent, as far as possible, the drastic change in gravity and the geoid which result when matter is simply removed from between the surface and a level surface through the point where gravity was measured.
Industry:Earth science
Those variations, in the value of gravity at a point, which are caused (a) by the attraction of the Moon, the Sun, and other planets, and (b) by the effects of those attractions on the land and seas.
Industry:Earth science
That point, usually near the center of a grid, where the line representing a parallel of latitude intersects the line representing a meridian.
Industry:Earth science
(1) A value of gravity acceleration given by a standard gravity formula. (2) A value of gravity used as the referent when calculating values of some function of gravity. E.g., the rate of precession of a gyroscope can be separated into (a) the rate in a field of standard gravity and (b) the variation of this rate with variations from standard gravity. (3) As adopted by the International Committee on Weights and Measures, a gravity acceleration equal to 980.665 cm/s². (4) The gravity acceleration at 45<sup>o</sup> latitude and mean sea level, equal to 980.616 cm/s². (5) A starting value for gravity acceleration as reduced to mean sea level.
Industry:Earth science
Geoidal height referred to an astrogeodetically oriented ellipsoid.
Industry:Earth science
A gravity correction δg <sub>i</sub> added to the theoretical value of gravity to correct for the (assumed) presence of additional mass below the geoid balancing excess mass above the geoid. The isostatic gravity correction is also called an isostatic gravity reduction. It is an ad hoc correction invented to make predicted values of gravity agree better with measured values. There are many different kinds of isostatic gravity correction. Each kind results from a particular assumption on how density varies within the crust and upper mantle. All the assumptions postulate that the crust and part of the upper mantle float on an underlying material of greater density. They differ in the depths to which the floating materials extend, the density of that floating material under the geoid, and in other particulars. Seismological regimes in the crust are only roughly related, if at all, to the regimes involved in explaining the isostatic gravity corrections.
Industry:Earth science
(1) Gravity considered as a whole, rather than as a force or acceleration at a particular point e.g., the Earth's gravity field. (2) A field having gravity as the function of location i.e., a space and a mathematical function giving gravity as a function defined over the points of that spaced.
Industry:Earth science
The height of the point to which an observation is directed above the ground or above the top of the marker it is associated with.
Industry:Earth science
A well-defined, naturally elevated region (a mound) of the sea bottom rising 600 to 900 meters above the bottom and several kilometers wide.
Industry:Earth science
(1) The quantity Δg <sub>f</sub> obtained by subtracting, from a gravity anomaly Δg, the free air gravity correction δg <sub>f</sub>: δg <sub>f</sub> ≡ Δg - δg <sub>f</sub>. The free air gravity correction (which is negative) is the total change in gravity which occurs in moving (without taking into account any mass along the path) from the reference surface to the point P at which gravity was measured: δg <sub>f</sub> = (δτ/δh) h, in which h is the geodetic height of P above the reference surface and τ is the theoretical value of gravity on that surface. (2) The same as the preceding definition, except that the free air gravity correction is calculated using the elevation H<sub>p</sub> of P above the reference surface instead of the geodetic height, and the rate of change of measured values g of gravity instead of the theoretical value: δg <sub>f</sub> ≡ (δg/δHp) Hp. This definition is used if the acceleration on the terrestrial ellipsoid or geoid) is to be calculated from the measured value of the station for comparison with the value calculated from a standard gravity formula. Free air gravity anomaly is sometimes called Faye's anomaly and incomplete Bouguer gravity anomaly.
Industry:Earth science
