- Branche: 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, ...
The great circle formed on the celestial sphere by intersection, with the celestial sphere, of a plane perpendicular to the vertical through any point such as the eye of the observer. This definition is practically the same as that of celestial horizon.
Industry:Earth science
The set of all points lying on one side only of a given plane. Alternatively, the set of all points such that if any two points are connected by a line, that line either lies entirely on a given plane, called a face of the half-space, or has no point in common with that plane.
Industry:Earth science
(1) The elevation of a point, calculated from the data of a leveling survey, the elevations of certain tide gage bench marks being given specified values. This concept dates from a period when mean sea level and the geoid were treated as equivalent terms. (2) The distance, measured vertically, of a point above the geoid. I.e., the elevation of a point. This definition applied in those instances, such as in the legends on maps, where heights are stated as distances above mean sea level but are actually elevations above the geoid.
Industry:Earth science
A gravity formula expressing gravity, on a surface of reference, as a function of geographic location, with the assumption that the potential function has a constant value on the surface.
Industry:Earth science
A coordinate in a grid coordinate system. In particular, in geodesy, a coordinate in a plane, rectangular, Cartesian coordinate system. A grid coordinate, in this sense, commonly means a distance measured from one of the axes of the system, and parallel to the other axis, to the point in question. In surveying, a constant amount is usually added to each measured distance to avoid negative numbers. Equivalently, the origin is assigned large positive coordinates. Grid coordinates can be transformed into geodetic coordinates and vice versa by simple arithmetic operations if tables (usually readily available) are used. The ordinary computations of surveying can then easily be carried out for regions of large extent using the methods of plane surveying that are otherwise usable only for regions of small extent.
Industry:Earth science
A transparent, plastic sheet for laying over a map, having a family of concentric circles and a family of corresponding radii drawn on it according to a scheme devised by J.F. Hayford, so that the angles between radii and the spacings between circles simplify the calculation of deflections of the vertical or topographic gravity corrections. A given template can be used only on maps of the scale and on the map projection for which it was drawn.
Industry:Earth science
(1) The mass of water present as vapor in a given volume of air. (2) The weight of water present as vapor in a given volume of air. Absolute humidity is usually expressed as grams per cubic meter.
Industry:Earth science
The elevation presumed to prevail in the locality shown on a particular photograph or group of photographs. It is used particularly to denote the elevation assumed to prevail in the vicinity of a critical point such as a peak or other feature having abrupt local relief.
Industry:Earth science
The value H <sup>H</sup><sub>N</sub> found for the elevation of a point P <sub>N</sub> by dividing the geopotential number W <sub>N</sub> by an approximation g <sup>H</sup><sub>N</sub> to the average value of the acceleration of gravity along the vertical between P <sub>N</sub> and the geoid: H <sub>HN</sub> = W <sub>N</sub> / g <sup>H</sup><sub>N</sub>. W<sub>N</sub> is calculated from the measured differences ΔH <sub>n</sub> of elevation between points P <sub>n-1</sub> and P <sub>n</sub> along a route from point P <sub>o</sub> on the geoid to P <sub>N</sub>, and the measured or otherwise determined values gn of the accel-eration of gravity at these points P <sub>n</sub>: W <sub>N</sub> = Σ g <sub>n</sub> ΔH <sub>n</sub> (n = 1 to N). The value g <sup>H</sup><sub>N</sub> is calculated from the measured value g <sub>N</sub> of the acceleration at P <sub>N</sub>, the Bouguer gravity correction δg <sub>B</sub>, and the free air gravity correction δg <sub>f</sub>: g <sup>H</sup><sub>N</sub> = g <sub>N</sub> - δg <sub>B</sub> - δg <sub>f</sub> / 2. This is approximately the value of the acceleration at a point on the vertical through P <sub>N</sub> and halfway between P <sub>N</sub> and the geoid. The Helmert elevation is also called the Helmert height.
Industry:Earth science
A horizontal, V-shaped notch cut into the trunk of a tree at about breast height.
Industry:Earth science
