- Industry: Earth science
- Number of terms: 93452
- Number of blossaries: 0
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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, ...
That condition which exists, in the Earth's crust, when the downward pressure of masses above the geoid is exactly balanced by the upward pressure of masses below the geoid, so that the crust neither rises nor sinks (except, of course, for changes produced by erosion, glaciation, etc. ). The equilibrium is between the crust and the subcrustal material and probably not within the crust itself. The existence of isostatic equilibrium is based on the idea that the mass of each unit section of the Earth's crust exerts the same pressure on the subcrustal material.
Industry:Earth science
A machine, also called a ruling engine, for precisely drawing or incising equally spaced lines in material. It is used to produce diffraction gratings, the graduated circles on telescopes, transits, and theodolites, and so on. In making graduated circles, only the prototype is usually graduated on the dividing engine. Copies are made by photoetching or some other photographic process.
Industry:Earth science
Engraving in which the original is scanned linearly and an engraver is controlled by the output from the scanner.
Industry:Earth science
(1) A particular instant of time from which an event or a series of events is calculated; a starting point in time, to which events are referred. In particular, a date and instant corresponding to the position of a coordinate system, all subsequent positions being referred to the position at that epoch. In this sense, it is equivalent to date (1). Many writers prefer to use date for this meaning, since it denotes an instant unambiguously. However, the two terms have different connotations. Epoch is associated with a point in time to which events are referred; date is associated with a value of time and is associated with only the event that occurred then. Also, date is commonly used for civil date, which is not an instant. (2) The angular lag of the maximum of the observed tide or tidal current behind the corresponding maximum of the same constituent of the theoretical equilibrium tide. When the theoretical, local equilibrium tide is meant, the epoch is represented by κ. When the corresponding equilibrium tide at Greenwich is meant, it is called the Greenwich epoch and is represented by the symbol G. (3) A specific period of time.
Industry:Earth science
The ratio (x+1):1 of the length of the diagonal of an enlarged copy to that of the rectangular original.
Industry:Earth science
An ellipsoid which best fits the astro-geodetic network in a particular datum and hence does not necessarily have its center at the Earth's center. More precisely, an ellipsoid whose dimensions and position have been selected to best fit the astronomic and geodetic coordinates of a particular geodetic network.
Industry:Earth science
That great circle of the celestial sphere which constitutes the primary circle of reference for galactic latitude and which passes approximately through the center of the Milky Way. It is inclined at about 62<sup>o</sup> to the celestial equator.
Industry:Earth science
Projecting and printing a small image, then copying and projecting it again to the required size. This is often necessary when the size of the copy or the limitations of the copying camera do not allow reduction in a single operation.
Industry:Earth science
