- 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, ...
A telescope attached to another and used for keeping that other telescope directed at a particular point in the heavens while observations are being made. Guide telescopes are most often used when the telescope to which it is attached is being used for photographing or spectroscopy and cannot be seen through. A guide telescope must have about the same magnifying power as the telescope it guides or the aiming will not be accurate enough. In particular, a refracting telescope attached to a telescope used for photographing.
Industry:Earth science
A curve, in three dimensions, having the property that its orthogonal projection onto any plane perpendicular to a given straight line (the axis of the helix) is a spiral. Alternatively, a curve, in three dimensions, such that r = f(θ) and z = g(θ), in which r, θ and z are cylindrical coordinates and r and z are monotonic functions of θ. Coiled springs (of the kind used in spring balances) and the threads of screws are shaped like helices.
Industry:Earth science
A gravity correction (δg <sub>tc</sub>) added to the theoretical value of gravity to account for the presence of matter outside the level surface used as reference. The complete topographic gravity correction may be calculated as the sum of the Bouguer gravity correction δg <sub>B</sub> and the topographic gravity correction δg <sub>t</sub>, or may be calculated from directly from the topography. If the first method is used, the gravity correction is also called the combined gravity correction.
Industry:Earth science
A change in gravity measured with an instrument fixed to the Earth's surface.
Industry:Earth science
The direct effect, on gravity, of masses of unit density extending to various distances above and below sea level (the geoid). The Hayford effect does not take into account differences of elevation (geoidal heights) between the reference ellipsoid (terrestrial ellipsoid) and the geoid.
Industry:Earth science
The quantity Δg <sub>2i</sub> obtained by subtracting from the measured value g the sum of the value τ of gravity on the reference surface, a free air gravity correction δg <sub>f</sub>' which is zero in oceanic and similar regions, the two dimensional value δg <sub>B2</sub> of the line integral of the gravity in oceans filled with rock and the two dimensional value (negative) δg <sub>i2</sub> of the effect of antiroot masses compensating for the filled in oceans.
Industry:Earth science
A quantity added to the depth obtained by echo sounding to compensate for the vertical displacement (because of waves) of the sounding vessel above or below the average surface of the water.
Industry:Earth science
A (straight) line having one point, called the end point, such that the line extends indefinitely from that point in one direction only.
Industry:Earth science
A precise method of determining astronomic latitude by measuring the difference in the meridional distances of two stars of known declination, one north and the other south of the zenith by approximately the same amount and at approximately the same right ascensions. The observations are made with a zenith telescope or with an astronomical transit (such as the meridian telescope and the broken tube telescope) which can be converted to serve as a zenith telescope. The two stars must have approximately the same zenith distances at the meridian and their culminations must occur within a few minutes of each other. The astronomic latitude of the point of observation will be one half the sum of the declinations of the two stars, plus or minus one half the difference of their zenith distances. This method is also known as the zenith telescope method of determining latitude and as the Talcott method. While Peter Horrebow (not Harrebow) published an account of his method in 1732, in Kobenhavn, the publication was buried in obscurity and there is good reason to believe that Captain Andrew Talcott had no knowledge of Horrebow's work when he announced his own discovery of the method in 1834
Industry:Earth science
