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The technology and practice of measuring and recording exact locations or boundaries. As a profession and a technology, surveying has several levels of application and accuracy. At its most precise level, geodetic surveying, or geodesy, focuses on the exact measurement of the earth and other heavenly bodies. At an intermediate level, control surveying establishes a network of monuments and control points with precisely estimated locations that enable cartographers to add meridians and parallels to air photos and large-scale maps (see topographic map). At the local and least precise level, land surveys provide legal descriptions of real property and help contractors mark the alignments of roads, pipelines and other transport facilities. In addition, hydrographic surveys use depth soundings to identify underwater hazards and delineate navigation channels, and aerial surveys provide earth scientists, foresters and planners with maps and measurements derived from aerial photography or satellite imagery (see remote sensing).
Although the simplifying assumption of a spherical earth is adequate for small-scale maps (see map image and map), a systematic series of large-scale quadrangle maps with minimal scale error requires that the projection accommodate a planet with an equatorial axis roughly 1/300 longer than its polar axis. Geodesists acknowledge this flattening at the poles by rotating an ellipse about the earth\'s axis (Jackson, 1980). The resulting ellipsoid is a practicable compromise between a sphere and the highly complex, largely theoretical geoid. Calibrated to coastal measurements of mean sea level, the ellipsoid provides a reference elevation for inland areas. Geodesists have calculated separate ellipsoids for Europe, North America and other continents as well as global standards such as the WGS 84 (World Geodetic System) ellipsoid, presented in 1984 (Snyder, 1987, pp. 11-13).
Because geodetic measurement is costly and time-consuming, control surveyors rely on networks of triangles anchored by a few highly accurate estimates of length and position. Triangulation based on carefully measured angles and trigonometric tables can carry across a continent the precision of a single carefully measured base line ten miles long. Similar efficiencies accrue to the direct astronomical and chronometric determination of latitude and longitude at a small number of first-order control stations. Triangulation also allows topographic and property surveyors to \'tie into\' and share the benefits of more precise, higher-order surveys. Land surveyors, who typically describe a parcel\'s boundary with a series of lengths and angles, use trigonometry to estimate the error of closure.
Technological advances have reduced the need to measure angles. In the 1970s, electronic distance measurement (EDM) equipment afforded direct measurement of distances and the adoption of trilateration, which bases network calculations on the sides, not the angles, of triangles (Bird, 1989). In the 1990s, increased civilian access to the constellation of geodetic satellites placed in orbit by the United States Department of Defense encouraged widespread use of Global Positioning Systems (GPS), which provide direct estimates of latitude, longitude and elevation (Hofmann-Wellenhof, 1997). In addition, electronic computing allows surveyors to assess and adjust for errors associated with individual measurements (Mikhail and Gracie, 1981). (MM)
References Bird, R.G. 1989: EDM traverses: measurement, computation and adjustment. Harlow, Essex: Longman Scientific and Technical. Hofmann-Wellenhof, B. 1997: Global positioning system: theory and practice, 4th edn. Vienna: Springer-Viennaerlag. Jackson, J.E. 1980: Sphere, spheroid and projections for surveyors. New York: John Wiley and Sons. Mikhail, E.M. and Gracie, G. 1981: Analysis and adjustment of survey measurements. New York: Van Nostrand Reinhold. Snyder, J.P. 1987: Map projections — a working manual. Professional Paper 1395. Washington, D.C.: US Geological Survey. |
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