Surveying

Surveying is the art, science, and technology of determining or establishing the three-dimensional (3D; x, y, z) position of points on or beneath Earth's surface. Because geographic surveying deals primarily with topography, two other terms, topographic surveying and field surveying, are interchangeably used. Geographic survey data are used to produce topographic maps, block diagrams, and cross-sectional profiles and to establish vertical and horizontal controls such as spot heights and benchmarks.

Conventional geographic surveying uses theodolites, levels, chains, links, steel (or invar, a steel alloy) tapes, altimeters, compasses, clinometers, tachymeters, stadia rods, plane-table alidades, ranging poles, tripods, and the total station. Three basic sets of readings are taken during a conventional survey: distances, heights, and angles. Horizontal distance can be measured with a tape using slope distance and reducing it to horizontal distance using the cosine of the slope gradient. Elevation (height) is measured using a leveling instrument with a level telescope and a stadia rod or by observing the vertical angle with a clinometer and deriving height. The horizontal angle is measured with a compass. Direction is expressed relative to a reference line (meridian). A North-South line is the true meridian, although a magnetic meridian may be used if compass azimuths are recorded without adjusting for declination.

The total station is the instrument of choice for many topographic surveyors because it integrates theodolite functions and an electronic distance meter (EDM) with a built-in or external computer for data storage. Its capability to automate and integrate the measurement of distances, heights, and direction greatly improves topographic surveying and mapping.

In any conventional survey, the starting or initial position(s) must be known and must be accessible. Subsequent positions are observed and determined based on preceding point base positions. There are four common approaches used in surveying—namely, triangulation, trilateration, triangulateration, and traversing. In triangulation, the location of a point is determined by measuring the angles to it from the initial points at either end of a fixed baseline rather than measuring the distances to the point directly. The point can then be fixed as the third point of a triangle with one known side and two known angles. Triangulation is used in surveying large areas for geodetic purposes. Trilateration is used in conjunction with triangulation to determine the intersections of three sphere surfaces given their centers and radii. Triangulateration is a combination of triangulation and trilateration. Traversing is the establishment of a series of consecutive lines (legs) and directions whose ends define points (also called stations, hubs, or corners).

There are four main types of traverse: link (azimuth), closed loop (polygonal), closed, and open (free). A link traverse follows along a single direction (azimuth). This method is common for slope profile surveys and is always perpendicular to the contours. Closed-loop traverses begin and end at the same station. In this method, interior and deflection angles can be established. A closed traverse uses a known endpoint, and its exterior angles can be measured. An open traverse ends in an unknown position.

Inaccuracy in surveys is a common problem. Sometimes even closed traverses will not “close” [Page 2736]because the initial point and the endpoint fail to coincide. This error is called the closure error (gap). Closed-loop and closed traverses permit calculation and adjustment for closure error. However, it is more difficult to maintain good accuracy in an open traverse, as it does not enable computational checks for error.

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