surveying
surveying method of determining accurately points and lines of direction (bearings) on the earth's surface and preparing from them maps or plans. Boundaries, areas, elevations, construction lines, and geographical or artificial features are determined by the measurement of horizontal and vertical distances and angles and by computations based on geometry and trigonometry.
Types and Branches of Surveying
Hydrographic surveying deals with bodies of water and coast lines, is recorded on charts, and records such features as bottom contours, channels, buoys, and shoals. Land surveying includes both geodetic surveying, used for large areas and taking into account the curvature of the earth's surface (see geodesy ), and plane surveying, which deals with areas sufficiently small that the earth's curvature is negligible and can be disregarded. Plane surveying dates from ancient times and was highly developed in Egypt. It played an important role in American history in marking boundaries for settlements; surveying was a profession of distinction—both Washington and Jefferson worked for a time as surveyors. Branches of surveying are named according to their purpose, e.g., topographic surveying, used to determine relief (see contour ), route surveying, mine surveying, construction surveying; or according to the method used, e.g., transit surveying, plane-table surveying, and photogrammetic surveying (securing data by photographs).
Instruments and Techniques
In surveying, measurements may be made directly, electronically, by the use of optical instruments, by computations from known lines and angles, or by combination methods. Instruments used for direct linear measurements include the Gunter's chain (known also as the surveyor's chain), which is 66 ft (20 m) long and divided into 100 links; the engineer's chain, 100 ft (30 m) long and also consisting of 100 links; the tape, usually of steel, which has largely superseded chains; and the rod. Tapes and rods made of Invar metal (an alloy of steel and nickel) are used for very precise work because of their low coefficient of thermal expansion. In many situations electronic instruments, such as the geodimeter, which uses light waves, and the tellurometer, which uses microwaves, provide a more convenient and more accurate means of determining distance than do tapes and rods.
The height of points in relation to a datum line (usually mean sea level) is measured with a leveling instrument consisting of a telescope fitted with a spirit level and usually mounted on a tripod. It is used in conjunction with a leveling rod placed at the point to be measured and sighted through the telescope. The transit is used to measure vertical and horizontal angles and may be used also for leveling; its chief elements are a telescope that can be rotated (transited) about a horizontal and about a vertical axis, spirit levels, and graduated circles supplemented by vernier scales. Known also as a transit theodolite , or transit compass, the transit is a modification of the theodolite, an instrument that, in its original form, could not be rotated in a vertical axis. A plane table consists of a drawing board fixed on a tripod and equipped with an alidade (a rule combined with a telescope); it is used for direct plotting of data on a chart and is suitable for rapid work not requiring a high degree of precision.
The stadia method of measuring distance, a rapid system useful in surveying inaccessible terrain and in checking more precise measurements, consists in observing through a telescope equipped with two horizontal cross hairs or wires (stadia hairs) the interval delimited by the hairs on a calibrated stadia rod; the interval depends on the distance between the rod and the telescope.
Surveys based on photographs are especially useful in rugged or inaccessible country and for reconnaissance surveys for construction, mapping, or military purposes. In air photographs, errors resulting from tilt of the airplane or arising from distortion of ground relief may be corrected in part by checking against control points fixed by ground surveys and by taking overlapping photographs and matching and assembling the relatively undistorted central portions into a mosaic. These are usually examined stereoscopically.
Bibliography
See W. H. Rayner and M. O. Schmidt, Fundamentals of Surveying (5th ed. 1969); R. F. Spier, Surveying and Mapping (1970); J. Anderson and E. Mikhail, Introduction to Surveying (1989); F. Bell, Surveying and Setting Out Procedures (1991).
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