Classification of surveying - classification based upon the nature of the field survey - land, topographical surveys, cadastral marine or hydrographic, surveying. Classification based on the object of survey - engineering, military, mine, geological and archaeological survey. Classification based on instruments used - chain, theodolite, traverse, triangulation,tacheometric, plane table, photogrammetric and aerial survey Classification of surveying Surveys may be classified under headings which define the uses or purpose of the resulting maps. Classification based upon the nature of the field survey Land Surveying Topographical Surveys: This consists of horizontal and vertical location of certain points by linear and angular measurements and is made to determine the natural features of a country such as rivers, streams, lakes, woods, hills, etc., and such artificial features as roads, railways, canals, towns and villages. Cadastral Surveys: Cadastral surveys are made incident to the fixing of property lines, the calculation of land area, or the transfer of…
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
Hydrographic 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 (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.
Learn primary divisions of survey plane surveying and geodetic surveying spherical triangle and great circle, earths surface Primary division of survey The earth is an oblate spheroid of revolutions, the length of its polar axis (12,713,800 metres) being somewhat less than that of its equatorial axis (12,756,750 metres). Thus, the polar axis is shorter than the equatorial axis by 42.95 kilometres. Relative to the diameter of the earth this is less than 0.34 percent. If we neglect the irregularities of the earth, the surface of the imaginary spheroid is a curved surface, every element of which is normal to the plumb line. The intersection of such a surface with a plane passing through the centre of the earth will form a line continuous around the earth. The portion of such a line is known as ‘level line’ and the circle defined by the intersection is known as ‘great circle’. Thus in…
Definition and object of Surveying and levelling Learn Surveying, definition of surveying,Definition of levelling, Object of Surveying Definition of Surveying Surveying is the art of determining the relative positions of points on, above or beneath the surface of the earth by means of direct or indirect measurements of distance, direction and elevation. It also includes the art of establishing points by predetermined angular and linear measurements. The application of surveying requires skill as well as the knowledge of mathematics, physics, and to some extent, astronomy. Definition of Levelling Levelling is a branch of surveying the object of which is (i) to find the elevations of points with respect to a given or assumed datum, and (ii) to establish points at a given elevation or at different elevations with respect to a given or assumed datum. The first operation is required to enable the works to be designed while the second…