leveling—Method and procedures by which the elevation of different points on the ground, or on or within structures, are measured. Leveling is usually done by sighting through a leveling instrument at a leveling staff or rod held vertically and in an orderly sequence from one point to another. See also Appendix A, Standards Geodetic Control Surveys.

leveling datum—The level surface to which height measurements are referred. The elevation of the datum is usually, but not always, zero, and it may be arbitrary or mean sea level.

leveling, barometric—A method of determining elevation differences from differences of atmospheric pressure measured with a barometer. By applying certain corrections and using the so-called barometric formula, a difference of atmospheric pressure at two places is transformed into a difference of elevations of those places. If the elevation of one station above a datum (as sea level) is known, the approximate elevations of other stations can be known by barometric leveling. By using barometers of special design, and including several stations of known elevation in a series of occupied stations, the accuracy of the elevations determined for the new stations is increased. Corrections are applied for temperature, latitude, barometer index• closure of circuit, and diurnal variation in atmospheric pressure.

leveling, differential—The process of measuring the difference of elevation between any two points by spirit leveling or other equally accurate leveling.

leveling, double-rodded—Leveling carried out with a single observer and a single leveling instrument but with two sets of leveling rods. Foresights are taken on each of the two rods at two separate turning points and backsights are taken on two other rods at two other, separate turning points, all sights being taken from the same location of the leveling instrument.

leveling, fly—¹The extension of a level circuit at the close of a working day to check the results of a level circuit measured in one direction only. ²Level circuits measured with the engineer’s ordinary leveling equipment but with a distinctly low order of accuracy. The error of closure may be perhaps I foot (or more) times the square root of the distance in miles, thus permitting long sights and rapid progress.

leveling, geodetic—Spirit or other equally accurate leveling of a high order of accuracy, usually extended over large areas, to furnish accurate vertical control as a basis for the control in the vertical dimension for all surveying and mapping operations.

leveling, indirect—The determination of differences of elevation from ¹ vertical angles and horizontal distances, as in trigonometric leveling; ²Comparative elevations derived from values of atmospheric pressure determined with a barometer, as in barometric leveling; and ³ Elevations derived from values of the boiling point of water determined with a hypsometer, as in thermometric leveling,

leveling, precise differential—Differential leveling conducted with instruments of a quality suitable for Pint-order leveling according to the National Geodetic Survey.

leveling, profile-¹ Determining elevations at closely-spaced points along an alignment, in order to determine the profile of the ground along that alignment. In the United States, the points are marked by stakes 25 feet, 50 feet or 100 feet apart. The points at 100 foot intervals, starting at the beginning of the line, are called full stations; all other points are called plus stations. ² Plotting changes in differences of elevation determined by comparing the results of two surveys done at different times.

leveling, reciprocal—Determining the difference of elevation between two points by taking foresights to leveling rods on both points from one set-up and backsights to both leveling rods from a second set-up. The method is used principally when it is not feasible to level between the two points by the ordinary method of differential leveling, as when the points are separated by a wide river or a steep valley. The method eliminates emirs caused by the curvature of the earth or by lack of horizontality of the line of sight but does not usually eliminate all of the error caused by refraction.

leveling, spirit—The determination of elevations by use of a leveling rod and an instrument using a spirit level to establish a horizontal line of sight; the term has now been broadened to include leveling by means of other types of precise levels, such as a pendulum level.

leveling, three-wire—Leveling in which a leveling instrument containing a reticle on which there are three horizontal lines is used. The scale on the leveling rod is read at each of the three lines and the average is used for the final result.

leveling, trigonometric—Determining differences of elevation by measuring vertical angles between points. The horizontal distances between the points must be known or the straight-line distances between them measured and converted to horizontal distances. The differences of elevation are then calculated using a trigonometric formula.

leveling, water—A method of obtaining relative elevations by observing heights with respect to the surface of a body of still water. The surface of a body of still water, as of a lake, is a level surface (equipotential surface), and the relative elevations of objects along the shores can be obtained by taking the differences of their heights with respect to the surface of the water. It is with this meaning that the term water levels is generally used.

leveling error of closure—See error of closure.

leveling rod—A straight rod or bar with a flat face graduated in linear units with zero at the bottom, used in measuring the vertical distance between a point on the ground (or other fixed location) and the horizontal line o sight of a leveling instrument. Rods are usually made of wood or metal. For more accurate leveling, the graduations are placed on an invar strip fastened to the foot of the rod and held against the face of the rod to maintain a constant tension in the invar strip. A self-reading rod is designed to be read by the observer at the leveling instrument. A target rod has a movable target that is raised or lowered by the rodperson in response to signals from the instrument man; the position of the target on the rod is then read with the aid of a vernier.

leveling rod, precise—A rod used for precise leveling. The graduations are on an invar ribbon which is maintained under constant tension and which, for all practical purposes, eliminates the need for correcting for changes in length. These rods are usually graduated in whole and fractional meters. The back side of the rod is graduated in feet and tenths of feet. Also called invar leveling rod: meter rod; U.S. Coast and Geodetic Survey (now, National Geodetic Survey) first-order leveling rod.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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