# Azimuth Definitions for Land Surveyors

azimuth—The determination of the horizontal angle between North (grid, geodetic, magnetic, astronomic) and an established mark. See also angle, azimuth; angle, zenith.

azimuth, assumed—An azimuth chosen as a matter of convenience, generally by assigning a north-south line in a survey as the meridian direction, or an east-west line as at right angles to the meridian direction. See also meridian, grid; meridian, assumed.

azimuth, astronomic—At the point of observation, the angle measured between the vertical plane through the celestial pole to the vertical plane through the observed object. A horizontal angle determined directly from the observation of the sun, moon, or stars. See also North, true.

azimuth, back—The azimuth of a line at the end opposite the reference end. Back grid azimuths differ by 180° from grid azimuths. Back astronomic and geodetic azimuths differ by 180° plus or minus the convergence of the meridian from the astronomic or geodetic azimuths.

azimuth, direction method—The determination of the astronomic azimuth of a line by measuring, with a direction theodolite, the horizontal angle between a selected star and a suitable mark, and applying that angle to the azimuth of the star computed for the epoch of the observation. Preferred over other methods in horizontal control surveys of the continental United States. A circumpolar star is observed at any hour angle, the mark being a signal light on a main-scheme station, or the “azimuth mark.” A correction for the inclination of the horizontal axis, depending on the altitudes of the star and of the mark, is applied to the observed angle. The curvature correction and corrections for variation of the pole and for diurnal aberration are also applied.

azimuth, forward—See angle, azimuth; angle, zenith.

azimuth, geodetic—At the point of observation, the angle measured between the true meridian of the ellipsoid representing the Earth and a plane normal to the ellipsoid through the observed object. A horizontal angle determined from astronomic observations with corrections to the astronomic azimuth for the difference between geodetic and astronomic longitude; or by correcting a state plane grid azimuth by the theta (0) angle in the Lambert Conic Projection system, or delta (A) in the Transverse Mercator Projection system. See also North, true.

azimuth, grid—See grid azimuth.

azimuth, Laplace—A geodetic azimuth derived from an astronomic azimuth using the Laplace equation.

azimuth, magnetic—At the point of observation, the angle between the vertical plane through the observed object and the vertical plane in which a freely suspended, symmetrically magnetized needle which is not influenced by any transient artificial magnetic disturbance will come to rest.

azimuth, micrometer method—Determination of the astronomic azimuth of a line by measuring indirectly with an ocular micrometer attached to a theodolite, or transit, the horizontal angle between a selected star at its elongation and a suitable ground mark (light) placed close to the vertical plane which passes through the star, and applying that angle to the azimuth of the star computed for the epoch of the observation. At elongation, the apparent motion in azimuth of a circumpolar star such as Polaris is very small for an appreciable period of time; as a result, a series of observations can be made by the micrometer method without re-orienting the instrument.

azimuth, normal section—For the normal section line from A to B, the angle at A between the meridian plane and the normal section which passes through B.

azimuth, true—See North, true.

azimuth angle—See angle, azimuth; angle, zenith.

azimuth equation—See equation, azimuth.

azimuth error of closure—See error of closure (definition 3).

azimuth line—In photogrammetry, a line radial from the principal point, isocenter, or nadir point of an aerial photograph, which represents the direction to the corresponding image point on an adjacent photograph taken in the same flight line. This line is used extensively in radial triangulation.

azimuth mark—A mark set at a significant distance from a triangulation or traverse station to mark the end of a line for which the azimuth has been determined, and to serve as a starting or reference azimuth for later use.

azimuth mark, astronomic—A signal or target whose astronomic azimuth from a survey station is determined from direct observations of a celestial body. The mark may be a lamp or an illuminated target placed especially for the purpose; or it may be a signal lamp at another survey station.

azimuth mark, geodetic—A marked point established in connection with a triangulation (or traverse) station to provide a starting azimuth for dependent surveys. The geodetic azimuth from the station to the azimuth mark is determined instrumentally; its distance is often determined only approximately but should be sufficiently great to render negligible the angular effect of the ordinary errors of centering the instrument and target. Since 1928, azimuth marks consisting of bronze tablets set in concrete or stone have been established in connection with the basic horizontal control survey of the United States. These marks are usually so located as to be readily available without special construction to elevate either the instrument or target. A station with an established azimuth mark can serve as the control for computing and publishing the geodetic azimuth and the grid azimuth of the mark on the state plane coordinate system.

azimuth mark, Laplace—An astronomic azimuth mark at a Laplace station.

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

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