horizon—A great circle on the celestial sphere whose plane is perpendicular to the direction of the plumb line.

horizon, apparent-¹An irregular line along which rays from the point of observation are tangent to the surface of the Earth. ² A line where the visible surface of the Earth appears to meet the sky. When the apparent horizon is formed by the surface of a body of water, it is sometimes used as a reference in observing vertical angles. See also horizon, dip of

horizon, artificial—A device consisting of a plane reflecting a surface such as a dish of mercury.

horizon, closing the—Measuring the last of a series of horizontal angles at a station required that the series be made complete around the horizon. At any station, the sum of all horizontal angles between adjacent lines should equal 360° (400 g). The amount by which the sum of the measured angles fails to equal 360° (400 g) is the error of closure. This error is distributed as a correction for closure among the measured angles to bring their sum to exactly 360° (400 g). The error and the correction are of opposite signs.

horizon, dip of—The vertical angle between the plane of the horizon and a line tangent to the apparent horizon. See also horizon, apparent.

horizon camera—See camera, horizon.

horizon photograph—A photograph of the horizon taken simultaneously with a vertical photograph for the sole purpose of obtaining an indication of the tilt of the vertical camera at the time of exposure.

horizon sweep—With the instrument pointing initially at the farthest visible known point, recording clockwise angles to tanks, spires, buildings, and signals, for the purposes of identification and subsequent use.

horizon trace—See principal plane [PHOTOGRAMMETRY].

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

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