# Horizon Definitions for Land Surveyors

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

**horizon, apparent-**^{1}An 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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