# Axis Definitions for Land Surveyors

**axis- **^{1}A straight line about which a body rotates, or around which a plane figure may rotate to produce a solid; a line of symmetry. ^{2}One of a set of reference lines for certain systems of coordinates. ^{3}The principal line about which anything may extend, as the axis of a channel or compass card axis. ^{4}A straight line connecting two related points, as the poles of a magnet.

**axis, bubble—**The horizontal line tangent to the upper surface of the centered bubble, which lies in the vertical plane through the longitudinal axis of the bubble tube; axis level vial.

**axis, collimation—**See *collimation axis.*

**axis, optical—**See *optical axis.*

**axis of homology-**^{1} In geometry, the intersection of two projectively related planes. ^{2}In photogrammetry, the intersection of the plane of a photograph with the horizontal plane of a map or the plane of reference of the ground. Corresponding lines in the photograph and map planes intersect on the axis of homology.

**axis of sighting—**The line drawn through the optical center and centers of curvature of the lenses of a telescope. See also *collimation, line of*

**axis of tilt—**A line through the perspective center perpendicular to the principal plane.

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

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