deflection angle—See angle, deflection.

deflection of the plumb line—The angle at a point on the Earth (geoid) between the vertical (direction of the plumb line) and the direction of the normal to the ellipsoid of reference through the point. Also termed “deflection of the vertical” or “station error.” It is also equal to the angle between the tangents of the geoid and the ellipsoid and is equivalent to the topographic deflection corrected for the effects of isostasy.

deflection of the vertical—The angular difference, at any place, between the upward direction of a plumb line (the vertical) and the perpendicular (the normal) to the reference ellipsoid. This difference seldom exceeds 30 seconds. Often expressed in two components, meridian and prime vertical. Also called “deflection of the plumb line;” “station error.”

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

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