A correction applied to a measured horizontal angle to eliminate any error that may have been caused by the horizontal axis of the instrument not having been exactly horizontal. If the horizontal axis of the instrument is not exactly horizontal, the line of collimation will not cut the horizon at a point directly underneath (or above) an observed point, that is a plane described by the line of collimation when the telescope is rotated about the horizontal axis will not be vertical. The inclination of the horizontal axis may involve one or both of two conditions: the horizontal and vertical axes of the instrument may fail to meet exactly at a right angle, and the vertical axis may deviate from the direction defined by the plumb line. So, determining the correction requires that both the inclination of the axis and the angular elevation of the observed point be known.