backsight-¹ A point used to determine the elevation and/or angular orientation of the surveying instrument. ²In leveling, the backsight is the visual intersection of the horizontal cross hair of the instrument with the backsight point or with a staff or rod held vertically over or under the backsight point when the instrument line of sight is level or when the vertical angle of the instrument line of sight is known or will become known from actual measurement. ³In angle measuring or line prolongation, the backsight is the angle measurement on the graduated horizontal circle of the instrument (set or to be read and recorded), when the vertical cross hair of the instrument visually coincides with the backsight point or with a marker in the vertical plane through the backsight point and the center of the instrument.

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

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