Why should the distances from a surveying level to two staffs be equal?

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surveying Figure 1: Photogrammetric photographs from two short, overlapping flight strips arranged for supplying mapping details. Photo-control points are shown on only one photograph; shading indicates a typical terrain feature such as a lake (see text).

In surveying, it is essential to keep the distances from the surveyor’s level to the two staffs equal to eliminate errors caused by the instrument's collimation (line of sight) adjustment.

If the line of sight is not perfectly horizontal—meaning it is slightly inclined upward or downward—the error in the reading is proportional to the distance from the instrument. By ensuring the distance to the backsight staff is identical to the distance to the foresight staff, the error introduced at each point becomes equal. When you calculate the difference in elevation between the two points, these equal errors cancel each other out, resulting in an accurate measurement of the height difference regardless of the instrument's calibration state.1