Target axis error
As target axis error or collimation is at theodolites , universal and passages instruments the deviation of the objective axis from the normal to the tilting axis , respectively.
In modern total stations , it is adjusted to almost zero at the factory , but can grow to a few milligons over the course of its useful life and due to improper handling of the instrument . Older measuring devices required readjustment every several weeks.
The effect of this instrument error on direction measurements increases with the elevation angle h , namely with the function sec h (= 1 / cos h) . Unlike the horizontal axis tilt it works already on targets that are at the same height, but does not increase so rapidly with , draw attention inclination to. By measuring in two telescope positions and averaging the directional measured values, both errors can be almost completely eliminated ("method of the 4-fold collimation error").
The adjustment is made by aiming at a distant line mark or mire in both circular positions, whereby a quarter of the angular difference that occurs on the thread network must be removed.