Alidaden eccentricity

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The alidaden eccentricity (also called circle division eccentricity ) is one of the instrument errors of a theodolite or a tachymeter . As a result of unavoidable manufacturing inaccuracies, the pivot point A of the standing axis for the horizontal circle , or the pivot point of the tilt axis for the vertical circle, and the center point M of the circle division (scale) do not coincide. A and M have the distance  e on the plane of the pitch circle . This distance is called the alidad eccentricity .

The alidaden eccentricity leads to angular deviations ε between the scale value α 'read on the pitch circle and the correct angle of rotation α, the legs of which converge at pivot point A. The readings on the pitch circle may be too large or too small. This depends on the set angle α and the circular position. Even very small eccentricities  e of a few μm cause deviations ε in the milli- gon range. The influence of the alidad eccentricity can be reduced by simultaneously tapping on two diametrically opposite pitch circle points. The influence of angle measurements in two telescope faces can be completely eliminated.