Standing axis error

As the vertical axis error or Stehachsfehler in which is Geodesy the deviation of the vertical axis ( vertical axis ) of a theodolite by the local perpendicular direction , respectively.

In contrast to the so-called target axis and tilt axis error of theodolites and related measuring devices, the standing axis error is not instrumental , but an operative error influence when setting up the measuring device on the tripod or measuring pillar (using the 3 foot screws on the lower part of the theodolite). It can never be eliminated entirely, but only as precisely as the dragonfly used allows it. In the usual Alhidaden -Libelle a Sekundentheodolits this is about 3 to 10 'or 1 to 3 mgon .

The effect of the standing axis error on the measurement of an elevation angle is largely absent if the reading on the elevation circle is corrected by a compensator (before 1970 "insurance vials" were used for this).

On the other hand, the directional and horizontal angles are noticeably influenced if the sight is not horizontal. The amount of error increases with the tangent of the elevation angle and can therefore assume the amount of the axial skew itself with a steep line of sight of 45 ° (in the 90 ° direction transverse to the axial inclination with the sign + or -).

Since this influence the measurement accuracy exceeds ia noticeably special are in modern, digital theodolites Neigungskompensatoren (vertical sensors) installed with which the reduction of the angle reading is possible. However, it is only exact if several target points are measured in both circle positions at each station.

The Geodetic Astronomy , are unavoidable in the Steilvisuren to stars, has particularly accurate for this problem area slopes - or rider dragonflies . They have par values (sensitivities) of only 1 ″ to 5 ″ and are hung or placed directly on the tilt axis . This means that a few tenths of a second can be achieved, apart from small temperature effects on the bubble level.

The standing axis error also plays a role in leveling , because the leveling instrument can only be leveled with the usual circular level . Even automatic levels can only compensate about 99% of the remaining standing axis error (about 10 to 50 ″); the remaining effect can even reach a few mm / km with precision leveling . It can, however, be reduced by alternately leveling up in the foresight and looking back ( “Rote Hose” method because of the possibility of always looking at the same of the two measuring assistants).

In the case of precision measurements, in addition to the standing axis error (which is constant for each station), a secondary axis error is also effective, which is also called a wobble error due to its mechanical cause . The vertical axis is implemented in the substructure of the theodolite as a cylinder axis or as a ball bearing and does not keep exactly the same direction when turning. In some designs, this slight wobbling of about 1 ″ has a periodic course and can be reduced by turning the theodolite or universal instrument 180 ° on the substructure.