Pillar rotation
In geodesy and astronomy, pillar rotation is the term used to describe slight rotations of measuring pillars or observatory piers , which occur as a result of temperature changes and can significantly impair the measurement accuracy. They have a strong effect on direction measurements , but less on vertical angles .
If the pillar rotation occurs gradually, it can be largely eliminated by special measurements (repeated directional controls , angular difference through autocollimation, control observation with Miren ). Jerky rotations, such as those that occur with concrete pillars due to nighttime cooling or solar radiation, are more critical . They can reach amounts of several arc seconds and can only be reduced by longer series of measurements.
In observatories , the pillars of large telescopes are traditionally built in brick , the temperature of which is kept constant within a day by rear ventilation (now also air conditioning). Instruments set up outdoors, such as the circumzenital or Danjon astrolabe, are not critical because only zenith distances are measured.
In engineering geodesy , the rotation of piers is particularly critical for deformation and control measurements on dam walls or major technical projects. The solar radiation is reduced by large umbrellas, sudden turns by continuous direction controls. In national surveying and astrogeodesy , larger effects are avoided by shading or waiting for dusk .