Circle reading

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The circle reading is the determination of the exact angle on the horizontal or vertical circle of measuring instruments of astronomy , astrometry and geodesy . In order to achieve an accuracy of 0.1 ", the previously common vernier on the meridian circle and theodolite was replaced by better methods in the 19th and 20th centuries :

  1. Reading microscopes
  2. Lighting (increasing the contrast of the fine graduation lines)
  3. Reading at several points of the vertical and horizontal circle
  4. Photographic circle reading or later
  5. Digital or electro-optical circle reading.

For exact theodolites from around 1920 :

  1. Transition to glass circles and
  2. Mirroring opposing circle locations,
  3. Coincidence micrometer (stacking of opposing glass lines)
  4. Digital or electro-optical circle reading.

In addition to aiming at the telescope, reading a circle is the most important process for achieving high precision. Therefore, one tries to make the reading more accurate than the aiming and at the same time to reduce the circle division errors (points 3 and 6-8).

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