Altitude index error
The index error ( index error , Engl. Vertical collimation error) is that inevitable, small instrumental errors by which the reading at vertical circle each theodolite must be corrected.
It corresponds to the deviation of the display from 0 or 90 ° (or 0 or 100gon) on the height circle if the telescope is pointed exactly to the zenith .
Most of the index error is eliminated by referring to the plumb direction :
- with older models by the height index vial , which has to be imported before each circular reading,
- for newer theodolites with a height compensator , which can be a special pendulum (accuracy <1 ") or a liquid compensator and deflects the beam path by the amount of the vertical axis inclination .
- Electronic total stations correct the inclination digitally.
A small remainder (about 2-10 ") remains, however, which has to be tested periodically and possibly eliminated. By measuring the elevation angle (zenith angle) in both telescope positions and calculating the difference between the measured values, it is not necessary.
In the case of penetrable telescopes, this remaining index error can be precisely determined by measuring a suitable target in both circles. If it is zero, the circle readings should add up to 360 ° (400 gon). However, changes in temperature or shocks change this state.
Index error on other instruments
With electronic theodolites, the last-mentioned process is completed by saving the 2-circle position measurement. The device uses this value to correct all measurements until the next check.
For astronomical instruments such as universal instruments , meridian circles or passage instruments , the procedure is analogous. If the height circle can be rotated (to reduce the circle division errors ), the index error takes on large values, but is eliminated in 2 circular positions.
An index error also affects nautical sextants . It is determined by the manufacturer together with the circle pitch error and eccentricity and stuck in the instrument box as a small " loading table ". This correction can vary by 10-30 "from 10 to 10 °.