Instrument failure
As instrument error deviations of the display are a meter from the true value designated caused solely by the meter. Some of these effects are also caused by improper use, but not by the observer himself.
For statements in measurement technology in general, see under measurement device deviation .
Especially in geodesy and astrometry, the following applies: The instruments used and their axes must be precisely adjusted in order to avoid systematic errors in the results. Specifically, four conditions have to be met for the three axes or their influences have to be eliminated by measurement. In addition, the vertical circle must be set precisely to the perpendicular direction .
With a theodolite or total station the following conditions apply for ideal adjustment:
- The sighting axis coincides with the optical axis of the telescope.
- Adjustment process: carefully moving the reticle under the control of a collimator or other telescopic sight . Older instruments have small adjusting screws for this purpose ; modern theodolites can only be adjusted by specialists.
- The target axis should be perpendicular to the tilt axis , i.e. it should enclose an angle of 90 °. The deviation from this condition is called a collimation or sighting axis error .
- The simplest adjustment process: a well-defined target at least 100 meters away and at the height of the tilting axis is sighted and the direction read on the horizontal circle, then after "cutting through" the telescope in the second position, the same point is aimed at and the direction read off. The difference in direction (minus half the full angle ) corresponds to twice the collimation error. If technically possible, it can be eliminated step by step (residual error usually 1–10 "). The residual error or an unadjusted collimation error can be eliminated with precision measurements or taken into account by calculation.
- The tilt axis should be perpendicular to the vertical or standing axis . The deviation from 90 ° is called the tilt axis error .
- In the case of modern, closed instruments, it is adjusted by the manufacturer. The remaining amount - which can change slightly with the temperature - is eliminated by measurement (e.g. with vials or through both circular positions).
- The vertical axis should coincide with the perpendicular direction. This is done when setting up on the tripod by means of bubble levels or plumbing sensors .
- If the target axis is vertical (directed towards the zenith), the reading on the vertical circle should be zero. Any difference is called an index error or an altitude index error.
- Determination of the index error: by aiming at a well-defined, distant point in the two circles. The sum of both readings should be 360 ° (or 400 gon ).
- This adjustment must not be carried out, but is mostly attached calculation - either in the evaluation program or in the menu total station .
The full measuring accuracy of a modern theodolite (approx. 1 ") can only be achieved after all four influences have been taken into account. Other effects of a thermal nature or small circle division errors on the horizontal and vertical circle usually remain below the 1" limit.
Analog effects occur with astronomical instruments. They are - extended to include the influence of the telescope bending and possible micrometer constants - mostly determined from the measurements themselves, in that their amounts are modeled as unknowns in an adjustment .