Wobble

Orbital error is a geodetic / surveying term from instrument science or measurement technology . It describes small errors in a device axis, especially the tilting and standing axes on a theodolite , total station or laser scanner . They can have a random or systematic characteristic and, depending on the design of the instrument, amount to about 1 to 3 arc seconds .

The causes can be:

• Axle or bearing play
• Out-of-roundness in the axle bearing or on the axle journal
• Deflection in ball bearing axles
• temperature-dependent axis deformation
• Irregular distribution of the grease .

Random or systematic effect

The wobble error has an effect of about 1 "on the measured horizontal directions , and to a lesser extent on vertical angles and distances. In most geodetic measurements, the errors in the point determination are insignificant. In precision measurements in engineering geodesy or in astrogeodesy , they are in the course of the compensation calculation minimized if the axis errors are of a random, irregular nature, whereas systematic components can be minimized by a suitable measuring arrangement .

It has been known for a long time that the wobble errors in theodolites with cylinder axes have a predominantly random characteristic and can therefore be reduced by repeated measurements. Nevertheless, the large triangulation theodolites such as the Wild T4 were manufactured with particularly long plug-in pins for mounting the vertical axis . The ingenious designer Heinrich Wild prevented many axis errors as early as 1935 by replacing the (vertical) standing axis with a horizontal ball bearing with a central guide. This made the DKM2 second theodolite and later the DKM3-A the most accurate instruments in their weight class.

In special laboratory tests, G.Gerstbach was able to show that this means that there are no more random axis errors, but a small 120-degree system of around 1 ". The cause is a slight deflection of the substructure (some 0.1 µm ) in the area between the three Leveling feet . But their influence can be eliminated if the theodolite to "half" of the observations on the tripod or measuring pillar is rotated by 180 °.

literature

• Karl Ramsayer : Geodetic Astronomy ( Handbook of Surveying Volume 2a). 10th, completely revised and restructured edition. JB Metzler-Verlag, Stuttgart 1970
• Gottfried Gerstbach : For azimuth measurement with second theodolites . Austrian Magazine for surveying 64/2, p. 53-68, Vienna 1977