Surveying
The error due to eccentricity of inner and outer axes can be eliminated by

reading both verniers and taking the mean of the two
taking both face observations and taking the mean of the two
double sighting
taking mean of several readings distributed over different portions of the graduated circle

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Surveying
Accidental errors

All of these
Do not follow any definite mathematical law
Are generally small
Cannot be removed by applying corrections to the observed values

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