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

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

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Surveying
A ’level line’ is a

line parallel to the mean spheriodal surface of earth
line passing through the objective lens and the eye-piece of a dumpy or tilting level
line passing through the center of cross hairs and the center of eye piece
horizontal line

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