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

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

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Surveying
A ’level line’ is a

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

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