Surveying
If the lower clamp screw is tightened and upper clamp screw is loosened, the theodolite may be rotated

on its outer spindle without a relative motion between the vernier and gra-duated scale of lower plate
on its inner spindle without a relative motion between the vernier and the graduated scale of lower plate
on its outer spindle with a relative motion between the vernier and graduated scale of lower plate
on its inner spindle with a relative motion between the vernier and the graduated scale of lower plate

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

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