Surveying
Which of the following errors cannot be eliminated by taking both face observations ?

error due to horizontal axis not being perpendicular to the vertical axis
None of these
error due to non-parallelism of the axis of telescope level and line of collimation
index error i.e. error due to imperfect adjustment of the vertical circle vernier

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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
reading both verniers and taking the mean of the two
taking both face observations and taking the mean of the two
double sighting

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