Surveying
The most convenient co-ordinate system for specifying the relative positions of heavenly bodies on the celestial sphere, is

Altitude and azimuth system
Declination and altitude system
Declination and right ascension system
Declination and hour angle system

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

double sighting
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

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