Surveying
The following points form a pair of homologous points:

Photo isocenter and ground isocenter
All listed here
Photo principal point and ground principal point
Photo plumb point and ground plumb point

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

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