Surveying
The following points form a pair of homologous points:

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

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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
index error i.e. error due to imperfect adjustment of the vertical circle vernier
error due to non-parallelism of the axis of telescope level and line of collimation

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