Surveying
The following points form a pair of homologous points:

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

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Surveying
Which of the following errors can be eliminated by taking mean of bot face observations ?

error due to line of collimation not being perpendicular to horizontal axis
error due to eccentricity of verniers
error due to imperfect graduations
error due to imperfect adjustment of plate levels

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