Surveying
The following points form a pair of homologous points:

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

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Surveying
Normal tension is that pull which

makes the correction due to pull equal to zero
is used at the time of standardising the tape
makes the correction due to sag equal to zero
neutralizes the effect due to pull and sag

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