Surveying
The distance between the projection centre and the photograph, is called

Principal distance
Focal length
Isocentric distance
Principal line

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Surveying
The difference between a level line and a horizontal line is that

level line is normal to plumb line while horizontal line may not be normal to plumb line at the tangent point to level line
level line is a curved line while hori-zontal line is a straight line
both are same
horizontal line is normal to plumb line while level line may not be normal to the plumb line

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Surveying
Theodolite is an instrument used for

measurement of vertical angles only
measurement of both horizontal and vertical angles
tightening the capstan-headed nuts of level tube
measurement of horizontal angles only

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