Surveying
If the distance between the projectors is altered by a movement along X-axis of one projector,

The scale of the model is altered
The length of the air base is increased
All listed here
y-parallax is not affected

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Surveying
A ’level line’ is a

horizontal line
line passing through the center of cross hairs and the center of eye piece
line passing through the objective lens and the eye-piece of a dumpy or tilting level
line parallel to the mean spheriodal surface of earth

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