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

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

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

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