Surveying
The relation between the air base (B), photographic base (b), flying height (H) and the focal length (f) of a vertical photograph, is

B = b/fH
B =f/bH
B = bH/f
B = H/bf

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

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

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