Surveying
If the horizontal distance between the staff point and the point of observation is d, then the error due to curvature of earth is proportional to

1/d2
d2
d
1/d

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Surveying
Dumpy level is most suitable when

fly levelling is being done over long distance
the instrument is to be shifted frequently
many readings are to be taken from a single setting of the instrument
All of these

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

horizontal line is normal to plumb line while level line may not be normal to the plumb line
both are same
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

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