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
level line is a curved line while hori-zontal line is a straight 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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Surveying
A ’level line’ is a

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

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