Surveying
Triangulation surveys are carried out for locating

Engineering works, i.e. terminal points of long tunnels, bridge abutments, etc.
All of these
Control points for surveys of large areas
Control points for photogrammetric surveys

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

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

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