Spirit (Optical) Levelling
Spirit levelling employs a spirit level, an instrument consisting of a telescope and a tube level like that used by carpenters, rigidly connected. When the bubble in the tube level is in the middle, the telescope's optical axis (collimation axis) will point exactly in the direction of the local horizontal.
The spirit level is placed on a tripod in the middle between the two points whose height difference is to be determined; the points are marked by markers or benchmarks in the rock or soil. A leveling staff or rod is placed on each point, with measured graduations, usually in centimetres and fractions thereof, or tenths and hundredths of a foot. The observer focuses in turn on each rod and reads the value from it. Subtracting the "back" and "forward" value provides the height difference.
Read more about this topic: Levelling
Famous quotes containing the word spirit:
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The stone has rolled away and death cannot imprison!
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—Shirley Erena Murray (20th century)