Chap 7 – Differential Leveling
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Transcript of Chap 7 – Differential Leveling
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Chap 7 – Differential Leveling Definitions Process Curvature and Refraction Verniers Errors and Mistakes Operational Procedures
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Definitions Benchmark
Relatively permanent point of known elevation Backsight
Shot taken on a point of known elevation Height of Instrument
Elevation of level plane of sight Foresight
Shot taken at the point of unknown elevation Turning Point
Temporary, identifiable point of fixed elevation
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Direct Differential Leveling Progresses from Known to Unknown ElevKnown + BS = HI HI – FS = ElevUnknown
Level moves ahead Previous Point is now known
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Direct Differential Leveling
Field NotesMath CheckClosure
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Curvature and Refraction
Difference between horizontal and level surfaces
Light bends down as it travels through air C = 0.574 M2, M in Miles C = 0.0657k2, k in kilometers C = 0.574(100/5280)2 = 0.000206’
C = .000206’ at 100’ C = .00185’ at 200’ C = .0206’ at 1000’
Shots less than 300’ – C is negligible
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Verniers Device to read measurements to smaller units
Scale marked to set units Divide (n-1) scale units into n parts
Least Count – smallest scale unit/No. of vernier parts
Scale to 1/10th, Vernier has 10 marks LC = 0.1/10 = 0.01
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Verniers
Determine Least Count Read Scale at the “0” on the vernier Read the vernier where the lines coincide
LC = 0.01 Scale = 3.0 Vernier coincides at 8 Record 3.08
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Rod Verniers
Scale = Rod markings, read to 0.01’
Vernier has 10 divisions LC = 0.01’/10 = 0.001’ Example = page 102
Read scale at 0 –> 2.94’ Verniers coincide at 8 Record 2.948’
Reading Rod Vernier Direct Indirect
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Mistakes and Errors
Mistakes Misreading rod Poor rod extension Mud/snow under the rod Reading wrong crosshair
Errors Rod not vertical – wave rod Settling of rod BS and FS not equal Instrument not level
Parallax Personal Errors
Heat waves Too long a shot
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Leveling Procedures Anchor legs firmly Sloping terrain – two legs on downhill side BS and FS the same distance Wave the Rod Hand/Voice Signals
Raise for Red High Rod OK, Not OK (Can’t See) Turning Point Raise/Lower target Plumb Rod Pick it Up! (We’re done!)