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Chap 7 – Differential Leveling

Chap 7 – Differential Leveling. Definitions Process Curvature and Refraction Verniers Errors and Mistakes Operational Procedures. Definitions. Benchmark Relatively permanent point of known elevation Backsight Shot taken on a point of known elevation Height of Instrument

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Chap 7 – Differential Leveling

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  1. Chap 7 – Differential Leveling • Definitions • Process • Curvature and Refraction • Verniers • Errors and Mistakes • Operational Procedures

  2. 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

  3. Direct Differential Leveling • Progresses from Known to Unknown • ElevKnown + BS = HI • HI – FS = ElevUnknown • Level moves ahead • Previous Point is now known

  4. Direct Differential Leveling Field Notes Math Check Closure

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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!)

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