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

Definitions Benchmark Backsight Height of Instrument Foresight 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

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

Direct Differential Leveling Field Notes Math Check Closure

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

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

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

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

Mistakes and Errors Mistakes Errors 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

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!)