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

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 NotesMath CheckClosure

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 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!)