chap 7 – differential leveling
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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!)