Chap 7 Differential Leveling n n n Definitions

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

Definitions n Benchmark n n Backsight n n Elevation of level plane of sight Foresight n n Shot taken on a point of known elevation Height of Instrument n n Relatively permanent point of known elevation Shot taken at the point of unknown elevation Turning Point n Temporary, identifiable point of fixed elevation

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

Direct Differential Leveling Field Notes Math Check Closure

Curvature and Refraction n n Difference between horizontal and level surfaces Light bends down as it travels through air C = 0. 574 M 2, M in Miles C = 0. 0657 k 2, k in kilometers C = 0. 574(100/5280)2 = 0. 000206’ n n C =. 000206’ at 100’ C =. 00185’ at 200’ C =. 0206’ at 1000’ Shots less than 300’ – C is negligible

Verniers n Device to read measurements to smaller units n n n Scale marked to set units Divide (n-1) scale units into n parts Least Count – smallest scale unit/No. of vernier parts n n Scale to 1/10 th, Vernier has 10 marks LC = 0. 1/10 = 0. 01

Verniers n n n Determine Least Count Read Scale at the “ 0” on the vernier Read the vernier where the lines coincide n n LC = 0. 01 Scale = 3. 0 Vernier coincides at 8 Record 3. 08

Rod Verniers n n Scale = Rod markings, read to 0. 01’ Vernier has 10 divisions LC = 0. 01’/10 = 0. 001’ Example = page 102 n n Read scale at 0 –> 2. 94’ Verniers coincide at 8 Record 2. 948’ Reading Rod Vernier n n Direct Indirect

Mistakes and Errors n Mistakes n n n Misreading rod Poor rod extension Mud/snow under the rod Reading wrong crosshair Errors n n Rod not vertical – wave rod Settling of rod BS and FS not equal Instrument not level n n Parallax Personal Errors n n Heat waves Too long a shot

Leveling Procedures n n n Anchor legs firmly Sloping terrain – two legs on downhill side BS and FS the same distance Wave the Rod Hand/Voice Signals n n n n Raise for Red High Rod OK, Not OK (Can’t See) Turning Point Raise/Lower target Plumb Rod Pick it Up! (We’re done!)