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

2. 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

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 = M2, M in Miles
C = k2, k in kilometers
C = 0.574(100/5280)2 = ’
C = ’ at 100’
C = ’ 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 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

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!)