Levelling: Sources of Error

1
Levelling Sources of Error
Errors in the Equipment
Collimation Error - Line of sight not horizontal
2
Levelling Sources of Error
Errors in the Equipment
Collimation Error - Line of sight not horizontal
Size of error depends on sight length
Correct reading
Actual reading
3
Levelling Sources of Error
Errors in the Equipment
Collimation Error - Line of sight not horizontal
Keep sight lengths from each instrument position
the same
Check collimation error using Two-Peg Test (see
later)
Check prism is not stuck (every reading)
Parallax
Focus eyepiece on cross-hairs, then focus
telescope on staff
4
Levelling Sources of Error
Errors in the Equipment
Staff Errors
Zero error - base may be worn - doesnt matter as
long as same staff is always used
May not be extended properly
Tripod Errors
Must be stable
5
Levelling Sources of Error
Field Errors
Staff not vertical
Use pond bubble on staff
Rock staff and take minimum reading
Unstable equipment
Watch out for soft ground under tripod or staff
Dont touch (or kick!) tripod
6
Levelling Sources of Error
Reading and Booking Errors
Keep sightings short to estimate mm on staff
accurately
Double check all readings
Write clearly
Carry out calculation checks
Weather
Wind causes level to vibrate, heat causes
shimmer