Levelling: Sources of Error

daughterduckUrban and Civil

Nov 15, 2013 (3 years and 11 months ago)

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Levelling: Sources of Error

Errors in the Equipment

Collimation Error
-

Line of sight not horizontal

Correct

reading

Actual

reading

Levelling: Sources of Error

Errors in the Equipment

Collimation Error
-

Line of sight not horizontal

Correct

reading

Actual

reading

Size of error depends on

sight length

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)

Parallax

Focus eyepiece on cross
-
hairs, then focus telescope on staff

Check prism is not stuck (every reading)

Levelling: Sources of Error

Errors in the Equipment

Staff Errors

Zero error
-

base may be worn
-

doesn’t matter as long as

same staff is always used

May not be extended properly

Tripod Errors

Must be stable

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

Don’t touch (or kick!) tripod

Levelling: Sources of Error

Reading and Booking Errors

Keep sightings short to estimate mm on staff accurately

Double check all readings

Write clearly

Weather

Wind causes level to vibrate, heat causes ‘shimmer’

Carry out calculation checks