Levelling Sources of Errors in the Equipment Collimation

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Levelling: Sources of Errors in the Equipment Collimation Error - Line of sight not

Levelling: Sources of Errors in the Equipment Collimation Error - Line of sight not horizontal Correct reading Actual reading

Levelling: Sources of Errors in the Equipment Collimation Error - Line of sight not

Levelling: Sources of Errors in the Equipment Collimation Error - Line of sight not horizontal Size of error depends on sight length Correct reading Actual reading

Levelling: Sources of Errors in the Equipment Collimation Error - Line of sight not

Levelling: Sources of 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

Levelling: Sources of Errors in the Equipment Staff Errors Zero error - base may

Levelling: Sources of 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

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

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’