Further Use and Understanding of the Total Station

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Further Use and Understanding of the Total Station
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The Jigger

• So far, we have used the total station (aka
  theodolite, jigger) to observe directions
• From these directions, we derive angles
• BUT, all instruments are subject to systematic
  error
• Also known as errors of calibration
• AND all observations are subject to random error

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Jigger Errors
Telescope errors
Observation errors
Levelling errors
Manufacturing errors
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The Telescope

• Parallax error
• Collimation error

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Parallax error

• The affect can be large
• The solution is simple
• Focus the telescope on an object at optical
  infinity (say over 200m)
• Then focus the eyepiece so that the cross-hair is
  crisp
• Move your head and watch if the cross-hair moves

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Collimation Error

• An error of adjustment
• Occurs when the centre of the cross-hair is not
  aligned with the optical axis of the instrument

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Collimation Error

• Can be detected by observing the difference
  between face left and face right
• For Horizontal angles, the difference is 180
• For Vertical angles, they should sum to 360
• Can be removed by adjusting the cross-hair
• Can be eliminated by taking the average of the
  face left and face right readings!
• http//www.sli.unimelb.edu.au/planesurvey/prot/fra
  mes.html

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Collimation Error

• Found in all surveying telescopes, including
  those on levels
• A similar approach is taken to its elimination
• Field work based
• More later

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Plate Bubble Error

• There are two level bubbles on most jiggers
• The pill or bulls-eye bubble
• The horizontal plate bubble
• (and the vertical index bubble)

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Pill bubble

• Is only approximate, to be used as a guide only
• Can be corrected by adjustment once the
  instrument is correctly levelled

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Plate Bubble

• Is the serious one
• Is subject to errors of adjustment
• The affect can be greatly reduced by averaging
  the displacement of the bubble during set-up
• See demo
• http//www.sli.unimelb.edu.au/planesurvey/prot/fra
  mes.html
• Further details are shown on the Learning Site

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Plumbing Errors

• Difficult to detect, we need to test the
  tribrach/instrument combination in the workshop
• The optical plummet is still a telescope, we need
  to focus the cross-hair and image correctly
• Errors are reduced by interchanging the jigger
  with targets when traversing

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Observation Errors

• These are treated as random and can be modelled
  by the Normal Distribution
• They can be reduced by repeated measurement
• This improves accuracy and precision

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Observation Errors

• Pointing to the wrong target is NOT an error!
• Reading the display incorrectly is NOT an error!

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Total Stations more than a jigger

• Total Stations measure distances as well as
  angles
• They can also store and process measurements
• The main telescope incorporates an Electronic
  Distance Meter (EDM)
• Infra-red laser, time-of-flight/phase measurement
  of beam
• More when we deal with distance measurement

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The Complete Surveyor

• Recent developments include a GPS unit with the
  total station
• Fully integrated data storage and data
  processing, Bluetooth data transfer or GPRS
• 45,000!
• But still doesnt make coffee