Basic GPS Surveying Techniques

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Basic GPS Surveying Techniques
Presented by Neil Gray, Teacher-in-Charge,
Columboola EEC. On behalf of ICT Innovators
Centre, STiS Project.
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Surveying Techniques

• 3 Levels of GPS Surveying
• Point Positioning uses stand alone receivers,
  taking measurements from the code carried on the
  signal. Many sources of error adding up to 10
  15 metres with Selective Availability turned off.
• DGPS (Differential) uses two receivers
  recording position data concurrently. One Base
  receiver is set up at a known position and the
  data from this is compared to rover data to allow
  a correction.
• Precision Surveying (Phase Observable)
  differential technique based on the carrier
  phase, not the codes. Direct radio signal
  measurement results in errors being reduced to
  the mm-cm level.

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• Point Positioning
• Uses recreational level GPS units. Any
  particular point measurement will be within 10-15
  m of a known point. (Under ideal conditions, this
  error range could be worse under less than
  favourable conditions.) This reading will cycle
  around the point over time.
• Accuracy can be improved by averaging, that is
  by taking a minimum of 10 readings for a required
  point over a 10-20 minute period and averaging
  the results to determine the point.
• The results of this process are displayed on the
  following slides.

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Surveying exercise completed for a single point.
Data displayed using MapSource.
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UTM positions can be seen in the data table for
the program. Note how similar most appear in
terms of at least one parameter.
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The same points displayed in DNRGarmin. Note how
different the points are now in terms of the
cartesian coordinates (x y proj.). This is
because the program records the position without
rounding off.
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The previous 10 records of the one position have
been sent to a GIS for display. Here it can be
easily seen how a GPS unit scatters the readings
even though the readout might show an unchanging
Cartesian coordinate!
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DNRGarmin can calculate a mean value, standard
deviation and circular error of probabilities to
enable statistical analysis and determination of
the point (mean).
Click Edit
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The mean value can be used to create a new point
in DNRGarmin. This can be saved to the GIS for
display. Note the mean point positioned at the
cross.
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• The mean (known) value is actually none of the
  measured points but a best fit for the analysed
  records. The scattering of the records is caused
  by inherent errors in the GPS system used namely
• Clock Errors
• Signal Multipath
• Dilution of Precision (SV constellation)
• Atmospheric Factors
• Human Error
• All GPS based measurements need some degree of
  processing to improve base level accuracy,
  professional grade units have this feature built
  in at a significant cost.

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• Other Surveying Techniques
• Recreational GPS is affected by buildings,
  vegetation and fences.
• Surveying to these objects may require more
  traditional measurement techniques.
• Some options include offset points or lines,
  measuring distances and bearings from known
  points.

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• Other Surveying Techniques
• DGPS
• Base units with the software and a subscription
  to a DGPS service are costly (1000s).
• Precision Surveying
• As above except the cost can now be very high
  (10000s).