10.3.6 GPS Surveying Specifications & Recommended Practices

CALIBRATION AND RESULT VALIDATION: THE QUALITY CONTROL ISSUES

GPS surveying is a relatively new and complex technology which, in addition, is changing rapidly. It is therefore not realistic to draw up a strict list of instrumentation and procedures. With many different sets of equipment becoming available and expected refinements and changes to the system, the emphasis should be on validation of, rather than strict specification, of equipment, etc. What can be done to ensure that a GPS survey has in fact been adequately carried out?

• The GPS surveyor can be tested (equipment, expertise, procedures and software) on a test-network (and licensed for various grades of GPS survey?). The issue of "legal traceability" of GPS surveys is being looked at closely in Australia.
  
• Multiple occupations of stations permit checks to be made on station coordinate results on a session-by-session basis. The amount by which the results deviate from each other and the proportion of stations with multiple occupancies can be defined in the S & S.
  
• Ditto for repeat baselines, in which the same pairs of stations are occupied more than once. The amount by which the results deviate from each other and the proportion of multiple occupancies can be defined in the S & S.
  
• Loop miscloses provide reliability validation information only if baselines comprising the loop come from at least two independent observing sessions. The acceptable misclose can be defined in the S & S, but this is not a statistical test!
  
• The relative accuracies derived from the minimally constrained solution should be checked against the standards defined for the class of GPS survey in question.
  
• The baseline results derived initially from the phase data adjustment can be compared with the derived results from the final network (constrained and unconstrained) solution. The acceptable discrepancy can be defined in the S & S.
  
• A study of the measurement residuals after adjustment should be made. Ultimately, if there is sufficient redundancy present, this is the best evidence of the quality of a GPS survey.

Further comments on QC and statistical (and other forms of) testing are made in the next sections.

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© Chris Rizos, SNAP-UNSW, 1999