Conclusion

The results presented here confirm the reliability of site transformations in achieving consistency between GNSS and ground coordinates. Where a standard, or even slightly compromised, geometry of marks was used in the site transformation the level of agreement between the transformed GNSS data and SCIMS coordinates in the valid area was, in the vast majority of cases, within the noise level of NRTK and therefore provided seamless positioning from the user perspective. When the selection of TPs deviated severely from the optimal configuration, marks within the valid area had errors within the NRTK noise limit, though any move beyond the valid areas resulted in rapid accumulation of errors.

One interesting aspect of the results is the very poor ability of errors at the TPs to indicate the quality of a transformation, when viewed in isolation. The residuals at the TPs are relied upon as a quality indicator by surveyors when they perform site transformations using on-board software in the field. Among the site transformations presented here, residuals alone are a very poor indicator of the quality of the results. To take the most extreme example, when viewed in isolation the linear error for the MCT and SQT, which have maximum values of 10mm and 6mm, give an inverted indication of their true relative quality, as shown in the table below.

Table of Linear Errors

The poor performance of linear error in indicating the quality of a transformation places the onus on surveyors to have an understanding that a certain level of error is expected in site transformations. The results also reinforce the importance of surveyors’ understanding of the spatial behaviour, and expected magnitude, of differences between transformed GNSS and SCIMS coordinates. This understanding is essential is to ensuring that when site transformation are performed numerical errors are not accepted at face value, but are weighed against the surveyor’s judgement and knowledge of the accuracy that can be expected from the technology and methods they are using.

 
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