Site transformations: A block shift in thinking

In order to use CORSnet-NSW in concert with local Survey Control Information Management System (SCIMS) marks in New South Wales (NSW), a site transformation (also known as a site calibration or localisation) is required. This site transformation accounts for any differences between the legal coordinate system as realised by SCIMS, and observations in the more homogenous GDA94(2010) realisation of the national datum as provided by CORSnet-NSW through Real Time Kinematic (RTK) or Network RTK (NRTK) services. This paper demonstrates that a simple block shift in Easting, Northing and Height, using AUSGeoid09, is sufficient to transform RTK or NRTK observations onto local SCIMS control for surveys requiring centimetre-level accuracy. At each of seven test areas distributed across eastern NSW, a minimum of 4 control marks (each with 7 observations) and a minimum of 11 test points (each with 10 rounds of observations) have been occupied using both RTK and NRTK. From the NRTK data, multiple unique site transformations are computed for each test area. Comparisons are made between a 7-parameter similarity transformation, a 4-parameter horizontal transformation plus separate height shift and a simple block shift, all with and without applying the AUSGeoid09 model. Compared to the other more complex transformations, the block shift returns similar or better agreement with SCIMS control marks and has a number of additional benefits. By using a block shift, transformation parameters are more intuitive, outliers in control are easier to detect, the site transformation can be computed with a single control mark if necessary, the geometry of the control marks does not affect the transformation results, and any errors in height control or height observations do not map into horizontal results.