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Site Transformations

A site transformation transforms global GDA94 coordinates (the coordinates given by GNSS CORS sites) into local coordinates which fit into the local network. The site transformation is done by occupying several monuments outside the projectís area and recording GNSS positions (see figure 1). Local coordinates are imported and used to create a four parameter rotation, scale and translation; producing an unweighted least squares adjusted solution (Henning 2010). This is necessary so that the GNSS results are compatible with results obtained through total stations connecting to local control (C Roberts 2010, pers. comm., 10th April). Figure 1 shows the setup of a site transformation. The red triangle represents a base station while the green triangles are the site transformation stations where the rover measures the global coordinates. A comparison is made between the global and local station coordinates which enables the transformation parameters to be calculated for any mark within the boundaries of the green triangles.

 

                                                                                             

Figure 1: Station setup for a site transformation (Roberts 2010)

For Leica GNSS users there are three types of site transformations: classic 3D, one-step and two-step transformations; the transformation steps for each are shown below. Below is a table highlighting the main differences between these transformations:

Characteristics

Classic 3D

One-Step

Two-Step

Transformation Parameters

7 parameters

4 parameters plus height shift

4 parameters plus height shift

Local Ellipsoid and Projection needed?

Yes

No

Yes

Area limitation

None

10km 2

None

Table 1: Comparison of classic 3D, one-step and two-step transformations

Figure 2: Steps in a classic 3D transformation (Leica Geosystems 2004c)

Figure 3: Steps in a one-step transformation (Leica Geosystems 2004c)

Figure 4: Steps in a two-step transformation (Leica Geosystems 2004)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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