# Projection

A projection is used to transform local geodetic coordinates into local grid coordinates. GNSS RTK measurements (both single base and VRS) are referenced to the Global reference datum defined for the job.. To work in local grid coordinates during a GNSS survey, you must specify a projection and datum transformation.

You can specify a projection:

- when a job is created and you have to choose a coordinate system (select from a list, or key in)
- during a survey (you calculate values by performing a calibration)
- in the Trimble Business Center software, when the data is transferred.

Enter an appropriate default height value for the software to correctly calculate a sea level correction and then apply it to the grid coordinate.

If a projection and datum transformation are specified, you can reduce any discrepancies between the Global coordinates and the local grid coordinates by performing a site calibration.

Projection grids

Use a projection grid to handle projection types that are not directly supported by the coordinate system routines provided by the Trimble Access software. A projection grid file stores local latitude and longitude values that correspond to regular northing/easting positions. Depending on the direction of the conversion, either projection or local latitude/longitude positions are interpolated from the grid data for points within the grid extents.

Use the Coordinate System Manager to generate the defined projection grid (*.pjg) file. For more information, refer to the Coordinate System Manager Help. Transfer the projection grid file to the controller.

To use the projection grid, in the Projection screen, select Projection grid in the Type field and then select the Projection grid file. If required, select a shift grid.

Shift grids

Initial projection coordinates are projections that are computed using specified projection routines. Some countries use shift grids to apply corrections to these coordinates. The corrections are generally used to fit the initial coordinates to local distortions in the survey framework, and so cannot be modeled by a simple transformation. You can apply a shift grid to any type of projection definition. Coordinate systems that use shift grids include the Netherlands RD zone, and the United Kingdom OS National Grid zones. The OS National Grid zones are treated as a standard Transverse Mercator projection plus shift grid.

Shift grid files are installed to the desktop computer running the Coordinate System Manager utility, which is installed with Trimble Business Center. Shift grid files can be transferred from the desktop computer to the controller using your favorite file transfer method.

To apply a shift grid to a projection definition, in the Projection screen, enable the Use shift grid switch and then select the Shift grid file.

SnakeGrid

SnakeGrid is a coordinate system with minimal scale factor and height distortion, even when projects extend for many hundreds of kilometers.

A job using a SnakeGrid coordinate system must use a custom SnakeGrid parameter file. These files are obtained through a licensing arrangement from the UCL Department of Civil, Environmental and Geomatic Engineering. Each SnakeGrid parameter file is customized for a specific project alignment envelope. Go to snakegrid.org for details.

The SnakeGrid parameter file name must be named SnakeXXXXX.dat and placed in the System Files folder on the device. See Data folders and files.

To choose a SnakeGrid projection, in the Projection screen, select Snakegrid in the Type field and then select the SnakeGrid parameter file.