To set up a ground coordinate system

If you need coordinates to be at ground level instead of projection level (for example, in areas of high elevation), use a ground coordinate system.

When you set up a ground coordinate system in a job, the software applies a ground scale factor to the coordinate system projection definition so that grid distances equal ground distances.

1. Tap and select Jobs.
2. In the Job properties screen, tap Coord Sys.
3. In the Select coordinate system screen:
• Choose the Select from library option to select a coordinate system from the library provided. Tap Next.
• Choose the Key in parameters option to key in the coordinate system parameters. Tap Next and then select Projection.

4. To use ground coordinates with the selected coordinate system, from the Coordinates field, do one of the following:

   • To key in a scale factor, select Ground (Keyed in scale factor). Enter a value in the Ground scale factor field.
   • To let the Trimble Access software calculate the scale factor, select Ground (Calculated scale factor).

5. If you selected Ground (Calculated scale factor), enter the Project location.

   Alternatively, do one of the following:

   • Tap Here to enter the current autonomous position derived by the GNSS receiver. The autonomous position is displayed in terms of WGS‑84.

   • Tap Point and then select a point from in the job or in a linked file to use the coordinates of that position.

     The Point softkey is not available until there are positions in the job. When creating a new job, you must create the job, then link files to the job or measure a new point, and then return to the Job properties and edit the coordinate system settings. The Point softkey is now available.

   The project height is used with 2D points to reduce ground distances in Cogo calculations. For more information, see Project height.

   These fields are used to calculate the ground scale factor. The computed ground scale factor is displayed in the Ground scale factor field. It allows for the projection scale factor at the Project location to ensure that the combined factor (point scale factor multiplied by the sea level factor) at the Project location equals 1.

   The software applies the ground scale factor to the projection.

6. To add offsets to the coordinates to differentiate ground coordinates from unmodified grid coordinates, enter a value in the False northing offset and False easting offset field.

When working with a ground coordinate system the reported ground distance may not be the same as the reported grid distance. The reported ground distance is simply the ellipsoid distance corrected for the average height above the ellipsoid. However the grid distance is computed between the ground coordinates of the points, and is therefore based on a coordinate system that provides a combined scale factor of 1 at the Project Location.