# 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. This means that the distance between the coordinates of two points equals the distance measured on the ground between those two points.

1. Tap and select Job.
2. Tap Properties.
3. In the Job properties screen, tap Coord Sys.
4. 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.

5. 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.

     The number entered should be the inverse of the combined scale factor for the location of the job.

     NGS datasheets in the United States report the “Combined factor” for control points. The Ground scale factor in Trimble Access is the inverse of the “Combined factor” number on the datasheet. So:

     Ground scale factor = 1 / Combined factor;

     Where: Combined factor = Elevation factor x Projection scale factor

   - To let the Trimble Access software calculate the scale factor, select Ground (Calculated scale factor).

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

   If you have not yet fully defined the underlying coordinate system, including any geoid models, datum grids, return to step 4 above and do that before continuing, as those parameters may affect the calculation of the ground scale factor.

   Set the Entry method to:

   - Local coords to enter the Local LLH coordinates of the project location. This is the recommended entry method.

   - Grid coords to enter the underlying projection grid coordinates for the project location. These are the grid coordinates on the projection of the currently selected coordinate system, which are not always the ground coordinates of 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 the Global reference datum.

     This should only be used if the Global coordinates are close to the Local LLH coordinates, particularly the Height.

   - Tap Point and then select a point from in the job or in a linked file to use the coordinates of that position. The Local LLH of the point chosen is used, transformed to or from grid as required, using the currently stored coordinate system in the current job.

     The Point softkey is not available until there are positions in the job. When creating a new job, you must create the job, finish selecting the underlying coordinate system for 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.

     Points keyed into the job as offset ground coordinates before the offsets are set up should not be selected using the Point softkey and used as the project location. Instead, you should select points stored as Local LLH.

   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.

   The computed ground scale factor is the inverse of the combined scale factor. The combined scale factor is the elevation factor computed at the Height of the Project location, multiplied by the projection point scale factor computed at the Project location using the projection of the currently selected coordinate system. The resulting combined scale factor, after the application of the computed ground scale factor at the Project location, equals 1.

   The software applies the ground scale factor to the projection.

7. Enter the details of the ground coordinates for the Project location into the fields in the Project location ground coordinates group box. Ground coordinates are often differentiated from the underlying projection grid coordinates to avoid confusion between the two.

   - To specify the ground coordinates of the Project location, enter the ground northing into the Northing field and the ground easting into the Easting field. When values are entered in the Northing and Easting fields, the offsets from the underlying projection grid coordinates are computed and shown in the Northing offset and Easting offset fields.

   - Alternatively, to add known offsets to the underlying grid coordinates to differentiate the ground coordinates from those grid coordinates, enter a value in the Northing offset and Easting offset fields. The Northing and Easting ground coordinates are computed.

   In a job with a ground scale factor, keyed in Grid coordinates are treated as ground coordinates in terms of the ground coordinate offsets. Points keyed into the job as Grid coordinates before the offsets are stored in the job are treated as being in terms of those offsets after the ground coordinate system is applied to the job. The Grid coordinate values of those points do not change.

8. Tap Accept.

- When working with a ground coordinate system the reported ground distance may not be exactly the same as the reported grid distance between ground coordinates. 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.

- When the ground coordinate system scale factor is being computed based on the Project location, any changes to the Project location will change the ground scale factor, and this in turn will mean that any GNSS site calibration based on this will need to be recomputed.

- Trimble Access does not transform ground coordinates to grid coordinates and vice-versa if the coordinate system setup is changed from ground to grid (or vice-versa). If the coordinate system, including the ground coordinate setup, is changed, Grid coordinates keyed into the job remain the same numerical values for Northing, Easting, and Elevation as were keyed in.