Ground coordinate systems

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. In the Coordinate system definition dialog, select the Parameters tab and then select the Local site tab.
  2. In the Type field, select one of the following:
    • Ground (Keyed in scale factor) and then enter a value in the Ground scale factor field.

    • Ground (Calculated scale factor) to let the software calculate the 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.

  3. 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.