# To calibrate point coordinates

1. Enter the grid coordinates of your control points. Key these in, transfer them from your office computer, or measure them using a conventional total station.

   Be careful when naming points that are to be used in a calibration. Before you begin, familiarize yourself with the Database search rules.

2. Place the calibration points around the perimeter of the site. Do not survey outside of the area enclosed by the calibration points, as the calibration is not valid beyond this perimeter.

3. Measure the points with GNSS.

   You can use up to 200 points for a calibration. Trimble strongly recommends that you use a minimum of four 3D points in local grid coordinates (N, E, E) and four observed GNSS points in Global coordinates. This should provide enough redundancy. If you do not specify the coordinate system, the Trimble Access software calculates a Transverse Mercator projection and a three‑parameter datum transformation.

   You can use a combination of 1D, 2D, and 3D local grid coordinates. If no projection and no datum transformation are defined, you must have at least one 2D grid point.

4. Perform an automatic or a manual calibration.

   If all the points have been measured, you do not need to connect the controller to a receiver during a manual calibration.

   Multiple calibrations can be performed in one job. The last calibration performed and applied is used to convert the coordinates of all previously surveyed points in the database.

5. To obtain the current list of points being used in the calibration, select Measure / Site calibration.

## Notes and recommendations

- The set of Global coordinates must be independent of the set of grid coordinates.
- You select the grid coordinates. Select the vertical coordinates (elevation), the horizontal coordinates (northing and easting values), or all of these together.
- The origin of the horizontal adjustment is the first point in the calibration when using one or two calibration point pairs. When there are more than two calibration point pairs, the computed centroid position is used for the origin.
- The origin of the vertical adjustment is the first point in the calibration with an elevation.
- When reviewing a calibration point in the database, notice that the Global values are the measured coordinates. The grid values are derived from these, using the current calibration.

  The original keyed‑in coordinates remain unchanged. (They are stored elsewhere in the database as a point with the Type field showing Keyed in coordinates and the As stored field showing Grid. )

- When you are calibrating a no projection, no datum job, (where ground coordinates are required after calibration) you must define the project height (average site height). When the job is calibrated, the project height is used to compute a ground scale factor for the projection, using the inverse of the ellipsoid correction.
- When you start a Scale Factor Only job and then introduce GNSS data, you must perform a site calibration to relate the GNSS data to the Scale Factor Only point coordinates.

When you select Site Calibration, you must specify whether the Scale Factor Only coordinates in the job represent grid coordinates or ground coordinates. The site calibration computations then set up a grid coordinate system or a ground‑based coordinate system, that best fits the existing data in the job to the GNSS data.