# Coordinate system

Trimble Access provides a comprehensive database of coordinate systems used around the world. The database is continually updated to reflect changes to different zones. To customize the list of coordinate systems available, see To customize the coordinate system database.

To select the coordinate system settings for the job from the coordinate system database:

1. Tap and select Job. The current job is already selected.
2. Tap Properties.
3. Tap Coord sys.
4. In the Select coordinate system screen, select Select from library. Tap Next.

5. From the lists, select the required System and Zone.

   Drag your finger up the list to scroll, or press the first letter of the country name on the keypad to jump to that section of the list.

   When you have selected the System and Zone, the following read-only fields are updated:

   - Local datum: The local datum for the selected coordinate system and zone.
   - Global reference datum: The datum of RTK measurements, such as the reference frame of base stations including VRS.
   - Global reference epoch: The epoch of realization of the Global reference datum.
   - Displacement model: The displacement model used to propagate RTX coordinates between ITRF 2020 at the epoch of measurement and the global reference frame.

   If you perform an RTK survey in the job, you must make sure the selected real-time correction source is providing GNSS positions in the same datum as that specified in the Global reference datum field.

6. If the job will contain GNSS observations and you want to use a geoid model or datum grid file:

   - If the controller is connected to the internet, enable the Geoid model switch and the Datum grid switch as required. The default geoid model and datum grid or shift grid for the selected coordinate system are automatically selected and are downloaded to the controller when you tap Store in the Select coordinate system screen.

   - To use a different geoid model and datum grid or shift grid from the default selection, or if the controller is not connected to the internet, you must have copied the required files to the Trimble Data / System Files folder on the controller. To select the geoid model or datum grid file:

     1. To select the geoid model, enable the Use geoid model switch. Select the file in the Geoid model field.

     2. To select the datum grid file, enable the Use datum grid switch. Select the file in the Datum grid field.

        The semi‑major axis and flattening values for the selected datum grid file are displayed. These details overwrite any already provided by a specified projection.

     3. To select the shift grid file, select the file in the Shift grid field.

   For more information on using geoid models and datum grids, see Coordinate system parameters.

7. Select the type of Coordinates to use. The default is grid. To use ground coordinates, see To set up a ground coordinate system.
8. Enter the Project height. See Project height.
9. Tap Store.
10. If prompted to confirm download the geoid model and datum grid or shift grid files, tap Yes.

Alternatively, you can define the coordinate system using one of the methods below.

Do not change the coordinate system or calibration after you have staked out points, or computed offset or intersection points. If you do, the previously staked out or computed points will be inconsistent with the new coordinate system and any points computed or staked out after the change.

Scale factor only

Use this projection type when the job will contain observations only from a conventional instrument and you are using a local scale factor to reduce distances to the local coordinate system.

If you are working in a small area and are not sure which coordinate system to use, select the Scale factor only projection and enter a scale factor of 1.000.

1. In the Select coordinate system screen, select Scale factor only.
2. Enter a value in the Scale factor field.
3. Tap Store.

Key in parameters

Use this method to key in your own parameters, particularly if you have your own projection files you want to use or if the job will contain GNSS observations and you want to key in a site calibration adjustment.

1. In the Select coordinate system screen, select Key in parameters. Tap Next.

2. Tap Projection.

   1. Fill out the details for the projection.

      Drag your finger up the list to scroll, or press the first letter of the country name on the keypad to jump to that section of the list.

   2. Select the type of Coordinates to use. The default is grid. To use ground coordinates, see To set up a ground coordinate system.
   3. Enter the Project height. See Project height.
   4. Tap Accept.
3. If the job will contain only observations from a conventional instrument, tap Store.
4. If the job will contain GNSS observations, or a combination of conventional and GNSS observations:

   1. To specify the datum transformation, tap Datum trans.

      To use a datum grid file, select Datum grid in the Type field and select the Datum grid file to use.

      The semi‑major axis and flattening values for the selected datum grid file are displayed. These details overwrite any already provided by a specified projection.

   2. To use a geoid model file, tap Vert. adjustment and select Geoid model and select the Geoid model file.

      The remaining fields in the Horizontal adjustment and Vertical adjustment screens are filled out when you perform a site calibration. See GNSS observations and local coordinate systems and Site calibration.

   3. Tap Store.

No projection / no datum

Use this method if you want to measure points using GNSS observations using a coordinate system with an undefined projection and datum, or if you do not know what the coordinate system settings should be.

1. In the Select coordinate system screen, select No projection/no datum. Tap Next.
2. To use ground coordinates after a site calibration, set the Coordinates field to Ground and enter the average site height in the Project height field. Alternatively, set the Coordinates field to Grid.
3. To calculate a geoid vertical adjustment after a site calibration, select the Use geoid model check box and then select the geoid model file.

- If no datum and no projection are defined, you can only stake out lines and points that have Global coordinates. Displayed bearings and distances are in terms of Global reference datum.
- Without a datum transformation, you can only start a real‑time base survey with a point that has Global coordinates.

When you perform a site calibration, the software calculates a Transverse Mercator projection and a Molodensky three‑parameter datum transformation using the supplied control points. The project height is used to compute a scale factor for the projection so that ground coordinates can be computed at elevation. See Site calibration.

Broadcast RTCM

Use this projection type when the Broadcast format is set to RTCM RTK and the broadcast datum definition messages are broadcast by the VRS network.

1. In the Select coordinate system screen, select Broadcast RTCM.
2. Select the appropriate projection parameters for your location.
3. Select the type of Broadcast RTCM messages to include. See Broadcast RTCM coordinate system messages.
4. Select the type of Coordinates to use. The default is grid. To use ground coordinates, see To set up a ground coordinate system.
5. Enter the Project height. See Project height.
6. Tap Store.

Coordinate system name

The name of the coordinate system indicates whether the coordinate system was selected from the library, was later modified, or whether the coordinate system is user defined.

When the coordinate system is:

- Selected from the library:

  - The Coordinate system field displays "Zonename (SystemName)".

    Changing the geoid model or project height does not change the name of the coordinate system

  - Editing any projection or datum parameters changes the coordinate system name to "Local site". To remove these changes and revert to the original name of the coordinate system, you must reselect it from the library. If you overlay a GNSS site calibration over this “Local site” the name of the coordinate system remains “Local site”.
  - Completing a GNSS site calibration changes the coordinate system name to "Zonename (Site)". If you disable the site calibration (by keying in parameters) the coordinate system name reverts to the original name.
  - Editing any horizontal adjustment or vertical adjustment parameters changes the coordinate system name to "Zonename (Site)". If you remove these changes, the coordinate system name reverts to the original name.
- Defined using Key in parameters, the coordinate system name is "Local site".
- Defined using No projection/no datum, completing a GNSS site calibration changes the coordinate system name to "Local site".