# Choosing the coordinate system

Before starting a survey, it is important to choose a suitable coordinate system. The parameters you must configure depend on whether the job contains observations from a conventional instrument or from a GNSS receiver.

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.

Conventional observations only

If the job will contain observations from only a conventional instrument, you can specify the coordinate system and zone by selecting them from the library or by keying in the parameters. With either method you can use grid or ground coordinates. Grid coordinates are computed at the grid level, which is usually at the ellipsoid level.

Because in a conventional survey measurements are usually made at ground level, you can choose to use ground coordinates and then key in the scale factor or calculate the scale factor the software will use when converting ground observations to grid. To use ground coordinates, see To set up a ground 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.

GNSS observations only

If the job will contain GNSS observations, the coordinate system settings will consist of a projection and a datum transformation. You can specify the map projection and datum transformation by selecting them from the library or by keying in the parameters.

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 shown in the Global reference datum field in the Select coordinate system screen of the job properties.

When you have chosen a coordinate system, search your survey archives for any horizontal and vertical control points in that coordinate system that are in the area to be surveyed. You can use these to calibrate a GNSS survey. Calibration is the process of adjusting projected (grid) coordinates to fit the local control. Slight discrepancies can exist between local control and GNSS‑derived coordinates. These discrepancies can be reduced using minor adjustments. Trimble Access calculates these adjustments when you use the Site calibration function. They are called horizontal and vertical adjustments. See Site calibration.

If you are surveying with VRS and the RTCM broadcast includes coordinate system parameters, you can set the job to use the settings included in the Broadcast RTCM messages.

With any of these methods you can use grid or ground coordinates. Grid coordinates are computed at the grid level, which is usually at the ellipsoid level. Because during a survey measurements are usually made at ground level, you can choose to use ground coordinates and then key in the scale factor or calculate the scale factor the software will use when converting ground observations to grid. To use ground coordinates, see To set up a ground coordinate system.

If you are not sure which coordinate system to use, select the No projection/no datum option.

Combining conventional observations with GNSS observations

If you intend to combine conventional observations with GNSS observations, choose a coordinate system that lets you view GNSS observations as grid points. This means that you must define a projection and a datum transformation.

You can complete the field work for a combined survey without defining a projection and a datum transformation, but you will not be able to view the GNSS observations as grid coordinates.

To combine GNSS measurements with two‑dimensional conventional observations, specify a project height for the job.