Reference frames and constellations

The reference frame RTK Premium corrections are delivered in for each region, the satellite systems they support, and the correction stream settings.

Coordinates are only useful in the reference frame your project works in. RTK Premium corrections are delivered in the regional frame listed below, and your field software transforms them into any national or local system your project requires.

Reference frames by region

Region	Frame the corrections are delivered in
Europe	ETRS89, realisation ETRF2000, epoch 2010.0
United States	NAD83(2011), epoch 2010.0
Canada	NAD83(CSRS), epoch 2010.0
Hawaii	NAD83(PA11), epoch 2010.0
Japan	ITRF2008, epoch 2005.0
Australia	GDA2020, epoch 2020.0
Other regions	Contact support for the frame used in your area.

In the United States and Canada, ITRF is available on request.

After setting up a transformation, measure a known control point before relying on the results. If every point is shifted by the same amount, the frame or the transformation is the first thing to check. See Inconsistent RTK accuracy on site.

Why coordinates can be offset

The size of a consistent offset usually points to its cause.

Consistent offset	Likely cause
About 1 to 2 m horizontally in North America	A global frame such as ITRF or WGS84 mixed with NAD83
Several decimetres to about 1 m in Europe or Australia	A global frame at the current epoch mixed with ETRS89 or GDA2020
A few centimetres	A different realisation or epoch of the same frame, or the quality of the control points
Tens of metres vertically	Ellipsoidal heights read as heights above sea level
A steady vertical offset of a few centimetres	The geoid model or its version, or the antenna height

To measure a service's offset against a surveyed point, see Testing a correction service. For heights, see Ellipsoidal and orthometric heights.

Where to set the coordinate system

Set the coordinate system deliberately at each step, and make every step match:

• Rover or controller: the field software applies the projection, the geoid model and any site calibration to the corrected position.

• Drone or mapping app: check which frame the app writes to the image or point metadata. Some apps label every position WGS84, whatever frame the corrections arrive in.

• Office software: set the processing project to the same frame and epoch as the field data and the control points.

Transformations

• Where a national grid file exists, for example OSTN15 in Great Britain or an NTv2 grid in several other countries, use it rather than a seven-parameter transformation. Grid files reach centimetre level; a general seven-parameter transformation can be off by metres for some national systems.

• Apply exactly one transformation between the delivered frame and your project system. Applying a second one, for example in both the controller and the office software, doubles the shift.

Control, cadastral and legal work

For control, cadastral and legal surveys, validate results against the national control network and follow the requirements of the responsible national authority. Measure at least one known control point at the start of every job.

Satellite systems

The corrections support GPS, GLONASS, Galileo, and BeiDou where the local network tracks them.

Correction stream

• Format: RTCM 3.x

• Mountpoint: COMMON

• Update rate: 1 Hz

• Position sending: send a GGA message every 5 seconds or so, so the network can build the correction for where you are. See Mountpoint selection and the GGA heartbeat.

Your receiver's position output rate, for example 5 Hz or 10 Hz, is set on the receiver and does not depend on the correction rate.

FAQ

Which datum are RTK Premium corrections in? It depends on the region; see the table above. In Europe, the corrections are delivered in ETRS89 (ETRF2000, epoch 2010.0). In the United States, they are delivered in NAD83(2011), epoch 2010.0, and in Canada in NAD83(CSRS), epoch 2010.0, with ITRF available on request in both. In Hawaii, they are delivered in NAD83(PA11), epoch 2010.0. In Japan, they are delivered in ITRF2008, epoch 2005.0. In Australia, they are delivered in GDA2020, epoch 2020.0.

My coordinates are about a metre off from my project control. Why? Most often one side of the comparison is in a global frame such as ITRF or WGS84 and the other in a plate-fixed frame such as NAD83 or ETRS89. Put the field data, the control and the processing project in the same frame and epoch.

Can I work in a national grid or local system? Yes. Set up the transformation from the delivered frame to your national or local system in your field software, then check it against a known point.

Which satellite systems do the corrections cover? GPS, GLONASS, Galileo, and BeiDou, where the local network tracks them.

Can I get corrections at a higher rate? The correction stream runs at 1 Hz. Your receiver's position output rate is set on the receiver and does not depend on the correction rate. If your application needs a higher correction rate, contact support.