Datum of NMEA output?

Hello helpful people- I’m using a pair of RS2s in RTK mode. The rover is set to deliver NMEA output via serial port or Bluetooth to external devices like GPR or the like. Normal, standard NMEA uses WGS84 as datum for the output lat-lon coordinates.

My question is what, if anything may affect the datum going out of the RS2 rover in the NMEA stream? I know that a rover can be set to use any number of country/region specific coordinate systems and their datums (like NAD83 and variants for N America). However my belief is that data collected using a regional CS is -only- stored in the RS2 itself and that the NMEA output will not use any datum specified above and will instead always be in lat-lon based upon WGS84 datum. This should be the case as most external devices presume incoming lat-lon from a GNSS to be WGS84 based.

In other words I want to be certain that setting a local CS on my rover has no effect on the NMEA coordinates being output. Can someone please clarify/confirm?

The datum will correspond to the base coordinates and elevation, the arp message.

The Rover is just along for the ride.

If you enter WGS84 coordinates at the base, your rover will match.

If you enter Nad83 coordinates at the base,
your rover will match.

It seems like magic

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Yes that is what I thought however that is not the problem, exactly. My external geophysical devices that capture the NMEA stream from the rover assume and expect lat-lon positions to use WGS84 datum, with no way to choose other datums.

So in NAD83 scenario you paint above, the NMEA streamed lat-lon positions from the rover to my device will in fact actually be using NAD83 datum, which will in my part of the world (BC Canada) be about 1.3 m different than lat-lon values based on WGS84. So there is a blunder introduced into the data set stored on my device, because the device’s software assumes WGS84 datum while in fact it is NAD83.

This is evil.

Still no issue, if they are all wgs84 only, just do everything in wgs84. Just remember to have the same point show up twice the points have to be taken at or translated to the same epoch.

As long as its all in the same datum you can use software later to convert to others.

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Hi Brian,

I agree with @PotatoFarmer. As you have your own base, you can set it in any coordinate system and control the coordinate system of the NMEA stream.

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