In reach view base mode, you can enter the NAD83 coordinates as either XYZ or decimal degrees (LLH). Whichever is simpler.

I believe the reach will transmit the base coordinates in RTCM as XYZ, but the software will convert between llh and XYZ using the WGS84 ellipsoid (I assume). The difference between the WGS84 ellipsoid and the GRS80 ellipsoid (NAD83) is about 2mm at the poles, but this conversion would generally result in a sub-mm error at user latitudes (meaning using the WGS84 ellipsoid when technically a GRS80 should be used if the coordinates are in NAD83). Note: what this is talking about is the mathematical process of converting NAD83 LAT LON ELEV (LLH) coordinates to Earth Centered, Earth Fixed XYZ coordinates. This is not talking about the process of converting WGS84 measurements taken in 2022 with NAD83(2011) more on that below.

Hereâ€™s why this works: RTK uses double differencing of the Base and Rover satellite observations. This equation returns a difference in X, Y, Z (delta X, delta Y, and Delta Z). So if your base is in NAD83(2011), the delta X, delta Y, delta Z from the observations is added to the base and gives you a new X, Y, Z for the observed point. This is where reach view 3 takes the new XYZ and converts it into your defined projection in your survey setup. But RV3 doesnâ€™t know from the correction RTCM stream what the Datum of the base is in. In this case NAD83(2011) because itâ€™s the datum from data sheet. So your new point is also NAD83(2011). Because the CRS you choose in RV3 expects a certain datum, it reminds you to double check.

For this specific reference frame, NAD83 (2011) all passive marks in the US were updated to their geographic location aligned with WGS84 on 1 Jan 2010. So if you traveled back in time and set your GPS on the passive mark on that date and ran PPP on the observation results, it would line up exactly (within equipment error). But if you returned to the same passive mark now 10+ years in the future and ran PPP, you would get the NAD83(2011) location plus the change in position by tectonic or other seismic movement since that time. This new location would be the Epoch that you measured it, so if now NAD83 2022.0. I believe the data sheets also gives the local velocity of the point so that if you needed to convert between observations taken in different years then you can. Depending on the movement of the tectonic plate you are on this can add up and is more than the error of modern GNSS equipment. Didnâ€™t use to be a problem when GPS was Â± 10m. This is why you cannot directly compare/convert WGS84 to NAD83 without knowing the Epoch that each observation occurred in and the local velocity grid.

The reason I bring all of this up is that it helps in understanding that the conversion happening in the base mode page between XYZ and LLH is just a mathematical ellipsoid transformation and not the same thing as conversion between WGS84 and NAD83 or even two WGS84 observations taken in different years. So you can enter it as either XYZ or LLH. Which is why Emlid states it can be geographic coordinates in any datum (with an assumption that the ellipsoid of your datum is close enough to WGS84 to not make a measurable difference in your final results) and your results will be in the same datum as you put in.