@proficnc as you have raised the issue as a forum thread and I have already posted this reply from a surveying consultant I work with on GitHub, I feel it is worth repeating here.
I think this subject is worth some in depth discussion as RTK is being requested a lot more now.
I’m not sure about the equator but here in Australia 1 second is about 30 metres depending on where you are (could be 25 could be 34 could be 28 – also differs in distance between north-south and east-west from location to location)
In Central Queensland we use MGA94 projection which is a Transverse Mercator (TM) projection of the (Geocentric) GDA94 datum (the geographic one). MGA94 zones are typically represented in metres. GDA94 uses the same spheroid as WGS84 and is ITRF compliant.
So using a seven parameter transformation between geographic coordinates (GDA94) to TM coordinates (MGA94 zone 56) on actual coordinates we get the following
Seven Decimal Places: 151.2943275E, -23.8655136S = 326317.031E, 7359616.631N
Above +0.000 000 1: 151.2943276E, -23.8655137S = 326317.083E, 7359616.631N
As you can see there is little difference in the Northing but about a 5cm diff in the easting when you use 7 decimal places and increment by 0.000 000 1. the differences obtained by 1/10 000 000 varies in the easting and northing values depending on where you are in Australia as longitudes converge at the poles and latitudes run parallel.
Also I’m not sure how the pixhawk/missionplanner handles decimal places (rounding or truncation) plus what is the precision of the EKF filters and does it change the value of the last decimal place (or last two decimal places.) should EKF be operating at 8 or 9 decimal places?
Fortunately, and that’s if the Pixhawk can actually talk to the Septentrio, we won’t be using the coordinates that the pixhawk/missionplanner stores as we will be logging direct to Septentrio using the coordinates of the survey grade L1/L2 GNSS module. That’s if the bloody Pixhawk will behave!
Hope this helps