GPS-RTK Hookup Guide
Nate 
NMEA and RTK
Can I Really Use NMEA with a High Precision GPS Receiver?
Yes! Except that NMEA sentences are right on the edge of enough precision. NMEA sentences look something like this:
language:bash
$GNGGA,012911.00,4003.19080,N,10416.95542,W,1,12,0.75,1647.1,M,-21.3,M,,*4F
NMEA outputs coordinates in the ddmm.mmmmm format. So what is the weight of the least significant digit? Said differently, what is the impact of one digit change?
language:bash
104 16.95542
vs
language:bash
104 16.95543
If we know 1 degree of latitude is 111.3km at the equator, we can glean the change of a fraction of a minute:
- 1 degree = 60 minutes
- 1 minute = 1 degree/60 = 111.32km / 60 = 1.855km
- 1 minute = 1855m
- 0.1min = 185.5m
- 0.01min = 18.55m
- 0.001min = 1.855m
- 0.0001min = .1855m = 185.5mm
- 0.00001min = 0.0185m = 18.55mm = 1.855cm
Using the NMEA sentence, the NEO-M8P will only be able to communicate a change of ~1.5cm location change for each digit in the 5th position. This is pretty close to the 2.5cm accuracy of the module. If you want additional precision, you should consider using the UBX protocol which can output up to 8 digits of precision in dd.dddddddd format which will get you down to 1.11mm of precision!