From your receiver's spec sheet for RTK. Many list about 8 mm + 1 ppm horizontal and 15 mm + 1 ppm vertical.
What "8 mm + 1 ppm" means
Receiver makers write RTK accuracy as a fixed part plus a part that grows with distance. "8 mm + 1 ppm" means 8 millimeters (about 0.3 in), plus 1 millimeter for every kilometer to the base. In US units, 1 ppm is about 1/16 inch per mile. So at 10 miles, 8 mm + 1 ppm is about 0.3 in + 0.6 in ≈ 0.9 in.
These are best-case numbers from the maker, with open sky and good signal. Trees, buildings and bad cell service make it worse.
NOAA's distance guideline
NOAA's National Geodetic Survey recommends keeping single-base RTK within about 12 miles (20 km) of the base for good results. Past that, fixes take longer and errors that the base and rover don't share (mostly from the air the signals pass through) grow.
Network RTK (VRS and similar) builds a virtual base near you from many stations, so the distance to one station matters much less.
Source: NGS User Guidelines for Single Base Real Time GNSS Positioning v2.1. More: baseline length and accuracy.
About NOAA CORS
The "nearest NOAA CORS" option uses NOAA's public list of 1,577 US reference stations (file dated Oct 14, 2026). Most CORS don't stream live RTK to the public. Many are also used by state and private networks. To see live correction services at your spot, use nearest base stations or our state pages.
Source: NOAA CORS Network coordinate list (ITRF2020). Distances are measured on the GRS 80 ellipsoid.
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