RTK accuracy and baseline calculator
RTK receivers are rated as a fixed error plus an error that grows with the distance to the base station, written like 1 cm + 1 ppm. One ppm is 1 mm per kilometer, so 20 km from the base adds about 2 cm.
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The formula
Expected accuracy = fixed error + (ppm × distance to the base station in kilometers) mm.
For a receiver rated at 1 cm + 1 ppm, working 15 km from the base: 10 mm + (1 × 15) mm = 25 mm, or 2.5 cm. Ratings describe good conditions: open sky, a fixed solution, and fresh corrections.
Typical accuracy by distance
For a receiver rated at 1 cm + 1 ppm horizontally and 2 cm + 1 ppm vertically, a common rating for survey-grade receivers:
| Distance to base | Horizontal | Vertical |
|---|---|---|
| 1 km (0.6 mi) | 1.1 cm | 2.1 cm |
| 5 km (3 mi) | 1.5 cm | 2.5 cm |
| 10 km (6 mi) | 2.0 cm | 3.0 cm |
| 20 km (12 mi) | 3.0 cm | 4.0 cm |
| 30 km (19 mi) | 4.0 cm | 5.0 cm |
| 50 km (31 mi) | 6.0 cm | 7.0 cm |
How far is too far?
The National Geodetic Survey's guidelines for single-base real-time positioning recommend keeping the base within 20 km (12 miles) for routine centimeter work, and within 10 km (6 miles) for the most demanding jobs. They also note that many manufacturers state RTK accuracy as 1 cm + 1 ppm horizontally and 2 cm + 1 ppm vertically.
Network RTK services avoid most of the distance error by creating a virtual base station close to you. If you connect to a network mountpoint, the distance to the nearest physical station matters much less. Our guide What are RTK and NTRIP? explains the difference.
Questions
What does "1 cm + 1 ppm" mean?
It is how receiver makers state RTK accuracy: a fixed error of 1 cm, plus 1 part per million of the distance to the base station. One ppm is 1 mm per kilometer, so at 20 km the rating becomes about 3 cm.
How far can I be from an RTK base station?
The National Geodetic Survey's guidelines for single-base real-time positioning recommend staying within 20 km (12 miles) for routine centimeter work, and within 10 km (6 miles) for the most demanding jobs. Farther away, a fixed solution takes longer and is less reliable.
Does network RTK have a distance limit?
Network services (often called VRS) model the errors across many reference stations and create a virtual base near you, so the distance error stays small anywhere inside the network. Use the distance to the nearest real station only when you connect to a single base.
Why is my real accuracy worse than the rating?
Ratings assume open sky, good satellite geometry, and fresh corrections, and they are often stated at 68% or 50% confidence. Trees, buildings, and reflections, an old correction stream, or a float instead of fixed solution all make real results worse. For 95% confidence, roughly double a 68% rating.