GPS explained: Position accuracy | Accuracy Values by Garmin Receivers
Note: The following was not something I wrote, but found on the web several years ago. The original site is no longer online, so I’m including it here for preservation. I can’t speak to the accuracy of the information, but it seems very accurate based on my extensive GPS experience.
GPS explained: Position accuracy. http://www.kowoma.de/en/gps/accuracy.htm Web. Accessed 22 September 2010.
Accuracy Values by Garmin Receivers
The declaration of the accuracy by Garmin GPS receivers often leads to confusion. What does it mean if the receiver states an accuracy of 4 m? This readout refers to the so-called 50 % CEP (Circular Error Probable). This means that 50 % of all measurements are within a radius of 4 m. On the other hand, 50 % of all measured positions are outside of this radius. Furthermore, 95 % of all measured positions are within a circle of twice this radius and 98.9 % of all positions are within a circle of 2.55 the radius. In the given example, nearly all positions are within circle with a radius of 10 m. The determined position is in the worst case accurate to 10 m.