What Does Your GPS Units Accuracy Reading Really Mean?

Some interesting info popped up in the New Zealand Recreational GPS Society forums regarding what the accuracy displayed on your GPSr really means. The website kowoma.de explains (emphasis mine):

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.

So the accuracy displayed is that for 50% of the readings. As it stands this seems to work absolutely fine for geocaching with the majority getting very close to GZ with those coordinates.

What I think is more important in having this knowledge is the enlightenment of geocachers regarding the accuracy and abilities of their GPS units. Comments from cachers referring to the accuracy of the coordinates of a cache often seem to disregard all the possible variables (GPS brand, weather, accuracy displayed etc) and sometimes seem quite frustrated with the CO.

Of course some cache coordinates are well out and need adjusting, but on the whole most are pretty good. I would tend to avoid commenting on anything that wasn’t more than 5 – 10m out.


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