# How can a GPS receiver know the estimated error?

I would like to plot a circle of possible error on the map in a GPS app.

From http://en.wikipedia.org/wiki/Dilution_of_precision_(GPS), we can see that GDOP is equal to "delta of output location" over "delta of measured data". How can we know the "delta of measured data" in order to calculate "delta of output location"?

If it is not possible, how do the receivers plot the circle of possible error on the map?

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You can't (in general) get this out. You can get it out in a specific case (where you can actually tell what the pseudorange error is - if you know exactly where you are). DOP is a mathematic construct to tell you how bad the error is, not a way to remove the error. Now, can you rephrase the question to tell us what you are really trying to do - exactly what is your situation, what information do you have, and what are you trying to do? – BradHards Feb 21 '13 at 10:39
What I want to know is that, why can a GPS receiver know the estimated error and plot a circle of possible error on the map? From NMEA message, there is no such field, but only DOP. Are they calculating the error from these figures? – Harold Chan Feb 21 '13 at 10:41
You get the estimated error in position by mixing the estimated error in pseudorange and combining that with PDOP. For most GPS, that isn't what happens though - the system will be optimistic, because that makes the users believe in the system. The true error pattern is almost never a circle - it'll be an ellipse in 2D. Please edit the question with all of the details I asked for, don't just follow-up. – BradHards Feb 21 '13 at 10:46