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End-user GPS receivers typically show an absolute confidence indication, such as Within 25 Metres, for the calculated position.

I am parsing NMEA data from a GPS unit and wan't to calculate a similar quality of service indication. The NMEA data exposes a Dilution of Precision indication (actually, one for both the horizontal and vertical position, HDOP/VDOP).

How do I go from DOP value(s) to a concrete confidence measurement (say, metres)?

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All the GPS receiver is doing, is multiplying the dilution of precision figure by the resolution of the device.

Of course, GPS positions can still be awry, with a high confidence of the location, if you have a good signal strength from satellites in a non-optimum configuration (sometimes referred as canyon effect)

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