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How does my gps calculate error estimate?

clutch400t 0 Points
edited November -1 in GPS Discussions
I've had a Garmin Colorado 400t for a few days now, my previous GPS was a Magellan Trailblazer XL (circa 94) or so. Big change in capabilities.

Anyway, when I go to the satellite page, it gives an error estimate. With WAAS turned on today in a swamp it was moving from 16 to 36 feet. How does it know that?




  • Tim 1500 Points
    The short answer is that the error estimate is a guess. After all if it actually knew how far it was off, why not just tell us the correct location? :)

    Estimated accuracy is calculated by looking at the signal strength of the satellites it is connected to, the number of satellites it is connected to, as well as the geometry of the satellites it is connected to. For the geometry part, the more widely distributed (far apart) the satellites are the better the accuracy... if you are reading from a bunch of satellites in a line or close together it won't be as accurate.

    Then, it will basically just throw out a guess/number based on that data. Each manufacturer "tunes" that number a bit different. Some are fine if the "estimated accuracy" is "correct" 50% of the time-- others want the estimated accuracy to be correct 95% of the time.
  • Boyd 2028 Points
    Interesting white paper on the National Park Service's website here:

    Based on this, sounds like you would have done better with WAAS disabled. These comments were based on the GPSMap 60 and 76 series, might be different on the Colorado?
    If you are using a WAAS (Wide Area Augmentation System) enabled receiver in an environment where you are getting intermittent signal from the WAAS satellite then the WAAS option should be disabled. Examples would be a forested, urban or canyon environment. If the WAAS option is left enabled in this type of operating environments then you very likely will get incomplete differential information from the satellite resulting in a position that has a much worse accuracy than an autonomous GPS position (no differential corrections). This is because the Garmin receivers will produce a differential position first even if it is a 2D position before it will give you a lower DOP (Dilution of Precision) position.
    The estimated positional error (EPE) is really HPE (horizontal positional error). On the new Garmins (GPSMap 76, 76s’s and V’s, etc.) the EPE is approximately at a 95% (or 2 sigma) confidence level when you are not using differential corrections (WAAS or NDGPS) this drops to a 68% (or 1 sigma) confidence level when you are using differential corrections.
  • gatorguy 328 Points
    Boyd, thanks alot for that post. Up till now the only reference I could find to Garmin's profile for estimated accuracy indicated they were still using CEP. The fact they are using 2 Sigma should give users a much higher degree of confidence in their position.
  • Thanks a bunch for that link Boyd. :D

    I'll play with it some more with waas turned off. While poking around I found that there is a waypoint averaging wherigo app for my unit. I'll play with that also.

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