Accuracy of Garmin GPSMAP receivers

Discussion related to the Garmin GPSMAP 66s/st GPSr
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Spiney
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Joined: Thu Apr 04, 2019 9:35 am

Accuracy of Garmin GPSMAP receivers

Unread post by Spiney »

With the 65 and 66sr multi-GNSS, multiband receivers now available, many people are interested in what additional positional accuracy this technology brings. I realised I didn’t know what statistical measure Garmin uses to determine the accuracy of the horizontal position fix as displayed on my GPSMAP 66st satellite screen i.e. what does the figure actually mean?

The latest 2020 GPSMAP66st User Manual Appendix page 17 states: “Accuracy of GPS: The margin of error for your exact location. For example, your GPS location is accurate to within +/- 3.65 m (12 ft). This is also the statement in the GPSrChive Data Field table, and is unchanged from the old 62 and 64 User Manuals. Does this +/- mean that the number is the 50 % CEP (Circular Error Probable)?

Today Garmin Support states: “Garmin GPS receivers are accurate to within 15 meters (49 feet) 95% of the time. Generally, users will see accuracy within 5 to 10 meters (16 to 33 feet) under normal conditions.” Which statistical measure of the 95% Confidence Interval is Garmin using? See Frank Diggelen’s interesting article on the accuracy of GPS devices and its measurement.

In 2014 GPS Review stated: “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.”

Back in 2009 GPS Review stated that a report from the US National Parks Service says “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.”

So back to my question: what does the horizontal positional accuracy figure actually mean?

In googling this question I came across an interesting program called DNRGPS published by the Minnesota Department of Natural Resources which can readily calculate the CEP from a GPX track. The CEP can be estimated from a track either located at a known surveyed location or if unknown, will do it by averaging the X and Y position from the coordinates.
Spiney
Posts: 65
Joined: Thu Apr 04, 2019 9:35 am

Re: Accuracy of Garmin GPSMAP receivers

Unread post by Spiney »

After eight weeks, I finally received this response from Garmin UK Product Support:

In regards to your enquiry:

We typically quote the 2-sigma (95th percentile) accuracy value.
So when we say your accuracy is 3.65m, we are saying that 95 percent of the time, you would be within 3.65m of the true location.
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GPSrChive
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Re: Accuracy of Garmin GPSMAP receivers

Unread post by GPSrChive »

Better late than never!

Thank you for the update!
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GPSrChive
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Re: Accuracy of Garmin GPSMAP receivers

Unread post by GPSrChive »

New: GPS Accuracy page now available at GPSrChive.com!
Spiney
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Joined: Thu Apr 04, 2019 9:35 am

Re: Accuracy of Garmin GPSMAP receivers

Unread post by Spiney »

An interesting new page, thanks for sharing.

Spiney
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