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PostPosted: Thu Jul 09, 2009 4:32 pm 
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The new Marine GPS units streamline their use. Unfortunately, some very useful features of the old, simple GPS 76 and the like are now gone. Does anyone know how to determine the number of Satellites and accuracy (EPE?) from these new models? Bob


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PostPosted: Thu Jul 09, 2009 6:40 pm 
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The EPE was always a con job and did not relate to the actual accuracy anyway, there was no way it could as it didn't know where it was in the first place.

Maybe some manufacturers are removing things that don't and never did serve much purpose?


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PostPosted: Fri Jul 10, 2009 6:23 pm 
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It seems like they could estimate your best possible accurcy based on the number of satellites the unit is able to see. I guess it may only be important if you have just 3-4.

However, I need this for a Govt. report of marine water depths. Here is how you can find it, thanks to Garmin rep:
Press Home key
Choose Configure and press Select
Choose System and press Select
Choose GPS and press Select
Choose Skyview and press Select
Can then see the number of Satellites and EPE.
Bob


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PostPosted: Sat Jul 11, 2009 3:34 am 
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Simply the number of satellites is not an indication of accuracy. More satellites does not necessarily mean better accuracy.

The EPE is based on more than number of satellites but still has no way of any direct comparison to accuracy. It can possibly indicate if accuracy improves or degrades but this is only in a relative sense.

EPE does not reflect anything to do with vertical accuracy anyway.


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PostPosted: Sat Jul 11, 2009 1:10 pm 
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It's been speculated that Garmin uses an accuracy reporting method referred to as CEP, Circular Error Probable. I've always multiplied the Garmin reported figure by 2.8 and taken that as pretty accurate as for how close I might be to where my Garmin thinks I am. For example, if your Garmin is reporting 10 ft, then going by a circle of 28' around you (with you at the center) will be 95% certain.


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PostPosted: Fri Jul 17, 2009 4:42 am 
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Many manufacturers use CEP as quoting accuracy basically because the accuracy "sounds" better but in fact what they are doing is deceiving the user who does not understand the difference between the many different ways of quoting accuracy.

EPE is a little different and in Garmin's case they claim the formula/method as propriety but there really isn't a lot of measures that aren't public knowledge.


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PostPosted: Sat Jul 25, 2009 11:22 pm 
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Think this may clear some things up. Courtesy of another forum member here, Boyd.

From the National Park Services website:

http://mms.nps.gov/gis/gps/gps4gis/how2 ... S-WASO.doc

Quote:
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.


Quote:
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.


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