GPS Accuracy - Is my GPS "off"?

[Guest DahMooser]

My friend has a Garmin Etrex (the yellow one), I have a Garmin Etrex Vista. When we go out geocaching together, our GPS unit readings are about 20-25 feet different. His seems to be closer to the actual location. Both GPS units show a 14-16 foot accuracy reading. Is this just random? Can anyone explain this in a scientific way? I'd like to know if my Vista is "messed up" or if that's simply the way it is. Obviously a third GPS unit would clarify this situation. Please respond, I am very curious and a little concerned!

[Guest Prime Suspect]

No, a third GPS would not clarify the situation. You'd probably just end up with another different reading.

 

Your GPS will try to calculate the position based the satellites it currently has a good fix on. With even a little bit of tree cover, two GPSs only a few feet apart can end up using different satellites. Even the way you hold the GPS can have an effect. Also, your Vista has the ability to use WAAS data, which can greatly improve accuracy. The basic eTrex doesn't support WAAS. However, if the unit only has WAAS data on a few satellites, it will attempt to use only those to determine a location. It's possible that those satellites are in a poor configuration (grouped too closely together). In that case, the little yellow eTrex may give a better reading. If that's the case, you have the ability to turn the WAAS function off, which would put you back on even footing.

 

And don't assume the posted coordinates are right. The hider's GPS was probably a bit off too.

 

If you want to check your accuracy, go hunt a USGS Benchmark.

[Guest DahMooser]

Alright! I will hunt one of those down. What are the numbers of the WAAS Satellites? is one number 35? if so i NEVER get it. Can you provide more detail? You've provided great information, more, more, and more is better!

Thanks for the post!

[Guest Prime Suspect]

AAS reception (I don't remember if the factory default for WAAS is On or Off).

 

[This message has been edited by Prime Suspect (edited 27 November 2001).]

[Guest RLO]

quote:

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Originally posted by DahMooser:

My friend has a Garmin Etrex (the yellow one), I have a Garmin Etrex Vista. When we go out geocaching together, our GPS unit readings are about 20-25 feet different. His seems to be closer to the actual location. Both GPS units show a 14-16 foot accuracy reading. Is this just random? Can anyone explain this in a scientific way? I'd like to know if my Vista is "messed up" or if that's simply the way it is. Obviously a third GPS unit would clarify this situation. Please respond, I am very curious and a little concerned!

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If both your units are reading 14-16 feet accuracy you could easily have readings that vary 28-32 feet difference. You could also have less.

[Guest Kerry]

The following links are to the 2 geo satellites that WAAS corrections are transmitted from. They are live and by clicking on the map your approx location the elevation will show if the Sat can be seen and how high above the horizon. Below Horizon or even low elevations then no go.

 

http://www.lyngsat.com/tracker/inmar3f4.shtml

 

http://www.lyngsat.com/tracker/inmar3f3.shtml

 

As for estimated accuracy that exactly what it is "estimated" and the only real information that might tend to mean something is the relativity between changing values and not the absolute value itself.

 

Most of these values as implemented by different manufacturers are non standard and have no comparative relation to each other (sometimes even between different models, same manufacturer) and the algorithms are propriety.

 

Cheers, Kerry.

 

[This message has been edited by Kerry (edited 27 November 2001).]