GPS Accuracy

Discussion about software such as GSAK, OziExplorer etc, as well as all things hardware, GPSrs, laptops, PDAs, paperless caching, cables etc
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gmj3191
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GPS Accuracy

Post by gmj3191 » 30 November 11 10:43 pm

Can anyone explain in simple terms (if that's possible) how a GPS receiver can report what it's current accuracy is?
I don't want to start a who has the best GPS discussion, but I'd just like to know how they can report their own current accuracy.
Thanks.

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mtrax
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Re: GPS Accuracy

Post by mtrax » 01 December 11 7:19 am

Sorry I but I'm just going to guess its the number of satellites your gps has got a strong signal from

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Mr Router
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Re: GPS Accuracy

Post by Mr Router » 01 December 11 10:22 am

the longer the gps is on the more data it has processed , thus reducing the inaccuracy, as will be witnessed if you use the averaging function on your gps while placing caches, that is a process in a process of making the gps it's most accurate

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Papa Bear_Left
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Re: GPS Accuracy

Post by Papa Bear_Left » 01 December 11 12:52 pm

The excellent gpsinformation site has this on the topic:

http://gpsinformation.net/main/epenew.txt

In general, it's a guess based on several factors and you should just work on the "lower is better" assumption and get a feel for what it means on your GPSr with experience.

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Re: GPS Accuracy

Post by Kerry » 04 December 11 10:09 am

In simple terms there is no GPS that can report "accuracy" as accuracy is based on knowing the real world coordinates and no GPS has this info.

The "accuracy" or EPE most receivers display are nothing more than an assumption and very heavily tied to propriety algorithms which typically display so called accuracy better than it might be.

Averaging also does not necessarily improve accuracy in regard to any practical averaging times and this is a long misunderstood myth. Averaging will give you a 50/50 chance, 50% chance of being better, 50% chance of being worse.

Number of satellites also is not an indication of accuracy, it is not the number but more so the geometry plus a few other factors
but I'd just like to know how they can report their own current accuracy.
Simple answer, they can't.

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CraigRat
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Re: GPS Accuracy

Post by CraigRat » 04 December 11 11:05 am

It's all about repeatability.

A device will report a better 'accuracy' as it gets more and more similar samples.

Averaging a waypoint will give you good confidence that it's the best set of co-ordinates you can produce at the time, but if there's only 3 birds in view it may not be that accurate over all.

Oh, and heres a good article, in our very own wiki: http://wiki.geocaching.com.au/wiki/Accuracy

In short:
EPE (the “Accuracy” shown on your GPS receiver) is an estimation, rather than a measurement.

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gmj3191
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Re: GPS Accuracy

Post by gmj3191 » 04 December 11 6:40 pm

Thanks for those who supplied comments.
An educated guess is probably the end result, interesting.

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