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Colorado "Accuracy"


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I decided to upgrade from a Vista HCX to a Colorado, to get the geocaching features mainly.

(Seems rather a backward step to me: much harder to work, and having to disconnect the USB then reboot each time to check if a cache has loaded spring to mind.)

 

However, why does my HCX indicate +/- 10 feet here, but the Colorado shows 18ft? Not =/- 18ft, but 18ft.

 

The Colorado has been on for ages, and has little "D" on most of the satellite bars.

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Not sure why you would see a big difference because they use the same chipset. A couple of things:

 

1) If you are using the CO for the first time it has to build an almanac, let it sit for 20 minutes with a clear view to the sky before doing any type of comparison.

2) WAAS is on on the CO, hence the little D's, trying turning that off to see if it makes things work similar to the HCX (assuming it has WAAS off).

 

GO$Rs

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However, why does my HCX indicate +/- 10 feet here, but the Colorado shows 18ft? Not =/- 18ft, but 18ft.

 

I also wondered why the EPE is represented differently on Colorado than on the Vista (non-H and H): as you say, the Vista shows "+/- xxx" whereas Colorado just says "xxx".

 

I was out with a friend who has the Vista (non-H version) and she was showing, for example, "+/- 10 feet" on her GPS where the Colorado would show "30 feet", etc. I rationalized that her "+/- 10 feet" really meant "20 feet" which was still better than the Colorado 30 feet.

 

I don't know how trustworthy the EPE estimate really is, and if Garmin calculates it differently on different units (apparently so).

 

J

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The EPE and Accuracy are only relative calculations and can not be used critically. You must find an adjusted benchmark to determine your real accuracy. The only thing you can determine with the EPE or Accuracy readings is that you’re getting a better signal in one place than another or you’re getting a better signal facing north than you were facing south etc.

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Thanks for the replies.

 

I guess that my question should really have read:

 

How come my newer, more expensive GPS is indicating a far higher EPE than the old one?

 

There should be no real need for +/- 10 feet, AIUI this error represents a circle radius 10 feet drawn round the indicated position. The actual position lies within the circle. The error is upto 10 feet, as the actual position cannot be further away from the indicated one by more than 10 feet.

 

There can be a 20ft error between marking a point and returning to it however, as the actual position can end up on opposite sides of the circle each time.

 

So the Colorado is claiming less accuracy, in this case, an 18ft radius. If it is a software thing, and it is making the whole thing up, why doesn't it make up a smaller number, to make me feel like it wasn't a waste of money?

 

It has had long enough to build the almanac, and with WAAS enabled, it should be between 10-16 feet anyway.

I've had the HCX indicate 1ft, usually not when anywhere near a cache though!

 

When I plot the tracks on Google Earth they are upto about 18ft out, but how accurate are the GE maps anyway?(Or maybe I really do walk down the middle of rivers)

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The Colo may simply be more accurate in its reporting. I've read in a number of places that the theoretical limit to the accuracy of any consumer GPS is about six meters, even with WAAS. My wife's Vista HCx regularly reports 10 ft accuracy, while my 60 CSx shows 18 ft, and we both end up being about the same distance from the cache.

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In any case, I believe that EPE is actually a probabilistic estimate of the error. It's not an absolute upper limit. As somebody stated in an earlier reply, best to treat it as nothing more than a relative measure of possible error.

 

One other note. If your Colorado shows an EPE of more than 50 feet or so, you might want to power cycle the unit. If the EPE drops (and your position shifts appreciably), you have been victimized by the infamous "drift" error. Higher than normal EPE is one characteristic symptom of this condition.

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Facade66

AIUI this error represents a circle radius 10 feet drawn round the indicated position. The actual position lies within the circle. The error is upto 10 feet, as the actual position cannot be further away from the indicated one by more than 10 feet.

 

Facade66 - look here : http://forums.Groundspeak.com/GC/index.php...=184158&hl=

post from May 14 2008.

 

Christopher

Edited by sokolo0
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First of all, you cannot compare EPE across units. It may well be calculated differently on each unit (although you wouldn't necessarily expect this across units from the same manufacturer).

 

The general concensus seems to be that Garmin normally uses the 50% CEP (circular error probable) definition of EPE. Basically, this means that there is a 50% probability that your actual position is within a circle of EPE radius around the reported position. And a 50% probability that it is not.

 

To obtain a 95% confidence level, you would need to multiply the radius by a factor of approximately 2.

 

So, if your reported EPE is 18 feet, there is a 50% probability that your actual position is within an 18 foot radius of the reported position. There is a 95% probability that your actual position is within a 36 foot radius of the reported position. This is typical performance for low-cost recreational receivers.

 

I only have a vague grasp of the math myself. A fairly comprehensible explanation is available here from the fine folks at gpsinformation.net.

 

Edited to add: No telling whether the info from gpsinformation.net applies to any particular unit. The fact that two Garmin units give appreciably different EPE in similar circumstances suggests that they are using different measures of EPE. But there is no telling for sure. I have never seen anything from Garmin -- manual or support FAQ -- which actually defines EPE on their units.

Edited by twolpert
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