General GPS accuracy?

[modicon2]

I just started geocaching and I've got an older Etrex Legend and Venture. We've made a few finds and everytime the cache was ~50-100+ feet away from where the GPS said. Is there something that I might be doing wrong/incorrect on the GPS? Are these units not accurate enough and newer units are?

[photomankc]

I had a friend with an etrex and his often disagreed with my 60CSx a significant amount about when we were right on top of a waypoint. It always seemed more pronounced when we were under tree cover in the forests.

[+StarBrand]

I just started geocaching and I've got an older Etrex Legend and Venture. We've made a few finds and everytime the cache was ~50-100+ feet away from where the GPS said. Is there something that I might be doing wrong/incorrect on the GPS? Are these units not accurate enough and newer units are?

The generally accuracy of most handheld units is in the 15 - 25 foot range under typical conditions. Sometimes better, sometimes a bit worse. When you condider any potential errors you have plus any errors the hider had - 50 foot is not a totally unreasonable error. However - 100 foot is a bit far.

 

Make certain your units are set for WGS-84 datum and decimal minutes as the coordinate format. Turn on WAAS and turn off battery saver mode. Let the unit "settle" down for a moment when you are getting close by not moving for a minute or so.

[+TheRedArmy]

i have an Etrex Legend HCx. yesterday while driving thru the open country, i had it on the dash (same as if i was walking around in an open field out there) the accuracy was reading generally around 10' all the time, once or twice it would drop to 9', but i've never seen it go under that. when i got out to grab a few caches in the bush, it was around 10-12', which i found pretty good.

 

when i have it set to metric (i'm in canada), the best i've seen it is 3m (

3 meters = 9.84251969 feet)

 

@ StarBrand.... by decimal minutes for the coordinates format, do you mean: hddd°mm.mmm'

 

also, where do i find BATTERY SAVE MODE on the legend HCx, i've looked around and don't see a setting for it.

 

thanks in advance.

Edited July 10, 2009 by brucered

[+Only1Bones]

Hey all...I have a Garmin Etrex Venture Cx that I'm having trouble with. I've emailed Garmin a few times but still having trouble with the accuracy of it. Originally, it would not acquire satellites. When I managed to get it to lock in, the accuracy was at about 10 feet. After updating the firmware and redoing the sat. locator, it's now anywhere between 50-75 feet. Garmin said to just run it and see if it will settle down but I'm not having much luck at all.

 

It's kind of hard to go caching when the GPS is so far off with the accuracy. I don't really want to look 150' radius..

 

Has anyone else had this problem and how can I fix it? I asked Garmin if it needs to be sent in to be recalibrated, but they didn't seem to think so.

 

Any ideas?

[4x4van]

Keep in mind that the "accuracy" that the unit reports, as in EPE (Estimated Position Error) or other terms, depending on brand/model, is nothing more than an estimate made by the unit itself based on the satelite arrangements, conditions, etc. It is also dependent on the way the particular unit calculates that "estimate". Different brands and different models will calculate that number differently. I also believe that the software's calculations probably tend to overstate the accuracy a bit, simply to make consumers happier. So the bottom line is, 10' EPE does not mean that the coordinates shown are within 10', just that they probably are within 10'.

Edited July 11, 2009 by 4x4van

[photomankc]

Exactly. People are taking that number like gospel and it isn't. If the manufacturer wants to they can make it say that the EPE is 3' but that won't make it so. My observations were against fixed points and the etrex usually reported significantly larger distances from the known point. I'm not sure what the model etrex was that he was using at the time though. This was about 2 years ago.

[Ken in Regina]

EPE is one thing, but the original poster said that he's always 50 - 100 ft away from the actual cache location when he finds it. That's quite a different issue.

 

He said he has "an older eTrex and Venture". He didn't say which one gives him this problem, the older eTrex or the Venture. And he didn't say if it's in clear open area or under heavy tree cover or in city "canyons" or other poor reception conditions.

 

Are the older eTrex models noted for poor accuracy in general? ... in specific difficult conditions like tree cover, etc.?

 

...ken...

[jmundinger]

EPE is one thing, but the original poster said that he's always 50 - 100 ft away from the actual cache location when he finds it. That's quite a different issue.

 

Correct, except that the estimated probable error also applies to the cache location. If the user is interested in checking the accuracy of his gpsr, he should be comparing the reading against the coordinates for a benchmark, not a geocache.

[modicon2]

Thanks for the replies. I just was reading about how to use a benchmark to test accuracy. I plan to do that ASAP. Both the Venture AND Legend had the same results. Our first find was in mostly clear view of sky and it was ~50ft off. The second find was in woods with a heavier canopy above and they both were about 100ft off. We are still learning, I notice that the GPS's distance to waypoints reading seem to jump around depending if you standing still/walking slow/walking fast. Just tring to figure out if there is a method to using the GPS to get better accuracy.

Thanks again. Really curious to see what the results of the Benchmark test are.

[Ken in Regina]

EPE is one thing, but the original poster said that he's always 50 - 100 ft away from the actual cache location when he finds it. That's quite a different issue.

 

Correct, except that the estimated probable error also applies to the cache location. If the user is interested in checking the accuracy of his gpsr, he should be comparing the reading against the coordinates for a benchmark, not a geocache.

Yes, of course that's the correct answer. Verify the accuracy properly.

 

In this situation a discussion of EPE wasn't particularly relevant without an explanation of how it relates to the problem at hand. It does relate but nobody bothered to explain that.

 

The way it relates is that every device has some inaccuracy. The manufacturer describes it in each device's detailed specs. Each manufacturer does the calculation for EPE differently, sometimes even between models from the same manufacturer. And when the EPE says a specific distance of inaccuracy, say 3 metres, that doesn't just mean it's in a circle with a radius of 3 metres centred on the current physical location, there's also a probability involved. That probability varies among manufacturers, just as the calculation method does. But you can usually be safe assuming the probability is at least 50% that the lat/long coordinates being shown will be within that circle. And nearly 50% that they won't.

 

So if the person who placed the cache is out by 3 metres, EPE, and the person seeking the cache is out by 3 metres, EPE, the person seeking the cache could be out by anywhere from 1.5 metres to 6 metres, depending upon which direction both EPEs were out from the real location.

 

But given the decidedly fictional nature of manufacturers' EPE calculations, I still suggest it's an irrelevant discussion when it comes to issues of accuracy. The only thing that will help anyone understand why they're not closer to a cache than they expected to be is for them to check their GPS against something with a known good accuracy, like a benchmark.

 

[\rant]

 

...ken...