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How good should I get? 17 feet accuracy 'acceptable'?


Geosaur

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What's the generally accepted "good" accuracy rate? I've been getting about 17' (~5 meters). While that's great it puts together a pretty wide error when looking through bushes. Is that about right? The unit is a Garmin eTrex Vista. Just wanting to set my expectations. I thought I've heard as good as 3 meters. Not wooded, minimal hills (I was at the topish of a hill when I got the 17' that's coming to mind).

 

Geosaur :: Geocaching Lizard

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17 feet is not bad at all, very few times have I got down to the 3 meter (10 feet) range. I always figure if it gets around 15 feet in open areas or 30 feet in tree cover is not bad at all. In heavy wood area (with leaves on trees) I sometimes am lucky to get under 50 feet.

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Don't forget that the person placing the cache has an element of error as well. They might be 25 feet off in one direction and your gps is reading 25 feet in the other direction. And some cachers aren't too careful when they mark a cache adding even more error. And don't even get me going about the tree cover! When I find myself withing 10 feet of a cache I figure it was a mistake or dumb luck.

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Yes 17' is a good accuracy reading, but this is just a software program that provides an estimate based on conditions. It is not gospel. Actually, often you will find you actual error will be a fair amount less. A good way of thinking of it is that it is telling you that your actual position will be within 17' of the stated postion 90% of the time. You may in fact be standing right on top of the actual position, but then again maybe it is that other 10% of the time and you are outside of the 17'. It simply gives you an idea of how your GPS is doing at the time.

 

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"See the wonderous works of Providence! The uncertainty of human things!" Geo.Washington

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I use a Legend also. 17' is very acceptable stated accuracy. Generally, I get maybe 15' accuracy. When I am able to lock onto a WAAS satellite, I get 7' accuracy, but these times are few and far between given the antenna the Legend uses. WAAS acquisition for me here in southern AZ seems to occur only on mountain peaks and buttes.

 

Veni, Vidi, Cachi ...

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accuracy

 

What EraSeek says is right on. It is an indication of how well your GPSr is reading the sats. On my Legend, the best I have seen the estimated accuracy is 8'. This was with WAAS on, on a clear spot, near a cliff, by the ocean, with a unobstructed view of the sky from east-south-west, on a bright sunny day.

 

This spot was at a NGS super accurate benchmark, where my true position was less than 6'off. On several tests at several of these benchmarks, I have found the true accuracy to be usually less than the shown "accuracy", often half. This is with open locations, receiving 8 or more sats, with WAAS or not.

 

Two more recent readings:

No WAAS: 15' "accuracy", true position within 3' (10 sats)

No WAAS: 13' "accuracy", true position within 6' (9 sats)

 

Do this a few times and you get a good confidence in your GPSr.

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I've gotten my Vista down to as good as 2m (6') accuracy, but I've had to sit at the top of a hill (or on a boat - anywhere with a nearly unobstructed view of the horizon all around you) for a while to get it. Obviously that's with WAAS enabled. Without WAAS, about 5m (15') is about the best I could do. Even along a beach (so say 180 degree horizon views) I can get 3 or 4 m (9 to 12 feet) accuracy.

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"Accuracy" ? I didn't know there was too many receivers that actually reported "accuracy", some attempt to "Estimate" their "Position" "Error" but that's basically a little different to accuracy.

 

The day the manufacturers actually provide some real information on the background behind these figures then one might be able to comment constructively, but they don't and they won't and they will remain just a (fairy) figure.

 

In some respect the relative trend might mean mean something but even when Selective Availability was terminated just about all manufacturers were caught out as the displayed error was no where in line with the actual improvement in accuracy. When the software is simply changed to reflect an error figure ("accuracy"?) that appears to be more representative of the times, then it really doesn't say much for the integrity of the error value in the first place.

 

Ah almost forgot, considering the world average system spec is less than 13m (~43 feet) @95% SIS then 17' is certainly well within spec.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by Kerry:

"Accuracy" ? I didn't know there was too many receivers that actually reported "accuracy", some attempt to "Estimate" their "Position" "Error" but that's basically a little different to accuracy.

...


Most Garmin GPSr's call it "accuracy", while Magellans (and maybe others?) call it "Estimated Position Error", or "EPE". EPE is obviously a little more scientific-sounding name, but essentially it's the same thing. Either way, your GPSr is "accurate" to within 17 ft or whatever the number is, relative to a fixed point of known positioning.

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quote:
Originally posted by Peregrine007:

Most Garmin GPSr's call it "accuracy", while Magellans (and maybe others?) call it "Estimated Position Error", or "EPE". EPE is obviously a little more scientific-sounding name, but essentially it's the same thing. Either way, your GPSr is "accurate" to within 17 ft or whatever the number is, relative to a fixed point of known positioning.


 

Ok "17 ft relative to a fixed point of known positioning" but in reality there is no fixed point to compare, so is that "accuracy"?

 

Definition of "Positioning Accuracy" , represents how well the position solution conforms to "truth". "Truth" is defined to be any specified user location where the position is known.

 

So how can EPE (as in being "estimated") be the same thing as accuracy, which by definition is a position of "truth", truth isn't an estimate.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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I would guess its estimated error based on several factors, two of which may be strength and quality of signal received. Based on if you DID have a true point, the estimated position error could be up to as much as what is displayed as the error?

 

I mainly just use mine to see whether it stays relatively stable, and whether it is a small number or a large number. Not what it actually is.

For example, if it says 23ft EPE (common for me), I dont expect to see the "point" I am hunting to be about 23ft away from where I'm standing.

 

Instead, I watch it to see if it jumps around, or stays relatively steady (mine usually will hit a min of 13 or 16ft, sometimes 23ft, 33ft, & 65-85ft seem to be most common). Then I see if its staying steady at a low number, which tells me I probably have a decent lock, and its prolly working as well as can be expected, which means I am as close as i'm gonna get.

 

Or if its at the higher end, or jumping around, I figure my lock isnt all that good, and I could be WAY off, which means I have to search a larger area.

 

Another thing I do, I usually use the map screen to get within 250-500Ft of a cache, then I'll switch to the compass screen and let that guide me until it starts giving me fluctuating distances or bearing. If I'm not on top of, or can see the cache within about 10-15ft, then I'll switch to the position screen and move around till i get the coordds to read the same as the coords I am looking for, then I'll put it away and start looking.

 

Usually, it gets me right on top of caches.

 

I had a cache today that I saw even before I stopped to look for it. The compass screen showed I had to keep going past the cache for about 80ft (unusual for my GPS).

i held the GPS above the cache, and the distance came down to about 35ft, with the bearing fluctuating.

When I checked the position screen, the coords were only off by 3ft (.oo1) north, and the same west.

 

Art

 

www.yankeetoys.org

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You should try to be as accurate as possible, especially when placing a cache (including elevation). Take several readings... walk out/into your cache from the four directions...pause...and then take the average reading. Sloppily placed caches really bite. Accuracy is part of the "game." I enjoy it when people comment on my caches (placed or found)and how accurate I was. icon_smile.gif

 

Never underestimate the stupidity of people in large groups.

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Kerry is right, it's not "accuracy", it's an estimation of the error.

Nevertheless, my Garmin unit reports it as "accuracy". I assume they choose that word just because it doesn't sound too technical. "Inaccuracy" would be a better word, in my opinion, but again, that may lead some potential buyers to favor a unit that says it's this and that accurate before one admitting to being inaccurate.

 

Anders

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My sportrak pro usually gets me within 20 feet of the cache -- sometimes right on top of it -- sometimes more than 20 feet away. But generally it's right in the neighborhood. Now all they need is to develop a gps with a laser that points right to it. That'd take the fun out of hunting through the bushes though.

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With my emap I usualy get between 15-22 feet accuracy. my method for finding caches is to get to the area and get the gps to go to as low a distance as i can. it usuay get to about 2 or 3 feet. then i set down my bad and put the gps on it so i wont loose it then i go to my gps info screen and get a reading on accuracy. then i start searcging in circles till i find the cache. it got me to within 5 feet the last 2 caches i did. gotta love a garmin icon_cool.gif

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