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Garmin Accuracy?


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I'm a fairly new cacher with some newbie blunders on my c.v. but tackling this problem is beginning to wear on me! I have a Garmin Etrex Vista and live in Anchorage, Alaska and lately I've noticed that not only is my accuracy between 20 and 50 feet, but that as I progress down the trail (not under the cover of trees) it seems to bounce around as I go.

 

One minute I am dead on, the arrow showing me heading directly for it and then it will begin to move. I'll be within fifty feet of said coordinates and then suddenly I'm two hundred feet away?

 

The GPSr has the 'track north' and 'track up' options and I've tried those without really knowing what it did, but to no avail. It's beginning to frustrate me because I'll be out on a hunt for a micro downtown and no matter which way I turn the indications on the GPSr remain skittish.

 

If any veterans out there have some advice I'd appreciate it.

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20 feet is pretty normal for most units. For starters the Vista (like all eTrex's) needs to be held flat, face up to get good reception. If you hang it around your neck, or clip it to your belt, or hold it dangling at your side you will probably get poor reception. You should be carrying it almost like a waiter would carry a tray of beer.

 

It also has to be pretty level for the compass to work well. If it isn't level, you will see the words "hold level" near the bottom of the screen.

 

If your unit is still new, make sure it gets a full almanac of the satellites. Take it to an open field with a good view of the horizon, turn it on and let it sit for about 15 mins. Another thing is to turn your unit on well before you enter the woods so it knows where all the sats are.

 

Another thing, since the Vista uses a magnetic compass, it can be thrown off by places with a lot of iron in the ground. There are several areas where I live that I have to make sure I turn off the compass to get the navigation screen it to work properly.

 

Also, all units will get varying readings because as you walk, you are losing some sats and getting others. Finally, if you are using the device around tall buildings, or in canyons, or near cliffs, or in ravines, you're going to get a little signal bounce.

 

If you have WAAS on, try turning it off. I read somewhere that under certain conditions it can degrade your accuracy. And if you have WAAS off, try turning it on.

Edited by briansnat
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The track north and track up are used on the map screen. I prefer track up, which has me as the little arrow, allways point to the top of the screen. If you used north up, then north would allways be at the top and your arrow would point whichever direction you were heading. Again, this is on the mapscreen.

 

Are you using the compass screen to find the cache? If not, try that as it will probably be alot easier than using the map screen (make sure to turn the compass off). North up and track up have no affect on the compass screen.

 

I know not everyone does this but it works for me when im in an area where my GPSr doesnt want to cooperate. As you near the cache area watch the arrow and the distance. Make note of what you see ahead and at about the distance it shows the cache to be. It will sometimes shift but for the most part it will keep you going in the right direction. If it does move around, say the last 50 feet to the cache, just keep going in the direction it was showing before. Nine times out of ten, the cache will be in the place your GPSr pointed to on the way in.

 

Just a note: it doesnt matter if you are standing perfectly still in a wide open field, the GPSr reading will still bounce around some!

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I had a Garmin Legend that when I was caching would one moment show I was 450 from the cache and the next 230 feet in the oppsite direction. Well I learned 1) that unless you moving the arrow DOES NOT point in the correct direction, that is why now I have the bearing on my pointer page. But most importantly 2) after calling Garmin and talking to Tech support the fact that the distance changed so irradically, and so much a few feet is okay but not 200, is the unit was FAULTY. I was fortunate in I had just gotten the Legend and so I took it back. I dont know if this help but it was my experiance and Garmins advice.

cheers

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I'm bettin' that you could buy any comparable unit and get the same results. without the e-compass, you'd have to back up and walk in again, change directions and approach for over there...you can still do that. Try just standing still with the unit held level for a full minute. I sure like my vista. On Saturday we had 4 Vistas all in a group.

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When you get near your target area, keep the GPSr still and let it 'settle'. When I am near where I think the cache is, I will usually find something to set my Legend on (a fence post, tree stump, etc.) and step away from it. I'll survey the area or something. If there is nothing to set it on, I'll just stand there and hold it for a minute or two.

 

This allows the unit to get a good lock on the sats it can 'see'. This is also a good practice in getting yourself to use your sense and be less dependent upon the gear.

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Fairly new cacher myself and using a Garmin Vista also. The normally accuracy I see out of the unit when moving about is at best usually about 16-25 feet. I've learned once I'm down to that range that I'm right in the vicinity of that cache and time to start looking at, in, under, around, above, behind what ever is within about 30 feet of me as that is usually where cache will be; and the least likely at that.

 

The absolute best accuracy I've had with the Vista has been 7' of accuracy with 11 sats locked, this was with being still in one spot for about 15 minutes and not moving.

 

I think you'll find that in most situations that it's going to put you right at or very near to the cache location even in tree cover.

 

Best advice i can give is this, when you start doing the bounce dance following the gps to where it's telling you is it's time to trust the instincts you were born with.

Edited by snowd
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The absolute best accuracy I've had with the Vista has been 7' of accuracy with 11 sats locked, this was with being still in one spot for about 15 minutes and not moving.

I have a Garmin Geko 301. I have also used a Garmin E-trex. I often see 5 m accuracy and rarely see 3 m accuracy. Don't believe it!!

 

Sure, 5 m accuracy means that I am getting good reception, but the system just can't do that for real. It's a fluke in the signal comparisons between the satellites that fools the accuracy algorythm.

 

I often cache in the viscinity of airports where I get differential reception via the WAAS system. If I am getting 5 m accuracy readings from the satellites, that consistently drops to about 8 or 10 m once the WAAS kicks in. And, the WAAS is surely improving the real accuracy of my GPS. Apparently it is also improving the accuracy of the GPS accuracy calculation.

 

But, most importantly, when my system reports 5 m accuracy and 0.?? m to go, it is very common for me to be able to move 10 or 15 meters away and get another reading that says 5 m accuracy and 1.2 m to go.

 

DON'T THINK THE ACCURACY ESTIMATES ON YOUR GPS ARE REAL!

 

The best they are is a statistical guess that some percentage of the time you are within x m of your goal.

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I live 1hr Souh of West Yellowstone In Idaho at 5300+ ft. elv. I do a lot of Camping, Hunting, Fishing, & 4x4 at elivations from hear to over 8000 ft. My E-Trex Ledgend with over 470 hrs. in one year since I got it. It maintaines 20 to 24 ft accuracy most of the time. And I have actually seen a lot of 8 to 12 ft readings. I have evin seen 6 ft once. I think elivation has a lot to do with it.

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first...WAAS. heres a map showing (generally) the effective coverage area for waas corrections (Solid red line) http://www.nstb.tc.faa.gov/vpl.html Parts of AK are covered.

 

Accuracy (or EPE), as displayed on your gpsr works like this. If it reads 20 feet, it means that your target has a 50% chance of being within a 20 foot radius of your current location. Double that number to 40 feet and you have a 95% chance of being within a 40 foot radius. With waas turned on, I've found that I'm usually much closer than the reported epe.

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The best they are is a statistical guess that some percentage of the time you are within x m of your goal.

The two Sept 18 logs at this cache: GCA7FE hint at us getting to see GPS accuracy calcs in action.

 

The Burrow and I were standing about 8m apart, both showing distance to the cach right near 0m, but with 8m accuracy under the trees.

 

We found the cache pretty much exactly between us, 4m away from each us.

Edited by geoSquid
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Check your satellite availability as well. I've had days where I've gotten to the cache site and the satellite screen only showed seven above the horizon, and only three of those were high enough in the sky to be able to lock on to them. Needless to say, I wasn't going to get very good accuracy then. I came back later that day and presto... eleven satellites available and I was able to get a lock on most of them and had great accuracy.

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I just skimmed this thread so forgive me if someone has already mentioned this. I have a Vista as well and one of the things the manual says you need to do is calibrate the electronic compass every time you change the batteries. Calibration is very easy and only takes about 30 seconds. If I don't recalibrate, I notice that I get more arrow bounce. The calibration doesn't make the reception better but it seems to hold a lock better.

 

In the Etrex family only the Vista and the Summit have the electric compass and it's a wonderful option. You don't have to be moving to get and hold a good position. I wouldn't trade mine for anything.....okay, maybe the 60CS. *smirk*

 

Scoob

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Accuracy (or EPE), as displayed on your gpsr works like this. If it reads 20 feet, it means that your target has a 50% chance of being within a 20 foot radius of your current location. Double that number to 40 feet and you have a 95% chance of being within a 40 foot radius.

Thank you. That is exactly the sort of thing I thought it was, but didn't know the actualy calculation. That needs to be repeated again . . .

 

"If it reads 20 feet, it means that your target has a 50% chance of being within a 20 foot radius of your current location. "

 

That means that half of the time the cache will be outside the stated accuracy range assuming that the person hiding the cache had 100% accuracy on their GPS.

 

So, doing the statistical error calculation, if both you and the person hiding the cache had the same accuracy (say 20'), that means that you actually have a 50% chance of finding the cache within 28' of where your GPS tells you it should be.

 

So, a good rule of thumb be that half the time, you should be able to find the cache you are looking for within 1.5 times the accuracy estimate of your GPS?

 

Interestingly, I think that is probably about what I have found in practice.

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The Puzzler makes a good point, one that I myself have made before. That is, the accuracy (or EPE) reading reported by your GPSr is nothing more than an estimated guess by the unit itself. It is based on the number of satelites, the orientation of those satelites, surrounding conditions, etc. etc., and most importantly, the unit's own software and how it interprets that data. Kinda like a Ford (or Chevy) telling you how good it is. Can you really believe it? No. The only thing you can believe is that if you are seeing a number of 12', your unit is giving you more accurate info than if it was giving you an EPE number of 50'. But are you really within 12 feet of the exact coords? I highly doubt it.

 

When I reach ground zero, the only thing that the EPE reading is good for is to let me know how good of a signal I am getting in relation to at other times. If my EPE is reading 12', I know that my location readings are better than if it was reading 30', but I certainly don't restrict my search to a 12' radius, or alot of caches would go unfound. And of course there's also the hider's relative accuracy to contend with as well.

 

I've had caches where my ST Pro zeroed out right on top of the cache, but it was showing me an EPE of 25 feet. And I've had times where it zeroed out 25 feet away from the cache, with an EPE of 10'. Sometimes mine will tell me to go left 15 feet, and my son's E-Trex will tell him to go right 15', and both units are giving an EPE of 10'. I also noticed that my son's yellow E-Trex almost always "tells" him that it's accuracy is a bit better than what my ST Pro "tells" me, even though the E is not WAAS capable, the ST Pro is, and we are in an area where I usually pick up the WAAS birds. I don't believe the yellow E is more accurate than the ST Pro, but rather that the software in the E is a bit more "optimistic" in it's displayed data than the software in the ST Pro. So I don't for a minute believe the actual number that shows up on the screen, but it is a good indicator of relative accuracy.

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Accuracy (or EPE), as displayed on your gpsr works like this.  If it reads 20 feet, it means that your target has a 50% chance of being within a 20 foot radius of your current location.  Double that number to 40 feet and you have a 95% chance of being within a 40 foot radius.

Thank you. That is exactly the sort of thing I thought it was, but didn't know the actualy calculation. That needs to be repeated again . . .

 

"If it reads 20 feet, it means that your target has a 50% chance of being within a 20 foot radius of your current location. "

 

That means that half of the time the cache will be outside the stated accuracy range assuming that the person hiding the cache had 100% accuracy on their GPS.

 

So, doing the statistical error calculation, if both you and the person hiding the cache had the same accuracy (say 20'), that means that you actually have a 50% chance of finding the cache within 28' of where your GPS tells you it should be.

 

So, a good rule of thumb be that half the time, you should be able to find the cache you are looking for within 1.5 times the accuracy estimate of your GPS?

 

Interestingly, I think that is probably about what I have found in practice.

That's a great way of looking at it. I'll have to remember that, as it pretty much fits what I have found in my practice as well.

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PATIENCE!

 

I keep walking along the trail until I am CERTAIN that I have passed the cache. Also, I look up once in a while. People always ask why I spot caches before others, it's because i am ooking at my surroundings rather than fixating on the GPS needle :D

 

20 -30 accuracy is not bad.

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It's hard to ask for much more than 20' from any brand of GPS receiver. I'm a dedicated Magellan user and find that's about the average accuracy of my unit.

A military GPS has much better accuracy; I've been told that it's within about 5 feet. But getting your hands on one is next to impossible. <_<

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I have not found my GPSr to have any accuracy at all under tree cover. I am not a veteran, but the best advice I have found on the forum is to use a compass. I normally walk down a trail without tree cover watching the location display, and I walk until the display shows that either the lat. or long. is the same as what is shown for the cache. I normally get a change of 1/1000th of a minute each time my left foot hits the ground. When I get a match, I know that I am either straight North, South, East, or West of the cache and out comes the compass. I can follow a compass bearing through the woods much easier than I can follow the arrow on the navigate display on the GPSr. When the navigate display starts flip-flopping among 200 ft. South-west, 80 ft. West, 120 ft. East, I know that I have arrived at the cache site. I put away the GPSr and start looking for the cache. <_< Hasn't failed me yet.

Ray

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