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15 Foot Radius??


D & K

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I see that if I purchase an entry level GPS such as a Garmin etrex or a Magellan 315 that the accuracy will be about 3 meters or so (15'ish)

 

If the person who placed the cache was using a similar GPS then this could compound the problem. Is this ever a problem when searching for a cache?

 

dik

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When someone places a cache, (hopefully) they walk around the area to get several readings and average them. This *should* provide a good average coordinate to start with.

When a person searches for a cache, the GPS doesn't point to exactly where the cache is, so people start at that spot and search outward.

Between the two, hopefully most GPS error is avoided.

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Yes, theoretically. But I have more problems with tree cover making my GPSr jump me around the area more than anything. "It's over here! No, wait... It's over there! No! Back over here I tell ya!".

 

I've had some cache coordinates to be just about right on target. Some others have been a good 50'-75' off. Then there's those times when you find miraculously fine cache given that the GPSr is still claiming it's 70' feet away. You put down the unit, fill out the logbook, do the trade, etc. The next time you look at the unit 10 minutes later, it now says your only 3' away. Doh!

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My experience has been that most caches show within 20' of the coordinates given when I find it and I'm holding hte GPS over the cache spot.

 

Every now and then the coordinates are off more than that but by and large it has never been a problem. People are more likely to slip a digit when inputing the cordinates than the GPS will lead you asray.

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I see that if I purchase an entry level GPS such as a Garmin etrex or a Magellan 315 that the accuracy will be about 3 meters or so (15'ish)

 

If the person who placed the cache was using a similar GPS then this could compound the problem. Is this ever a problem when searching for a cache?

 

dik

If you'll look at the specs, you see that there's generally little or no difference in accuracy between the "entry level" models and the higher end consumer models. The cost difference is in the bells and whistles - the ability to upload maps, built-in compass and altimeter, more memory, etc. The only exception might be whether or not it supports WAAS. But a good many people believe that for the type of use that geocaching often requires, having WAAS turned on can actually cause worse readings than with it off. That's been hashed out many times in the GPS Units forums, so there's no need to get that argument started again.

 

The basic point is, just because you're spending less doesn't mean the unit is less accurate.

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"The basic point is, just because you're spending less doesn't mean the unit is less accurate."

 

Once again, Prime Suspect offers sound advice. I spent a lot more on my GPSr than I needed to - I rarely have need of all the "bells and whistles" that I paid for.

Edited by gallahad
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  My Garmin eTrex displays, on it's “Sky View” page, a claimed accuracy.  Once it gets a good lock, this is usually a number in the high teens or low twenties, of feet.  However, I have had experiences which make it clear that the readings are not always withinthe stated degree of accuracy.

 

  For example, on one occasion, as I left my car in a large parking lot, I marked a waypoint to help me find my way back to my car.  At that time, my eTrex was claiming an accuracy of 13 feet.

 

  Later, I returned to my car, with my eTrex still claiming an accuracy of 13 feet.  However, standing in the exact same place where I was when I set the waypoint, my eTrex was now claiming that I was about sixty feet away from it.

 

  What I suspect is that this claimed accuracy is not an absolute limit on how much error may be present, but rather a statistical thing, that perhaps it calculates somewhere that there is an n-percent chance that the error is less than the number which it displays.

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It can be a problem or not - there's no way of knowing in advance. The potential problem is in the accuracy of GPS technology, which is not controlled by the receiver manufacturers. The specs say that accuracy is within about 15 meters or 49 feet. But that's at what is called a 95% confidence level. That means 5% of the time, the readings can be way off. I've had times when the position would jump by a hundred feet one way then another. Also, remember that overhead folliage or large boulders or buildings can throw off readings because the GPS signals get reflected and confuse the readings. However, most cache placements seem more or less on target and it's not worth worrying about, since there's nothing you can do about it.

 

Hmm, maybe it's time to go do another cache...

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I'm relatively new to GC; my expeirence agrees with these posts that you'll get within 20-30 ft. of the cache. I follow the compass arrow and distance until I reach the waypoint, then usually end up circling around a bit, with my GPS giving me several locations in the same general radius that average around a 'most likely' spot. Then I peel my eyes and try to think like sneaky cache-hider.... :blink:

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accuracy.jpg

 

At Sax Man's campout we were all given a waypoint and told to place a flag where our GPS thought the waypoint was at. The vast majority were all clustered around where the little girl is walking. If you had sat on the ground with a walking stick you could have knocked most of them over with the stick. Except the one on the left...

 

At the next event I want to try it again, giving everyone two flags, one for WAAS and for for WAAS off. I'm also thinking about color coding them by brand and antenna type.

Edited by bons
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