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Accuracy of GPS unit - new to Caching


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Hello, I just started caching with my daughters (10 and 8) last week. We've been out twice. We have found 4 caches of the roughly 10 that we have attempted. For at least 3 of the 4, my coordinates were off by a fair bit for each of them. I was not sure if we were off for the 4th one.

 

We are using a Magellan Explorist 210. I am pretty certain I am proficient enough in the basic operations. As we struggle to find the actual caches I have switched to the coordinates view and am often off by more than I think I should be.

 

One example:

 

N 37° 14.935 W 080° 58.540 Reported cache location

 

N 37° 14.927 W 079° 58.534 Observed on my GPS when we got there.

 

Is this sort of variance to be expected? I've seen the "WAAS" noted on my machine at times, is that something I need to turn on?

 

I understand this is only one example, but I've talked with a neighbor about a cache we could not find and was told it was "easy to find and right there" when they were on the coordinates, and we covered a 20-30' radius around where our GPS told us we needed to be and found nothing.

 

Is this a lack of experience issue or do I need to check into my GPS unit's accuracy? If the latter, how do I do that, do I look for a survey marker and see if my GPS shows an accurate reading there?

 

Thanks for your input!

Edited by squirrels nuts
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Hello, I just started caching with my daughters (10 and 8) last week. We've been out twice. We have found 4 caches of the roughly 10 that we have attempted. For at least 3 of the 4, my coordinates were off by a fair bit for each of them. I was not sure if we were off for the 4th one.

 

20-30' sounds about right for the typical minimum search radius. The GPS unit will tell you how accurately it is reporting coordinates, which gives you a search radius, but then you have to assume that the hider's GPS unit also was operating within such an error radius, so you have to double that figure. It's possible to be quite a bit off, depending on a lot of conditions, but I don't often hear of coordinates being off by more than 100 feet or so.

 

Any large-scale inconsistencies could also be related to the datum your GPS is set to use. Are you using WGS84?

 

I would suggest when you plan to hunt a cache that you look at the gallery of posted photos from previous finder logs, and get an idea of what the area looks like, and possibly what the container looks like. Once you know exactly what (and roughly where) you are looking, it will help you to narrow down the potential hiding places when you get there.

Edited by GreyingJay
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A typical novice mistake is expecting to find the cache near or at where your GPS says it is. You have to remember that your GPS has an error of roughly 15-30 feet and so did the GPS of the person who hid the cache.

 

Because of that its rare that the cache will be right where your unit zeroes out. 15 to 30 feet off is very common and 40, 50 or even more feet away is not unheard of.

 

Many seasoned geocachers put their unit away when it reads 40 feet and they start looking. If they come up empty they may take it out again and see if there are any changes (sometimes you get a signal bounce and your readings will fluctuate, even when standing still).

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By my calculations, your location and the reported were ~17m apart. That's on the outside of GPS accuracy under the best of conditions. Were you under tree cover? That will degrade the quality of the signals and therefore the quality of your position. The trick is the get as close as you can, and then start thinking "where is a likely place?" Look for UPSs (Unusual Piles of Sticks). Look for nooks and crannies. A lot of the time there can be dozens of potential hiding spots that could all give the same coodinates. You kind of have to develop an eye for potential hiding spots.

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One example:

 

N 37° 14.935 W 080° 58.540 Reported cache location

 

N 37° 14.927 W 079° 58.534 Observed on my GPS when we got there.

 

That's over 55 miles apart. How the heck did you find that :P

 

Seriously, assuming your 80 was supposed to be a 79, that is only a bit over 50 feet which is not unusual. Its at the far end of what can be expected though.

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We are under tree cover for most of these ones that have gone unfound. These are some great tips, I appreciate your feedback.

 

I'm new to the sport, as well, and under tree cover I've found that my GPS (an inexpensive Garmin eTrex Legend) can go quite a bit off. Also, standing still/moving slowly seems to confuse it - IOW, if I'm walking at a brisk pace, it seems to operate more accurately than when I slow down and start looking.

 

One thing I've started doing, to reduce frustration when hunting in the woods, is to waymark something which is NOT under the tree cover. Then I use the map view - IOW, good old-fashioned map reading <G>- to get a better idea of where the cache is (supposedly) located.

For example, this weekend I was having a frustrating time searching in underbrush for a micro-cache. I began to be fairly sure my GPS wasn't giving me an accurate locations, so I walked back out of the woods to a nearby footbridge which is clear of tree cover, and marked a waypoint for the bridge.

I then looked at the map view on my GPS, which told me that the cache should located on a line no more than 10-15 feet parellel to the bridge, rather than about 80 feet parallel as the unit was reading under the trees. I put away the GPS, went back into the woods along that parellel line, and sure enough found the cache within a few minutes.

 

You can also use an ordinary compass to help you figure out what direction to go... as well as to find your way out if you get discombobled while searching.

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Yep, triangulation, map reading, all kinds of ways to learn to harness the power of the GPS.

 

If you want some practice, go out into an open field (or wherever you want, really) and waypoint a landmark, like a tree, or boulder. Then walk away a few hundred feet, set a "goto" to that waypoint, and see how close the GPS takes you. Learn to recognize how it starts to behave as you get close. For further enlightenment, go back to that tree another day in different cloud cover or in the rain.

 

Once you learn the limitations of the GPS, you'll be able to focus more on the hide, and not so much on following the little arrow, as others have said.

 

That said, don't always distrust it. I've had days where I followed the arrow until the distance read 0, and I was standing right in front of the cache. Generally I will follow the GPS until I can get it to show me 0, 1, 2m, then start from there and spiral may way outward.

Edited by GreyingJay
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I have been geocaching for a while now and am just getting up the courage to hide my own. Given the accuracy limits of the GPSr, can anyone recommend the best way to mark the most accurate waypoints for the hide so most people can find it.

 

I have tried marking the exact point of the hide, walking fifteen feet in the four directions perpendicular to each other with the hide at the center, adding all the numbers in the last position for latitude and dividing by five to get the average latitude.The same for longitude. Then record the averages as most likely. Any better ideas? :ph34r:

 

I have also tried using two different gpsr's a Garmin Etrec Legend and a Garmin Etrec Summit. They of course disagree even with each other.

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I have been geocaching for a while now and am just getting up the courage to hide my own. Given the accuracy limits of the GPSr, can anyone recommend the best way to mark the most accurate waypoints for the hide so most people can find it.

 

I have tried marking the exact point of the hide, walking fifteen feet in the four directions perpendicular to each other with the hide at the center, adding all the numbers in the last position for latitude and dividing by five to get the average latitude.The same for longitude. Then record the averages as most likely. Any better ideas? :ph34r:

 

I have also tried using two different gpsr's a Garmin Etrec Legend and a Garmin Etrec Summit. They of course disagree even with each other.

 

Go to the cache site, make sure you have a good lock, let your GPS settle for a couple of minutes, then mark the waypoint and you're done. Averaging, unless you have a unit that does it automatically, is a waste of time.

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Go to the cache site, make sure you have a good lock, let your GPS settle for a couple of minutes, then mark the waypoint and you're done. Averaging, unless you have a unit that does it automatically, is a waste of time.

Averaging will improve the expected accuracy of your coords; It's good idea to move away from the cache position, come back to the cache, and check to see that you can reproduce the coordinates.

 

I have noticed people with more experience hiding and finding caches tend to have more reliable coordinates on the caches they hide. That's because they learn how to use their units better and better.

 

As for Brian's insistence that the entire mathematical edifice of statistics is wrong, I don't know what to do about that. He's wrong, again.

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