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Returning to cache, finding that coordinates are off?


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I just placed my first cache this weekend, and I returned to the area today to do some hiking that I didn't have time to do when I placed the cache. I decided to follow my GPS to the cache and see if it would lead me straight to it, but when I reached the cache, my GPS said I still had 48 feet left to go.

 

Is this a common problem, having the same GPS be that far off when taking readings at the same place, but at different times? The GPS is a Garmin eTrex Legend. When I created the waypoint for the cache on Saturday, the GPS stated that the accuracy was 21 feet, and today it said 30 feet. I had a lock on the same number of satellites both days.

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I just downloaded and plotted out your cache on a TOPO map. You picked a great area to drive GPS units nuts. Looks like nice steep terrain on three sides, a few cliffs or rocky areas, lots of potential for what is called multipath. Thats where the signal gets to the receiver by indirect means. In canyons like that, it's hard to get a good direct line fix on the satellites sometimes, and easy to get reflected signals. One thing you can do that might help in areas like that is rotate your body so that there is nothing between your receiver and the sky in the direction where you have the best view of the sky. Let your body somewhat block the area with the steep terrain. In tough reception areas like that, as others have mentioned it often pays to take several reading at different times, then average them out.

 

Looks like a great spot.

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48 feet is well within the expected margin of error. Your GPS probably had an EPE of around 15-30 feet when you placed it and a similar EPE when you went back. Add one to the other and the potential is that you'd be as much as 60 feet off, so 48 feet is quite normal. I bet if you go back again, there is a good chance it will bring you closer...or further.

 

I disagree with the others here who espouse averaging. I never do it anymore and my coordinates are quite good, so I'm told. I averaged when I first started placing caches. I'd take 20-30 readings, but I found that my coordinates were no better than if I took 1 reading...and sometimes were worse.

 

Save yourself some time. Take one reading and post it. If you get several complaints about the accuracy, go back and take another. But I bet that 90 percent of the time, your coordinates will be fine.

 

"You can't make a man by standing a sheep on his hind legs. But by standing a flock of sheep in that position, you can make a crowd of men" - Max Beerbohm

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The first cache I hid I posted one number upside down in the coords. Thats not even dylexic, which I'm not. I had taken a bunch of readings and still messed up. Human error. It got fixed and my cache was a hit.

 

You'll always remember your first hide. If its too far off the FTF will let you know.

 

And sattelites do move around and I notice that more now with my new etrex legend. I decided not to pay that much attention to the accuracy with ft when it averages 30-33. Close enough. My old garmin said if you locked in 5 Sats it was a go. Sometimes I only got 3 and that was ok too.

 

Course, I havn't found any of these infamous geocaches yet. Just hidden a few (3).

 

~This is the place~

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quote:
Originally posted by Touchstone:

I've had similar problems and the above comments on averaging is good advice. It's also nice that people post thier coordinates when reporting the find, so you can get a feel for different GPSr brands. I always skim the logs to see if anyone has posted different coordinates and jot them down just in case.


 

That's a nice idea, may start doing that on my logs. icon_smile.gif

 

~robert

www.CacheGear.com

 

mdgpsbanner.gif

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quote:
Originally posted by New England n00b:

Do you think it is appropriate (or at least a good idea) to post your accuracy indicated at the time of the reading?


 

that's certainly a good idea, and definitely up to you. if the EPE is more than 20' or so you may want to let the finders know so they can widen their search a bit. use your own discretion as to what that number should be... 20', 13' whatever. doesn't hurt. icon_smile.gif

 

~robert

www.CacheGear.com

 

mdgpsbanner.gif

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I just placed my first two caches and found the same phenomena. My displacement on the return visit was about 30 feet. I had WAAS accuracy both times and had done a 2 minute average on the first recording of coordinates. I was pretty suprised to find an error. I've been working on averaging and decided that I would begin recording and submitting coordinates when I visit other caches. I hope others will do that for me too so I can see what kind of varance is normal. My GPS is a Magellan Map 330. I'll admit the terrain and tree cover don't help. It's all part of the game.

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30 feet? That's right on the money in my book, especially under marginal reception conditions.

As for placing caches, the only time I will take multiple readings is if reception is iffy. In that case I will walk to the cache from 100 feet or so away from 4 different directions, then post the average of those coordinates. But if I have a clear view of the sky and no nearby obstructions I'll let the Vista settle for a few minutes and run with that.

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