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Coordinates and accuracy


chiqui74
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I went geocaching today for the first time. I had some trouble with the reception in the woods and for a few different reasons, I had to abort and come home. My GPS receiver was giving me readings all over the place and the accuracy was never better than 25 feet. So, how can peple be sure the coordinates given by the person who hid the cache are the right ones for the location? Is it not possible that their GPS wasn't very accurate and we have the wrong coordinates? Just curious.

 

June

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I went geocaching today for the first time. I had some trouble with the reception in the woods and for a few different reasons, I had to abort and come home. My GPS receiver was giving me readings all over the place and the accuracy was never better than 25 feet. So, how can peple be sure the coordinates given by the person who hid the cache are the right ones for the location? Is it not possible that their GPS wasn't very accurate and we have the wrong coordinates? Just curious.

 

June

 

The first time I went caching I thought I would narrow it down to 0 feet and be standing on top of the cache. My eTrex will give me anything from 3 to 20 feet of accuracy. Sometimes I will get as close as I can then walk away say 20' and see what heading I now get. I do this 2 times and I can get the general area. Time to turn off the GPS and use the eyes.

 

I started only looking for micros just so I could train my eyes to look for small items. Then i went to look for regular caches which stick out like a sore thumb sometimes due to my eyes looking for something small. :laughing:

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I went geocaching today for the first time. I had some trouble with the reception in the woods and for a few different reasons, I had to abort and come home. My GPS receiver was giving me readings all over the place and the accuracy was never better than 25 feet. So, how can peple be sure the coordinates given by the person who hid the cache are the right ones for the location? Is it not possible that their GPS wasn't very accurate and we have the wrong coordinates? Just curious.

 

June

 

At 25 ft you were doing not too badly. The spec for GPS (without WAAS or differential correction) is 9 meters or about 30ft. though typically we all do better than that. With WAAS correction you will get about 3 meters (10ft) or better. Without going into the lengthy explanations, there are lots of threads you can find that have those, there is error in the GPS system (as stated above).

 

When a cache is placed there is a certain amount of error and when you go out to find it there is also a certain amount of error. The amount on any given day depends on lots of factors including the number and orientation of the satellites above you. So, long story short, being off a bit is not a bad thing and 10m (or 33ft) is quite acceptable. You did well.

 

JD

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Yeah, that's pretty common.

 

Also, due to chance, sometimes your GPS will take you right to the cache.

 

Other times, you will be 10, 20, 30 or more feet from it.

 

Tomorrow, your GPS will take you 30 feet from where it took you today.

 

Hey, what would be the fun if the GPS always took you right to the cache ... NONE!

 

So, once you're in the general area, set aside your GPS and use your imagination. Where would YOU hide a cache? Remember, the person who hid the cache didn't just drop it somewhere randomly (usually not). There was probably some unique tree, fence, structure that caught their eye and that's why they hid it there.

 

Pretend someone led you blindfolded into the woods. Upon releasing the blinfold, you are told to search a 40 foot radius for the cache. Got the idea?

 

Happy geocaching.

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As others have mentioned, what you are experiencing is normal. GPS units are only accurate from about 10-30 feet so you can't be certain that the hider has provided perfect coordinates, nor can you be certain that your unit is giving you perfect data.

 

Because of this the cache is usually between 10 and 30 feet from where your GPS is telling you it is and 40, 50+ feet is not uncommon. Its part of the challenge. Where is the fun if you walk right up to it every time?

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I guess the main problem I was having was that my GPS was saying I was at a set of coordinates that were roughly the cache's yet it was saying the cache was 200ft away, then 28ft, then back to the hundreds. I didn't know whether to believe the coordinates or the other information.

 

June

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I guess the main problem I was having was that my GPS was saying I was at a set of coordinates that were roughly the cache's yet it was saying the cache was 200ft away, then 28ft, then back to the hundreds. I didn't know whether to believe the coordinates or the other information.

 

June

Not really unusual either, depending on your reception (number of satellites, constellation which means orientation, whether you are under trees etc). If you watch you satellite screen you will see the number of satellites being received. There are times, especially under trees and other difficult reception areas, when you will see the number of satellites fluctuating rapidly. The number and orientation can greatly affect the accuracy of your GPS position calculations which can change every second (the typical refresh rate).

 

To me its part of the fun of learning about the equipment. After a while you learn how to analyze the accuracy you are receiving and you develop techniques to double check and improve the accuracy. The sport wouldn't be any challenge at all if all you had to do was walk up to the cache. Eventually you'll get better understanding of how to adjust to the reception conditions and that feels like an accomplishment once you've done it.

 

JD

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I guess the main problem I was having was that my GPS was saying I was at a set of coordinates that were roughly the cache's yet it was saying the cache was 200ft away, then 28ft, then back to the hundreds. I didn't know whether to believe the coordinates or the other information.

 

In tree cover you'll get erratic results.

 

If I move from an open trail into tree cover, I've found walking a little slower and stopping very few feet will help. Give the GPS a chance to settle out before moving from your current location. Also, I'd recommend doing a search on <2 difficulty and <2 terrain for your first few caches. Look for caches in a parking lot or something easy (for the GPS to see the sky non-stop) to help learn how to use your GPS.

 

Most people expect the GPS to literally lead them to the cache. As the other posters have mentioned, this isn't usually the case.

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I guess the main problem I was having was that my GPS was saying I was at a set of coordinates that were roughly the cache's yet it was saying the cache was 200ft away, then 28ft, then back to the hundreds. I didn't know whether to believe the coordinates or the other information.

 

June

 

Don't bother trying to match coordinates. That is a futile endeavor. Just use your navigation page and follow the needle and pay attention to the distance. Once you're around 40 feet away start looking.

 

It also sounds like you're getting some signal bounce. This can happen with caches near buildings, cliffs, in ravines, on hillsides and similar places. Sometimes you just have to focus on where the unit is pointing most of the time.

Edited by briansnat
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...It also sounds like you're getting some signal bounce. This can happen with caches near buildings, cliffs, in ravines, on hillsides and similar places. Sometimes you just have to focus on where the unit is pointing most of the time.

 

To add for the benefit of the OP: When you are getting signal bounce and you can't seem to get a fix on the cache location you may need to trianglate. Check out the FAQ in the getting started section. Especally the link to Markwell's FAQ. He's got a good description of how to triangluate.

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One thing I do, Is when I get close, say under 100 meters, I take a bearing w/ my compass then walk it in on the compass rather than the gps. I did my first weekend of hunting - found 13, couldn't find 2. I think on my worst spot, I was 12-15' off the mark and was in a pine thicket.

 

When your readings "jump" like that, go over to your satellite screen, you'll find that you've lost some lock-ons and you'll need to wait to get them back. If you cannot get them back, go somewhere you get a good read, take a bearing and use the compass.

Edited by jcblough
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