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What Can Affect The Accuracy Of A Gps?


DubbleD70
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Went looking for a couple of "easy" caches Sunday on the way back from a friend's house. Here's the scenario...

 

It's dusk in Lake Havasu City, AZ... The GPS (Magellan SporTrak Map), in the past, has always (last 10 caches) seemed to lead me to within a foot or to of the cache. This particular evening, I couldn't even get close... The hint for one cache said that "You don't need to cross the fenceline...", but the coords led me about 30ft past the fenceline and the compass needle on the GPS was bouncing all over the place while I was standing still. The other gave me a good hint, but again the coords were showing 30ft beyond what the hint was describing.

 

The only thing alike about the two sights was that there was lots of metal around. One place had tall metal pipe fenceposts at the gate, and the other had a bunch of metal pipes laying around...

 

Any help or words of encouragement would be appreciated...

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<snip> the compass needle on the GPS was bouncing all over the place while I was standing still.

Your sporttrak map does not have a magnetic compass, so for you to get the needle to point to the cache you must be moving. If you are standing still the needle may flip around a lot.

Edited by Stunod
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Went looking for a couple of "easy" caches Sunday on the way back from a friend's house. Here's the scenario...

 

It's dusk in Lake Havasu City, AZ... The GPS (Magellan SporTrak Map), in the past, has always (last 10 caches) seemed to lead me to within a foot or to of the cache. This particular evening, I couldn't even get close... The hint for one cache said that "You don't need to cross the fenceline...", but the coords led me about 30ft past the fenceline and the compass needle on the GPS was bouncing all over the place while I was standing still. The other gave me a good hint, but again the coords were showing 30ft beyond what the hint was describing.

 

The only thing alike about the two sights was that there was lots of metal around. One place had tall metal pipe fenceposts at the gate, and the other had a bunch of metal pipes laying around...

 

Any help or words of encouragement would be appreciated...

Perhaps the hider's coordinates were off.

Are you sure you were on the right side of the fence to begin with?

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I have had similar problem with my Meridian Color. I have been standing at a cache and my GPSr shows it 60 feet away. The two other GPSr's at the cahe were showing 7 and 10 feet off. One GPS was a Garmin the other a Magellan. I check all of my setting against the other magellan and they were set the same. Somedays it is right on and other days they are not even close.

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We usually start looking at 20-30 feet from where we should be according to the GPS. The only things we've found to interfere with the GPS signal are: very large trees (especially conifers), forests with very large trees (in excess of 100 ft. tall), buildings, and hills. If metal were a problem a GPS would go crazy in a car.

 

Some of the caches we've found have been 60 or more feet off from the given coords, but most are within 10 feet or so. We've only walked up to about half a dozen caches using the GPS...I assume this is due to the time of day the coords were taken was similar to the time of day we found the cache (i.e. the sats were in a similar position as to when the original coords were taken, hence good geometry).

 

One other thing we've found is that low battery power can make the GPS less accurate. We use a Garmin Legend, so it may be different for a Magellan.

 

I hope you figure out the problem.

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lots of GPS info

 

All the above posts are correct. Theres a bunch of sources for error. It's a wonder that we find stuff at all. :( the GPS unit gets you very close in ideal situations: little atmospheric interference, good satalite constellations, no nearby obstructions, etc. Most times you're not in these conditions. Thats when you put the receiver down and rely on yourself, start looking in the type of places that you would put it. Don't get frustrated, if the conditions stink, they stink.

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After 5 DNFs in a row a couple of weeks ago, I think I put out my own personal magnetic field that interferes with anything electronic--seems to happen at work as well--someone will run a job all day without any trouble (punching sheet metal), then when I get there, I'll have material bunch up in the machine, my punches will shear off, etc.

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Somedays it is right on and other days they are not even close.

This may well not apply in your case, but ... Make sure that the "map datum" is set to the standard setting used by geocaching.com, which is WGS84 If you have it set to something else, you may find that your navigation is consistently off.

 

Also make sure that your GPS receiver has had several minutes with a clear view of the sky before you use it for navigation especially if you have not used it recently. It uses this time to collect position-estimates for averaging purposes, and to make sure that its "almanac" database is up-to-date.

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just go to there web site and search around for fermwear. When you find it they have step by step directions. Its relitivly easy to do. One prob with it is it resets your gps so you lose your wey points. I did this many mounths ago so I'm trying to rember the inprovments one I noticed a better acuracy and 2 the desplays changed slightly to make them easyer to read.

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People and things do have their own magnetic fields. It's just that they're usually so small that they don't interfere with electronic equipment. Our whole bodies are run by electonics...just on a very extrememly low level. So, maybe some people do have strong magnetic fields around them???? Who knows!?!? Maybe some of us are just lucky in the opposite way as usual?... B)

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Just as a matter of curiousity, does a cell phone affect a GPSr? I know that cell sites, the towers with those odd looking antennae, can have an effect. But does a cell phone itself do anything, or is it too low powered?

I don't know about those cell phones but I'd like to know more regarding the effects of the cell towers on gps signals. Could you please direct me to the source of that information? Thank you.

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Hello,

I am wondering as I have been searching for 2 days now. (for this answer)

When entering cords on GPSr, do I just do North XX YY. ZZZ and then West XX YY.ZZZ

OR, should I leave out ZZZ ?

 

Let me know as I have scoured the forums for this answer-I am using a Garmin V.

Heading now to look for Firmware updates and also bought the newest map version- from Vs. 4 to Vs. 6.

 

We went on the 27th for the first time (caching) and found 3 of 6. Our fist search was boucing us all over. Got it, but we were off by 100 feet from our initial walk into the area. We actually found it when we were walking away after saying- "time to look elsewhere", Our GPS somehow got itself righted and got us w/i 5 feet. Help...

 

I am new to this and I appologise for going off track here on my questions.

 

Thanks.

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Some more info:

 

http://www.safnet.org/archive/702_howtogps.cfm

 

Other things to consider...

 

Were you wearing a daypack? If so, how many metal things were in/on the pack? Water affects the signal. Your body is a big bag of water, and if you have a hydration pack on your back, then....

 

Clothing generally does not affect signal to any significant degree, unless there are many metal zippers or have stuff in the pockets. I have heard unconfirmed anecdotal reports that gore-tex lined clothing may reduce the signal. Doesn't seem likely, but I cannot completely ignore the possibility. In the case you were headed to a retro-70's disco party, shiny metallic clothing can do it. Or if you are a diehard Black Sabbath or Metallica fan and have more rings and studs in your head than in your car, then maybe. In these cases, simply shifting your body to get out of the way could improve things.

 

Also -- was it an electric fence?

 

Any one of the factors mentioned in any of the posts may not be enough to materialy affect the accuracy (30 feet is not bad anyway), but you could have a cumulative affect.

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Hello,

I am wondering as I have been searching for 2 days now. (for this answer)

When entering cords on GPSr, do I just do North XX YY. ZZZ and then West XX YY.ZZZ

OR, should I leave out ZZZ ?

 

. . .

 

I am new to this and I appologise for going off track here on my questions.

 

Thanks.

I'm new to this also, but a while back I asked what each digit in the coordinates represented and learned the last one is approximately six feet (particulaly speaking of latitude). The next one is approximately 60 feet and the third one is approximately 600 feet.

 

So, yes, you should include all the .ZZZ numbers.

 

Last week I finally completed a three-stage multi I had been having difficulty with. Since I couldn't find the second stage, I went back to the beginning and checked my math. Sure enough, I was had made a mistake, but it only made my distance off by 12 feet. :laughing:

 

Therefore, I found myself back in exactly the same place looking for the "missing" altoids tin. :rolleyes:

 

If I had made the mistake in the second or third column, I would have been looking in an very different area.

Edited by idiosyncratic
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When entering cords on GPSr, do I just do North XX YY. ZZZ and then West XX YY.ZZZ

OR, should I leave out ZZZ ?

 

. . .

 

.

 

.

 

So, yes, you should include all the .ZZZ numbers.

Thanks for the straight answer and fast reply.

We are going to Florida (Disney) next week and "might" seek some cashe down in the park areas ? :rolleyes::laughing:

 

Glad to have this confirmation on inputting correct information.

 

Thanks again.

 

D

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I've had the needle spin when I'm at the cache. I never expect it to point right at the box when I'm at the coordinates. Since the GPSr is supposed to be accurate to 3 meters, that means I should be able to walk around a bit (couple of steps) without the coordinates changing.

 

I always interpreted the needle spinning as meaning I've arrived in the area. A long time ago geocachers called this "loose bearings", not sure if it was the typical misspelling of "lose" as in "I lose my bearings". Regardless, what direction should it point once you are there? I found switching to the coordinate screen at this point is much more helpful.

 

I've also experienced multipath more often than I would have liked, and strange behavior with weak batteries too.

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I went back to both of the caches in the thread starting post with fresh batteries and walked up on both of the dang things right away... I think it was a combination of the weak battery syndrome, coupled with the fact that the second cache was hidden underneath about 4 or 5 piled up old 8 or 10 inch water pipes, ranging in length between 15-20 feet and about a half inch thick metal...

 

The gate at the first one was also made of thick metal pipes that had relatively tall posts on either side.

 

Thanks for all the replies and pointers... :rolleyes:

 

Edit: Dang, is that a massive runon sentence?!! :)

Edited by BRUZRS_Daddy & Sugar-pie
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Edit: Dang, is that a massive runon sentence?!! :rolleyes:

You're English teacher is either:

 

A) Cursing your bad grammar.

:) Spinning in his/her grave.

C) Out caching anyways, he/she isn't read this.

Oh man, I hope you're joking with all your grammar mistakes.

 

On Topic:

 

The original post said that the "problem" reading had him off by less than the normal accuracy anyway. If your GPSr says you're within 30 feet of the cache when you finally find it, you're having a good day.

 

I hate logs that suggest the coords to my caches are off when they say things like, "Found it finally, but the coords are off by 20 feet".

 

It just makes me laugh.

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When I started this game/hobby/sport/obsession, I just went out with my GPSr and no instructions.

 

I could have been one of those people who put that in your log. :)

 

:D I didn't know the GPSr wasn't supposed to tell me exactly where the cache was . . . :o:):rolleyes:

 

It was a few weeks later when I read in some threads in the "Getting Started" Forum about the 30-foot distance and the arrow spinning around. Finding caches was a lot less frustrating after learning those basics . . . :D

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Another possible problem is the brand of batteries. We have a Garmin Etrex Vista, and it will not run on Energizer batteries. Don't know why, but everytime we have put Energizers in it, it locks up or works but tells us we're in China!?? :)

 

It seems to prefer Duracells, and has worked without a problem when we use them. We haven't been brave enough to try any other brands at this point. :rolleyes:

Edited by Apple Dumpling Gangg
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