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Accuracy of my GPS


carolmil
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I'm new to the world of geocaching. We've had an okay success rate so far but thats mainly to my hubby routing around every tree or stump within a 30 ft radius.

 

I have an e trex venture hc but for some reason it doesnt seem to be that accurate when finding a cache. It can indicate that we have anything from 30ft down, or, it says something like 1ft away when we know that it's still a little way away because of the clue.

 

I was wondering if maybe i had a setting wrong on my gps or whether there might be another reason?

 

xx

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I'm new to the world of geocaching. We've had an okay success rate so far but thats mainly to my hubby routing around every tree or stump within a 30 ft radius.

 

I have an e trex venture hc but for some reason it doesnt seem to be that accurate when finding a cache. It can indicate that we have anything from 30ft down, or, it says something like 1ft away when we know that it's still a little way away because of the clue.

 

I was wondering if maybe i had a setting wrong on my gps or whether there might be another reason?

 

xx

 

That's the way it works for all of us. The GPS only gets you close. You have to find it.

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Based on my experience with 50+ finds using a Garmin Trex everything is working fine. You should have another number on your display in the upper left that tells you the relative accuracy based on how many satellites are being used etc. The approach your husband is using is the same one I have been using sense I started, once the GPS says you are within 20 feet it is time to put the GPS away and start looking in all the likely places. Good luck on future searches and have fun.

Edited by belle610
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the accuracy depends on things like cloud cover, occlusion from trees, hillsides, upper atmosphere thermal layering, You can name another dozen. and thats on the day the coords were entered by the person that placed the cache. then figure in the factor of your unit doing the same thing, they may use a Garmin you might have a Magellan.

now you can see where the 30 foot discrepency comes from.

as lost in the ozone as i am, i'm lucky i can find my way back to my vehicle

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the accuracy depends on things like cloud cover, occlusion from trees, hillsides, upper atmosphere thermal layering, You can name another dozen. and thats on the day the coords were entered by the person that placed the cache. then figure in the factor of your unit doing the same thing, they may use a Garmin you might have a Magellan.

now you can see where the 30 foot discrepency comes from.

as lost in the ozone as i am, i'm lucky i can find my way back to my vehicle

Accuracy is not affected by cloud cover for GPS. The frequencies used were chosen specifically because they are not affected by clouds.

 

Garmin versus Magellan is one of those wonderful urban myths. We have 2 Garmins and 2 Magellans and they all differ from each other, nothing specific to manufacturer. Its just what satellites are being used for the calculations by the specfici GPS.

 

JD

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Accuracy also depends on the maps that your GPS uses. If you want to trust those maps you have to know how they were created, what map datum was used and so on.

The free Tiger Census street database which was the basis for many GPS maps is c-r-a-p. Just look at a nearby intersection and see how far off that is.

 

but in geocaching this is of much less concern that the 1 sigma 10 meter GPS system accuracy.

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I have noticed that on occasion you can turn the GPSr off and back on to get a new fix on your location. There is something else to consider however, and that is the resolution of the GPSr you are using and the one used when the cache was hidden. Some older GPSrs can were only accurate to within 60 feet. Newer ones tend to be more accurate. 30 feet is actually pretty good.

 

The trick is to stop when you are at the Co-ords, thane look around for good hiding places, and anything that doesn't look right, such as UPS (Unnatural piles of sticks/stones) after a while you will develop a knack for this.

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Don't forget battery strength. Weak batteries in your GPSr can have an effect on it's accuracy.

 

Do you have source for that claim? I would think the processing power of the GPS is relatively steady until it just plain didn't have the juice to power them.

 

My friend and I have been standing next to each other with identical units and differing power and have almost the exact same readings, but we've only done that a time or two.

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Accuracy is not affected by cloud cover for GPS. The frequencies used were chosen specifically because they are not affected by clouds.

 

Garmin versus Magellan is one of those wonderful urban myths. We have 2 Garmins and 2 Magellans and they all differ from each other, nothing specific to manufacturer. Its just what satellites are being used for the calculations by the specfici GPS.

 

JD

 

Yeah, it amazes me how often I see the cloud cover myth mentioned...

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Do you have source for that claim? I would think ......

 

My own observations. I have seen some highly suspicious readings and wierd accuracy problems on 4 different Garmin units when the battery life was very close to being out. I mean within 15 minutes or so of being out. I finally concluded that the unit just gets a bit wacky as the voltage drops off at the end.

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Do you have source for that claim? I would think ......

 

My own observations. I have seen some highly suspicious readings and wierd accuracy problems on 4 different Garmin units when the battery life was very close to being out. I mean within 15 minutes or so of being out. I finally concluded that the unit just gets a bit wacky as the voltage drops off at the end.

 

Ok, I can see that...however, the statement as I read would imply that it would drop with battery life.

 

I would expect that close to battery death you'd see some issues.

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Don't forget battery strength. Weak batteries in your GPSr can have an effect on it's accuracy.

 

Do you have source for that claim? I would think the processing power of the GPS is relatively steady until it just plain didn't have the juice to power them.

 

My friend and I have been standing next to each other with identical units and differing power and have almost the exact same readings, but we've only done that a time or two.

 

The only source I have is personal expieriance. Just before the batteries are dead my Garmin GPSMap 60CSx goes whaky. :unsure:

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Don't forget battery strength. Weak batteries in your GPSr can have an effect on it's accuracy.

 

Do you have source for that claim? I would think the processing power of the GPS is relatively steady until it just plain didn't have the juice to power them.

 

My friend and I have been standing next to each other with identical units and differing power and have almost the exact same readings, but we've only done that a time or two.

 

I've had direct experinece with weak batteries causing a warm start to just not happen at all. Just a few seconds later after waiting a couple minutes trying to get a warm start with 90% dead batteries and replacing the batteries with new ones cause a warm start just to occur in the expected time of 5-30 seconds.

 

Now as for Magellan vs. Garmin, I can say that Magellan firmware has a rubberbanding issue which will cause you to sometimes be up to 200 feet away from where you actually are. Check a link in my profile for info on that and how to avoid the issue altogather. ALL people with Magellan Golds, perhaps all Meridian series, perhaps all Magellan GPS's altogather should read that info.

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I use a Magellan and can definitely vouch for the "rubber-banding" thing. The trick i have found is to slow way down when I get within about 150 feet or so. i have never used a MODERN Garmin so I don't know if they are any better in this regard... the OLD one I have isn't really made for walking use and it pretty much gets lost in the woods. I still found nearly 100 caches with it in my first month in the game.

 

Once you get "in the ballpark" (75' or so) simply start looking for "sign" (piles of sticks, unusual trails, hollow trees, fallen logs, etc). I often have cache in hand before the GPSr gets below 50'.

 

Others will disagree, but it has been my experience that 30' is EXCELLENT accuracy.

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Accuracy is not affected by cloud cover for GPS. The frequencies used were chosen specifically because they are not affected by clouds.

 

Garmin versus Magellan is one of those wonderful urban myths. We have 2 Garmins and 2 Magellans and they all differ from each other, nothing specific to manufacturer. Its just what satellites are being used for the calculations by the specfici GPS.

 

JD

 

Yeah, it amazes me how often I see the cloud cover myth mentioned...

 

Clouds specifically don't affect GPS accuracy that much, but the entire atmosphere the signal is passing through does. An overhead satillite's signal travels through less error inducing atmosphere than one near the horizon. This is the main reason that even the miliary and land surveyors can only get +/- 5 metre accuracy with only a single GPS receiver, so 20' accuracy is indeed good. To get results better than that, either the readings need to be taken for long periods of time and then averaged (Static GPS) or be compared with another independent GPS unit measurement from a known point to compute a correction vector (RTK or Post processing). With either of these methods (and big $$$) the accuarcy is down to a couple centimetres.

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Another thing that really makes a warm start take forever, depending on which GPS model you use is your spped when you turn it on. If you are careening down a highway and turn on a Magellan it might never get a fix.

 

Yes clouds have little effect, but rain does, and if your GPS is on your dashboard which has a nice coat of streaming water on it you will have terrible reception.

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Don't forget battery strength. Weak batteries in your GPSr can have an effect on it's accuracy.

 

Do you have source for that claim? I would think the processing power of the GPS is relatively steady until it just plain didn't have the juice to power them.

 

My friend and I have been standing next to each other with identical units and differing power and have almost the exact same readings, but we've only done that a time or two.

 

I've had direct experinece with weak batteries causing a warm start to just not happen at all. Just a few seconds later after waiting a couple minutes trying to get a warm start with 90% dead batteries and replacing the batteries with new ones cause a warm start just to occur in the expected time of 5-30 seconds.

 

Now as for Magellan vs. Garmin, I can say that Magellan firmware has a rubberbanding issue which will cause you to sometimes be up to 200 feet away from where you actually are. Check a link in my profile for info on that and how to avoid the issue altogather. ALL people with Magellan Golds, perhaps all Meridian series, perhaps all Magellan GPS's altogather should read that info.

 

I would fully expect weak batteries to be somewhat problematic...it seemed like maybe the poster was implying that accuracy fell at a parallel rate to your battery life versus a spike of inaccuracy at the end which I'd expect, but re-reading that it maybe wasn't what he meant.

 

I was just curious if he an actual study link...also like reading data that bores most people.

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Accuracy is not affected by cloud cover for GPS. The frequencies used were chosen specifically because they are not affected by clouds.

 

Garmin versus Magellan is one of those wonderful urban myths. We have 2 Garmins and 2 Magellans and they all differ from each other, nothing specific to manufacturer. Its just what satellites are being used for the calculations by the specfici GPS.

 

JD

 

Yeah, it amazes me how often I see the cloud cover myth mentioned...

 

Clouds specifically don't affect GPS accuracy that much, but the entire atmosphere the signal is passing through does. An overhead satillite's signal travels through less error inducing atmosphere than one near the horizon. This is the main reason that even the miliary and land surveyors can only get +/- 5 metre accuracy with only a single GPS receiver, so 20' accuracy is indeed good. To get results better than that, either the readings need to be taken for long periods of time and then averaged (Static GPS) or be compared with another independent GPS unit measurement from a known point to compute a correction vector (RTK or Post processing). With either of these methods (and big $$$) the accuarcy is down to a couple centimetres.

 

I know...I do some elementary, basic level presentations on occasion about GPS and other technology to kids. I was more commenting on just the nature of how common that misconception still seems to linger. :P

Edited by egami
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Another thing that really makes a warm start take forever, depending on which GPS model you use is your spped when you turn it on. If you are careening down a highway and turn on a Magellan it might never get a fix.

 

Yes clouds have little effect, but rain does, and if your GPS is on your dashboard which has a nice coat of streaming water on it you will have terrible reception.

 

I have used Magellan since I started in one form or another and have never had problems with "slingshoting". Always had a good fix whether walking through the woods in heavy cover or carreening down the highway. In fact sometimes I have had better luck than some Garmin's when out on a group hunt with keepin signals in heavy tree cover.

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