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Need help with Meridian accuracy


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I bought a Meridian GPS unit recently, upgraded the firmware, uploaded some cache waypoints, and then took it geocaching. As I got out in the park and started searching for the cache, though, I noticed that the accuracy was WAY off.

 

The unit took near the cache, but when I saw the cache, the unit said I was still 32 feet away. And when I was signing my name to the log, the unit's distance started dropping, but finally stopped at 12 feet. Yes, I was under some trees, but I should still be within 2-3 feet, since I was standing *on top* of the cache.

 

I then backtracked to my car (which I had previously marked), and when I got there, my unit said I was 52 feet away! And, as I made notes about the cache I had just found (without moving anywhere), the distance fell again, down to 12 feet, but it never got down to 0 feet.

 

Does anyone have any good ideas why the unit would display the information like this? The whole time I was geocaching, I was getting strong signals from 7 satellites.

 

--John

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A couple of things are likely. There's always some degree of error with commercial GPS units.

 

When a cache is placed, there is some error introduced by the GPS unit of the cache-hider, and when you go looking, more error is introduced by the cache-finder. This can result in having the actual cache not quite where you think it 'should' be. This is normal.

 

I'm still a newbie with GPS, but I wouldn't expect my Magellan 315 to get me closer than 15 metres from a cache. If it gets me closer, great, but I'm certainly not going to expect it.

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GPS is not an absolute positioning (nothing is) and expecting to be within a few feet (and especially zero) is a little over ambitious.

 

Under the circumstances sounds like things are as would be expected. Average worldwide accuracy specification is less than 13 metres (43 feet) 95% of the time and that's signal-in-space accuracy, which doesn't allow for things like trees, obstructions etc tec.

 

Signal strength is not as important as the geometry of those signals (satellite positions) but unfortuneately that's something many recreational receiver manufacturers don't bother to display.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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GPS is not an absolute positioning (nothing is) and expecting to be within a few feet (and especially zero) is a little over ambitious.

 

Under the circumstances sounds like things are as would be expected. Average worldwide accuracy specification is less than 13 metres (43 feet) 95% of the time and that's signal-in-space accuracy, which doesn't allow for things like trees, obstructions etc tec.

 

Signal strength is not as important as the geometry of those signals (satellite positions) but unfortuneately that's something many recreational receiver manufacturers don't bother to display.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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JH,

 

Ditto what everyone else said.

 

Remember, the coordinates of a cache were figured with a GPS just like the one you have, and when you combine the possible error of the two readings, there can be quite a bit of room to look.

 

Anyway, my point--

 

I too have a Meridian, and here's what I've done which seems to work miracles.

 

I follow the arrow on my GPS until it says that I'm very close. Then I turn off my unit, and turn it back on. What I find almost every time is that the "fresh" coordinates will get me closer to the cache.

 

Example: I'm walking down a trail watching the distance measurement on my GPS get closer and closer to zero. Pretty soon the distance reads 15 feet. I turn off the Meridian and turn it back on give it a few moments to get a signal. Now the distance measurement says I'm 40 feet away, so I'll follow the arrow to this "new" location. More often than not, this second reading is substantially closer to the cache than the reading I got from the GPS as I approached the cache.

 

I've used this technique for my last 20-or-so finds, and I've found that virtually every time, the GPS seems to put me closer to the cache after I've turned it off and then back on again.

 

Jamie

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quote:

I follow the arrow on my GPS until it says that I'm very close. Then I _turn off_ my unit, and turn it back on. What I find almost every time is that the "fresh" coordinates will get me closer to the cache.


My money is that the improvement isn't in the actual cycling of power. It's in the two minutes that it takes to regrab the lock while the unit is still.

 

I won't divulge my secret cache-finding voodoo, but if anybody finds a magellan 330 tied in a tree by the lanyard near a cache I've recently hunted, I'd appreciate a note.

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quote:
Originally posted by Jamie Z:

 

Then I _turn off_ my unit, and turn it back on. What I find almost every time is that the "fresh" coordinates will get me closer to the cache.

 


 

I suspect that cycling power clears the internal histogram used for averaging. That is, when you first approach the cache, the unit is averaging your position based on new measurements and, to some degree, prior measurements. After you cycle the power, you are getting a fresh measurement without any averaging.

 

If you just stand still for awhile, the histogram would presumably fill with samples all near your current position and give you a similiar measurement.

 

-jjf

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I just got a Platinum about 2 weeks ago...and it appears to be working fine. I also a have new Map 330 which is a replacement for one that I sent back to Magaellan. Well, I took them both out for a test...each unit was showing the same readings in the car, walking, and for sure the caches that I placed using the Map330, the Platinum was right on. This is my experience and I am very pleased...now, I just hope I dont' see those "spots" that I am hearing about. icon_smile.gif

 

de mortuis nil nisi bonum

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I just got a Platinum about 2 weeks ago...and it appears to be working fine. I also a have new Map 330 which is a replacement for one that I sent back to Magaellan. Well, I took them both out for a test...each unit was showing the same readings in the car, walking, and for sure the caches that I placed using the Map330, the Platinum was right on. This is my experience and I am very pleased...now, I just hope I dont' see those "spots" that I am hearing about. icon_smile.gif

 

de mortuis nil nisi bonum

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quote:
Originally posted by jfitzpat:

quote:
Originally posted by Jamie Z:

 

Then I _turn off_ my unit, and turn it back on. What I find almost every time is that the "fresh" coordinates will get me closer to the cache.

 


 

I suspect that cycling power clears the internal histogram used for averaging. That is, when you first approach the cache, the unit is averaging your position based on new measurements and, to some degree, prior measurements. After you cycle the power, you are getting a fresh measurement without any averaging.

 

If you just stand still for awhile, the histogram would presumably fill with samples all near your current position and give you a similiar measurement.

 

-jjf


 

I use a similar trick. When I get clsoe to a cache, I wrap my GPS unit in red meat (sirloin works well, but porterhouse will do) and I seem to find more caches this way.

 

P.S. Does anyone know how to get a GPS unit out of a large dog's stomach?

 

--majicman

 

(Always trade UP in both quantity and quality and Geocaches will be both self-sustaining and self-improving!)

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Thanks for the tips.

 

I had been geocaching with a friend (and his Magellan 315) and it always got us within 2 or 3 feet of the cache. I was hoping that my GPS unit would be even more accurate than his older model.

 

But, can anyone explain why the unit said I was 50 feet away from my car... which I had just marked (with the same unit) 10 minutes earlier?

 

Anyway, I'll try the tips- maybe turning it off and on will help. Maybe wrapping it in red meat will help... I don't see how, but, heck, I'll still new to geocaching. icon_smile.gif

 

--John

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Thanks for the tips.

 

I had been geocaching with a friend (and his Magellan 315) and it always got us within 2 or 3 feet of the cache. I was hoping that my GPS unit would be even more accurate than his older model.

 

But, can anyone explain why the unit said I was 50 feet away from my car... which I had just marked (with the same unit) 10 minutes earlier?

 

Anyway, I'll try the tips- maybe turning it off and on will help. Maybe wrapping it in red meat will help... I don't see how, but, heck, I'll still new to geocaching. icon_smile.gif

 

--John

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quote:
Originally posted by majicman:

 

I use a similar trick. When I get clsoe to a cache, I wrap my GPS unit in red meat (sirloin works well, but porterhouse will do) and I seem to find more caches this way.

 

P.S. Does anyone know how to get a GPS unit out of a large dog's stomach?


 

You lost me there, I don't even remember posting in the infamous 'red meat' thread. In fact, the only enchange that I can think of that you and I have had convinced me a) you have a rapier wit and ;) I'm really, really glad I don't do your laundry...

 

-jjf

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You probably did this but.... If you upgrade the firmware the Magellan procedure does not automatically clear memory. This is usually not a problem, but.... With some upgrades in some circumstances stuff gets left behind in the wrong location (stuff like WAAS corrections being used for the wrong satellite, ephemeris being used for the wrong satellite).

 

It is always best to clear memory after an upgrade. If you don't (because, like me, you don't want to enter all your waypoints and setup again....) then if you see weird/inaccurate behaviour you should clear memory...

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