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What The... I Can't Get This Thing To Work

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Ok so I got into this really cool thing called geocaching long story short decided I needed to prove to myself(read wife) that this would be fun so we found 4 right by our house w/out the gpsr. we have now used the gpsr for about 4 caches that needed it and we have opnly found one of those caches while using the unit the other three were thru deductive reasoning. I just can't seem to find anything WITH the silly thing I understand about the double error ( one person measuring than another trying to get the same result) but everytime I use the unit and I find the cache it is way more off than the status screen leads me to believe. I'm wondering if I am expecting too much, using the unit the wrong way, or if my unit is just that inacurate. PLEASE HELP!!!! The unit I am using is a Lowrance global mp 100 built in 97 or so w/ 12 channels I always seem to have approx 4 or 5 sats in use at a time (when I feel that I can rely on it) and have gotten the accuracy to 55 ft while searching for a cache in a relativly clear area (if you haven't guessed there is a cache that is driving me crazy trying to locate it simply because it is about a half mile walking distance from my house ect...) without any major obsticals to the signal.



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I did some work with a similar unit last year, and found that accuracy levels never got much better than 30 ft or so, and would quickly bounce to 100 ft or more when we started moving, and even got as bad as almost a half mile at one time.


My initial thought was that since your unit was built pre-2000, which is when S/A (selective availability--the intentional degredation of the GPS signal for almost all non-military uses) was turned off, your unit was not capable of decoding the newer, more accurate signals. However, 55 ft is much better than the standard accuracy with S/A turned on (100m) so this is not the cause.


4 or 5 sattellites is not very good reception, 4 is the absolute minimum for 3-D locating. Usually, if I'm not working on at least 6 or 7 I consider it bad reception.


When you say relatively clear area, do you mean wide open space with no tall buildings or trees nearby, and a reasonably clear view of the horizon in all directions? Also, what is the longest you have left the device turned on with a clear view of the sky at any one single time? If your unit has not been turned on for a while, I'd say a week or more, it has to reaquire the almanac data so it knows where the satellites are. The unit calculates it's own location by figuring the time lapse of the signal coming from each satellite and triangulating it's own position from that. To do this, it has to know the current orbital pattern of each satellite. It can take as much as 15 minutes to download this data, and has to be done each time the unit has been idle for a period of time or has moved more than a couple hundred miles since it was last powered on.


Another possibility would be WAAS (Wide Area Augmentation System) It's a new(er) system being developed by the FAA to increase horizontal and vertical accuracy enough to be able to guide airplanes in on approach. Your reciever is not able to recieve these signals, but I can't really say if it would make that big of a difference in accuracy--I only use mine with WAAS enabled. Others here can provide more information on that.


Can't provide any definite concrete helpful information, but my advice would be go to a WIDE open space, turn your GPS on and set it on the hood of your car, then just sit back and chill for 20 minutes and see what happens from there. Hope you get it working better.

Edited by dkwolf
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The 55 accuracy (EPE) figure probably doesn't mean squat. Your unit is programmed to calculate that it is 95% confident that the coordinates it is displaying are within a certain radius. As such the unit can be flat dead on with half mile EPE or with 15 foot EPE.


A likely source of your trouble is the datum the unit is set to. Make sure it is set up to use the WGS84 datum. Hold the unit horizontally. Stand still with it for a few minutes in an open area to get good signal and allow the unit to settle. The unit will get you close enough to find most caches.

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My initial thought was that since your unit was built pre-2000, which is when S/A (selective availability--the intentional degredation of the GPS signal for almost all non-military uses) was turned off, your unit was not capable of decoding the newer, more accurate signals.

The current signal structure is precisely that same as it was when S/A was on. You wont' see any new civil user signals for another 3-4 years.




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The setting thing looks really promising. When I get home (im at work) Ill check what it is set to then go up on the roof w/ it for awhile and hope for the best.


I did get the manual but sometimes when I read a manual my eyes glaze over, :blink: but now I know that all of those settigs mean something i'll work over the manual.


Ill get back to this thread with an update and am still willing to hear any and all suggestions except for the ones where you have to spin around three times with your tongue stuck out during a full moon...




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Just an update yesterday I found that my datum setting needed to be changed just like the suggestion I haven't had a chance to test the unit on a cache but I'm dying to. Now for another idiotic qustion what the heck is the datum my gpsr must have a hundred different datums I can choose from some are logical and others I can only scratch my head at. Are these the different maps that the unit is basing its data from I personally thought that lat and long were internationally recognised as being standard and not variable but then of course measuring accurately was always difficult.


Hopefully the answer will also explain why the gpsr will be off by hundreds of feet if the wrong datum is selected.


Thanks for the help

blenz B)

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The simple explanation is that the datum is a model of the earth. A simple model would have the earth as a flat map, but that is a poor description, especially over a large area. A better description is a sphere, and that could serve as a basic datum for longitude and latitude.


But the Earth isn't a sphere either. It's flatter on the poles. Over small areas, you can can fudge things a little and get a pretty good fit. In 1927, the USGS proposed the North American Datum (NAD27) that gave a pretty good fit in the US, and most USGS paper maps still use this datum. They improved things in 1983 with the help of satellites, and in 1984, some international body adopted the World Geodetic Survey of 1984 (WGD84) as the model of choice.


If you have a USGS paper 1:24,000 map, and you are trying to get the coordinates on the GPSr to match those on the map, it is handy to be able to switch datums to NAD27 (the datum is printed on the map). For geocaching, unless you are doing some puzzle cache that makes you switch datums, you can set your unit to WGS84 and leave it there.


People who know lots more about this stuff than me can add details/corrections, but that is basically it.


I'm betting that you'll be please with the improvement in accuracy.


Happy caching!

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:rolleyes: Finaly got a good chance to check out the new setting and WOW it really did the trick when it said I was on the spot it ment it if it said I was 9ft away it was right even in light tree cover or out of sight of the horizon thank you for the help it made it from a future fishing lure to an actual measuring tool thanks.



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I started out with the GN212 which I think is the same unit as yours except it didnt have mapping on it. I found almost 200 caches with it before I went to the magellan gold.

I gave the Lowrance to my 15 year old grandson because he liked going out caching with me. My gold has waas the gn212 didn't, about half the time when we found a cache the older 212 was reading closer then the gold.

I hope you enjoy your Lowrance they are good units, not as popular as garmin or magellan but still a good gps. :rolleyes:

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I can't be this bad at this............


I spent 6 hours this AM looking for a cache. I am using an old GPS 300 and the readings were contantly changing even when I wasn't moving. Anyone have any experience with this particular unit? Also, you have to convert from decimels into feet and inches, but the worst part is that you have to "round" because it won't accept the degree of acuracy you need!

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