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"location" changes throughout the day


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I'm still pretty much a newbie at this but wanted to place a cache using my Garmin Etrex Summit today. First I went in search of a place to hide a cache of my own. I set a waypoint next to a tree right on the edge of a clearing then went home to get some lunch and the cache. My accuracy was about 8 meters. When I returned I used the "goto" function to take me back to the cache, but it led me several meters from it. My accuracy was still at 8 meters. Granted I was within 20 meters of the original spot, but you'd think that there wouldn't be any variances since I was using the same GPS to find a specific spot with a clear view of the sky. Any suggestions as to what I might be doing wrong, or how I can improve my accuracy? I'm worried that I'll post a location that is too far off course for anyone to find.

Thanks for the help. icon_confused.gif

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What you are noticing is a normal. All GPS's do it. If your GPS will "average" you might try letting it do an average for about 30+ min. Another method is to make a waypoint at the cache site and then walk back and see if it takes you to a close spot near the cache. Try several waypionts until you find the one that works best. Close is about as good as it gets! You will never find the perfect one. Get someone within 50 feet and he will probably find your cache.


"The more original a discovery,the more obvious it seems afterwards"

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The accuracy (or EPE, Estimated Position Error) of 8 meters normally means that you are 50% sure that you aren't more than 8 meters away from your position. Double that, and you are 95% sure that you aren't more than 16 meters away.


That's just mathematics, but seems to make sense compared to your experience.



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I put a GPS [Garmin 12xl] on the railing of my deck -- and watched the position "change" over a matter of minutes before it stabilized. One guess is that the circuitry is slightly sensitive to temperature. That means that the indicated position will change as the GPS warms up when you turn it on. [There is enough of a current even in microprocessors to cause them to warm slightly.]


There's also the issue of geometry. The positions of the "visible" satellites are changing constantly -- and it may also be that the antenna is not completely non-directional, so holding the GPS with one orientation may yield a slightly different position than holding it in another. The first effect would introduce errors over long time periods, while the second effect would introduce errors over much shorter periods.


And finally, there seems to be the issue of how the unit computes position. The 12xl seems to use a combination of signal and prediction -- so when I've been moving, there is a tendency for the indicated position to keep moving even after I stop, but it will settle down in a few minutes.


Lots of good science fair projects here.


When you figure the error as a percentage, the surprising thing is that they are as accurate as they are. We're talking about only a few thousandths of a minute of angle here.


As for finding caches, I use three stages: good street mapping software [Delorme] to get me to general area; the GPS to get me "pretty close"; and both a compass and common sense for the last few feet. [And sometimes the clues.]


Hope this helps.

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GPS accuracy relies on extremely precise timing -- the gps knows where it is since it takes a reading from several satellites whose positions are known and the exact time of signal transmission is known. From that the gps calculates the time the signal from each satellite took to get to the unit based on the speed of light, the speed at which the signals were travelling.


There's a problem involved. The satellites have atomic clocks in them, so they know precisely what the time it is down to you don't want to know how many gazilionths of a millionth of a second. Your gps, on the other hand, doesn't have an atomic clock, so the time calculation is less precise. You wouldn't be able to afford a gps with an atomic clock, nor would you want to bring it along with you geocaching both due to its high value and cumbersomeness.


Hence the inaccuracy in calculating position. 8 metre inaccuracy is actually pretty good, unless you're a cartographer or land surveyor.


So you want to get a good reading for determining your cache's location?


Take ten readings at the location you want. Between each one walk at least three or four metres away and come back to take your next reading. Then take an average. Beyond that you'll still never know whether or not your value is the location of your cache or some other tree 8 metres away.


For what it's worth:


A) I get a number of "your coordinates were dead on", though of course I also get the occasional "My readings were off by 20 metres".


;) I did a little test the other day on one of my multipoint caches. After a few problems deciding on some coordinates I decided to go out checking each and every one; taking only four samples this time at every waypoint, I averaged them. Of this second set, one of the five coordinates had an accuracy of about 12 metres from the original, and the other four each had an accuracy of about 2 metres. YMMV but it demonstrates how accurate a handheld commercial gps can be if used properly.

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I havn't done this myself, but it would be informative to take a reading in your front yard (or somewhere convenient with good signal reception) every day and at different times of day for a month or so, to get a feeling for how much it varies due to satellite constellation or whatever. Here is a tutorial re. how GPS works.

http://www.trimble.com/gps/ Here is another: http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

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Originally posted by Pete&Julie Bridwell:

I'm worried that I'll post a location that is too far off course for anyone to find.

Thanks for the help. icon_confused.gif


As has been mentioned, 20m isn't terrible accuracy. If you still can't seem to get good coordinates, you can compensate. First, put in your description that the coordinates may be off to warn potential seekers. Second, ask the first finder to verify your coordinates and adjust them accordingly. Third, give hints that will make it possible to find the cache once in the general area. Remove the give-away hints once the coordinates are fixed.


Good luck.



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I'm no GPS expert, but from what I have read the source of the inaccuracy is not just the non-atomic clock in the GPSr.


When the GPSr calculates its distance from multiple satellites using its "inaccurate" clock the spheres where the GPSr must be situated do not overlap properly. The GPSr can then add or subtract a "fiddle factor" to its notion of time until the spheres overlap at a single point. This should be a relatively simple process of "successive approximation".


However... I'm not sure where the inaccuracy DOES come from. One thing is for sure... it is a VERY small error given the complexity of the system and the fact that radio propogation is involved (reflections off objects etc.).

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I have found that the accuracy reading from my Garmin GPS 76 is not reliable until the unit has been still for about a minute. I often notice that if I walk with the GPS and then stop, it will take about a minute for the position to settle down. During this "settle down" perod I have found that both the accuracy and the position should not be taken too seriously. I have tested the GPS 76 on a position where I know the coordinates, and after the unit has been still for a minute or so, the GPS position is always within the accuracy radius.


Also, if you record a position with an accuracy of 8 meters, and then you use the GPS to try to return to the same position, with the GPS accuracy again at 8 meters, you may be more than 8 meters off the original position, because the double error of 8 meters will result in accuracy of about 11 meters. In other words, both the original reading and the return reading may be within 8 meters of the true position, but they still may be more than 8 meters apart.


The way to improve the accuracy of a position is to average a number of readings taken at least 30 minutes apart. For better results, only take readings when the GPS indicates good accuracy.




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