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Tracking GPS Accuracy


bcj19
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In the short time I've been involved with geocaching, I've often wondered how accurate my GPS readings are.

 

I know that this is not an exact science and that there are many outside factors that can infulence the readings we receive from our GPS units. However, when I locate a geocache site, I tend to wonder what coordinates other geocachers get at the exact same location. Is anyone else ever curious about this?

 

I was thinking about building a web page to track this info and provide the average deviation from the posted coordinates of a geocache site and the coordinates posted by the geocachers that have found the site. Would something like this be of interest to anyone, or would I be wasting my time?

 

Any feedback is appreciated.

 

Happy hunting!

 

~ bcj19

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logscaler...

 

Thanks for the info, but that's not quite what I had in mind. I am more interested in comparing the coordinates displayed by my GPS unit at various cache sites with the coordinates reported by others at the same cache sites.

 

For example... Suppose that I track down a cache that has been posted at N39 15.587 W094 30.050 and when I get there, my GPS unit reads N39 15.589 W094 30.049. I'm wanting to compare MY coordinates with what others get when they visit the exact same cache site.

 

In the above example, there is a deviation of N0 0.002 E0 0.001. (My coordinates are slightly NE of the posted coordinates.)

 

If this still doesn't make sense, check out this link that shows a web page I've thrown together to help illustrate what I'm getting at:

 

http://bcj19.homeip.net/gps/gps2.asp

 

I hope this makes sense.

 

Happy Hunting!

 

~ bcj19

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bcj19 :

 

I do understand what your after.

 

You are wanting other cachers to submitt the readings they get at varies cache site. As not everone will hit the cache at the same time but at different time's of day and time's of year, you will be able to run an average deviation, and maybe show a better cache page number for cache placement. The more numbers you get, the better refinement and tighter the numbers will get.

 

I just thought that having a central point for cachers to check all the different GPSr's against each others, the newbies would have a place to check and make sure they where close to what everyone else has.

 

The cache page I posted above would work out the same, if a person wanted to build a spreadsheet for all the numbers.

 

Heck, the cache owner "could" install that right to the cache page and others would be able to watch things change as other cachers log in.

But more trouble then it is worth I would think.

 

logscaler.

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I think it would be a waste of time. There are just too many variables involved. With a clear view of the horizon in all directions and your antenna properly oriented, todays receivers will give an average accuracy of about 6 or 7 meters. With proper use of WAAS corrections, that error is reduced to about 3 meters. If your unit does averaging and you average for 30 minutes you can reduce the error to about 2 meters(with WAAS). The biggest variable that affects accuracy is satellite geometry. Poor geometry is caused by signals being blocked or severely reduced in strength by many variables including tree cover, steep terrain, structures, multipath, improper orientation of the receivers antenna and the user themselves blocking signals. The user must know how to interpret the "satellite" screen on their receiver to determine that they are getting the most accurate fix possible under the current conditions. Plus, I don't think that everyone is all that concerned about getting the most accurate fix possible for their cache. As far as I am concerned, as long as it's within about 50', then that's fine. If it's more than that based on my calculations (which aren't exact either) then I'll post my coordinates in the log. Like you say, it's not an exact science, so why try to make it one? Part of the fun is searching for the cache once the GPS gets you to the general area within a resonable distance.

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quote:
Originally posted by bcj19:

In the short time I've been involved with geocaching, I've often wondered how accurate my GPS readings are.

 

I know that this is not an exact science and that there are many outside factors that can infulence the readings we receive from our GPS units. However, when I locate a geocache site, I tend to wonder what coordinates other geocachers get at the exact same location. Is anyone else ever curious about this?

 

I was thinking about building a web page to track this info and provide the average deviation from the posted coordinates of a geocache site and the coordinates posted by the geocachers that have found the site. Would something like this be of interest to anyone, or would I be wasting my time?

 

Any feedback is appreciated.

 

Happy hunting!

 

~ bcj19


 

I would be fascinated to see this kind of info, but the truth is, yes there are a lot of variables involved and it may not mean that much in actual fact. I would still like to see it.

 

In case you have not discovered it yet, here is a good GPS related info source for stuff like this: http://gpsinformation.net/

 

4497_300.jpg

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Thank you, everyone, for your input. I think Poindexter hit the nail on the head in his description of all the variables that come into play when our GPS units calculate a specific location. Logscaler, on the other hand, was also dead on in his (her) description of what I am interested in accomplishing.

 

That said, I like the way Eraseek thinks... It might be a waste of time, but the information (however practical or impractical) could be of a basic trivial interest to many people.

 

http://bcj19.homeip.net/gps

 

Happy Hunting!

 

~ bcj19

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On one cache, my GPS got me within 2 feet.

one others I have been 15 or 20 feet to the South.

One or two others were 29 feet, or even 70 feet.

 

It would be nice to know if I am off, the hider was off, or both of us. But, since I've not recorded new locations on any of my finds, I don't have the right to complain.

 

But it would be nice info to have.

 

DustyJacket

...If life was fair, a banana split would cure cancer.

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I have the Garmin Street Pilot 3 designed ideally for auto routing, it talks, full color, has 128 megs memory, etc. The second unit is the Garmin Etrex Vista which has a lot of neat bells and wistles of its own - including WAAS feature which the SP3 does not have. Together, they do mostly everything.

 

My own test is intersecting the seconds part of my coordinance both North and West to the exact 700x700 location. I tried at many times of day and night, with many and few satalites and under varying weather conditions and in all cases m was never more that 8 feet square from the coordinance I entered. That being 4 feet in all diections from a standing still position.

 

I will end by saying that I am very please with this result and would not imagine a condition where I would need to get closer to that using simple handheld GPS devices.

 

Please send your gps adventures to my site for posting and sharing with the world at www.confluencex.com - and hopefully we can can share our trips and adventures,

 

Joh

Lakehurst, NJ

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BCJ19- I think thats a real cool Idea! Give me the trivial info...I'll use it. Better yet' come up whith a spreadsheet type thingy I can use and I will use it on all my caches(not that I have any NOW,wait till this summer!)and I know others that probally would also.

 

PS-NO MORE BEER, MAN I'M SHINEY! icon_biggrin.gif

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Again, thanks to everyone for your input.

 

It sounds to me like there is a potential "market" for this type of information. As a result, I've spent some time working on building a site for the sole purpose of collecting and reporting the average deviation from the coordinates of a posted cache site and the coordinates reported by the geocachers that have visited the site. Based on this deviation, "new" coordinates for the cache site will be calculated and displayed.

 

You can view this site (as somewhat of a work-in-progress) by visiting the following URL: http://bcj19.homeip.net/gps

 

Before anyone says anything, I do realize that this information is trivial, at best, and that all GPS readings are approximations and can be affected by any of a number of outside forces (see Poindexter's previous post...5th from top, I think). That said, any questions, comments, etc would be greatly appreciated.

 

Happy Hunting!

 

~ bcj19

 

PS: The Face... You're close, but that's not a Golden. He's a Vizsla (aka Hungarian Pointer).

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I found this article on WAAS accuracy.

 

http://users.erols.com/dlwilson/gpswaas.htm

 

The other thing I'd like to add to the accuracy discussion is the theoretical maximum. I don't know about you, by my GPS (a Megellan Platinum) displays minutes to three decimal places.

 

I created two waypoints using EasyGPS. The one was at N80:00.000 W79:00.000 and the second was at N80:00.000 W79:00.001. I also created a third at N80:00.001 W79:00.000. That allowed me to calculate a theoretical maximum of about 6 ft of Latitude and 4 ft of Longitude.

 

This will give you an idea of just what .001 minutes means. Of course, you'll need to re-calculate Longitude for your area.

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