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GPS Calibration?!?!


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Hello all

I have my GPS unit mounted on my motorcycle, and always park it home in the same spot. I have the spot way pointed, so I can always find my way home icon_wink.gif Anyway, I've noticed that last year my GPS showed me within 3 or 4 feet of the way point when parked, but this year it shows me off by 20+ feet, consistantly.It doesn't seem to be off when I use it to find caches, I mean I get within the normal distances when I use it. I was wondering if anyone knew why this could be? Has the Earth shifted on it's axis? Have the satellites been sabotaged? Have I moved and don't remember? Do GPS's need "calibration"? Is my unit whacked and it's a good excuse to buy a new one? Thanks for any help...Edgy

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No calibration required. Well, the units with an electronic compass need to be calibrated, but that's just for the compass not positional accuracy.

 

Some other possibilities for the change:

Software upgrade - latest version may calculate accuracy differently?

metric units - maybe it was 3-4 meters.

WAAS - on or off?

New obstacle (building/tree)blocking part of the sky?

Alien interference?

 

On the other hand, 20ft accuracy is about as good as I ever see. The best accuracy I've seen is ~15ft with good coverage by several satellites. I'm amazed that you were able to get an accuracy of 3-4ft.

 

I'm not lost!

I just don't know where I am.

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quote:
Originally posted by Kerry:

3 or 4 feet was simply coincidence


 

Err, I'm sorry, but does coincidence cover dozens and dozens of "parkings"? not in my world...maybe yours! And like I said, when I cache with it...still accurate to within a few feet. Lowrance GlobalMap100, not one of the toy models icon_wink.gif Said without malice and in good humor...don't wanna start a flame war...

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icon_eek.gif no flame wars aren't good icon_biggrin.gif but reality and impression is rather different to that.

 

I suppose the real question is (which will answer a lot of questions) is do you actually believe 3-4 feet is reality in the first place icon_confused.gif

 

Cheers, Kerry.

 

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

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quote:
Originally posted by edgydrifter:

 

And like I said, when I cache with it...still accurate to within a few feet. Lowrance GlobalMap100, not one of the toy models...

 


 

When you locate a cache, you should be seeing accumulative error (the error of the GPS used when placing the cache, plus whatever error in yours). So, no matter how good your receiver, you are still somewhat at the mercy of the lowest-common-denominator (ie, the 'toy' that placed it).

 

Kerry is correct, 20' is a pretty reasonably value, based upon the total system. If you were seeing superior accuracy, it was most likely luck of the draw, not a superior receiver.

 

This isn't flaming, this is just geometry, the physics of radio waves through the atmosphere, and the tolerances of commercial grade receiver components.

 

A really small environmental change can account for an EPE change like you report. For example, if your normal 'parking' time changes 20 minutes, the geometry of the sats can be considerably different. A neighbor might have repainted a house, changing your receiver's EMR 'view'. A building blocks away might now block your view of a sat near the horizon...

 

If it is working OK for caching, I wouldn't worry about it.

 

-jjf

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Wow, I wasn't to concerned till now, now it seems that my GPS is affecting reality and is responsible for painting my nieghbors house? Somehow I find that hard to believe, but you seem to know of what you speak so I'll take your word for it. It started out as a mild curiosity and now has blossomed into a thread on radio wave interferance patterns, quantum mechanics and the nature of reality...geocaching rules! icon_wink.gif

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quote:
Originally posted by edgydrifter:

Wow, I wasn't to concerned till now, now it seems that my GPS is affecting reality and is responsible for painting my nieghbors house? Somehow I find that hard to believe, but you seem to know of what you speak so I'll take your word for it. It started out as a mild curiosity and now has blossomed into a thread on radio wave interferance patterns, quantum mechanics and the nature of reality...geocaching rules! icon_wink.gif


 

Well, I was going to say that the distance is changing because the universe is expanding, or that you are shrinking with age, but... icon_wink.gif

 

Actually, this is all buzzing in my head now because of a class I recently gave to some junior high kids. We basically turned the class into a human NavStar system, tossing time stamped ping pong balls and almanac strips around. I don't know if anyone learned anything, but it was fun.

 

-jjf

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icon_eek.gif the earth moves icon_confused.gif, yes that's a worry (for some of us) and at the present rate over this way when cival dual frequency becomes available it will probably have a greater accuracy capability then the actual distance this country will have moved since the new datum reference was adopted in 1994.

 

Cheers, Kerry.

 

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

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When did he ever say any thing about EPE? He said when he went to a location he marked and returned to the same area it showed 3-4ft, now it is 20 ft when he does a goto. I think the whole EPE stuff is bogus and did not miss it when magellan hid it with WAAS averging.

 

Wyatt W.

 

The probability of someone watching you is directly proportional to the stupidity of your actions.

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quote:
Originally posted by phantom4099:

When did he ever say any thing about EPE? He said when he went to a location he marked and returned to the same area it showed 3-4ft, now it is 20 ft when he does a goto. I think the whole EPE stuff is bogus and did not miss it when magellan hid it with WAAS averging.

 


 

Positional error seemed implied, since the original waypoint and the new sample being compared both have some margin of error.

 

FWIW, I've plotted a lot of points on NAPP aerial data and have always found the Garmin EPE estimate to be pretty close to the actual quality of the acquired data.

 

-jjf

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