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GPS Accuracy

 
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Moe the Sleaze
Geocacher


Joined: 10 Jan 2003

Posts: 1145
Location: Champlin, MN

PostPosted: Mon Jan 19, 2004 7:04 pm    Post subject: GPS Accuracy Reply with quote

I've been noticing a lot of logs lately where the finder takes issue with the posted coordinates because their GPSr indicated they were off by 20 feet or less! What some people don't seem to realize is that because accuracy can be affected greatly by tree/leaf cover, nearby buildings/bluffs, or simply by the constellation of satellites, 20 to 25 feet of accuracy is often as good as one can hope for. Now, the hider may have 20 feet of accuracy when he hides the cache and the finder may have similar accuracy in the opposite direction which would result in a perceived error of 40 feet!

Personally, I rarely post "corrected" coordinates unless my reading is off by more than 0.01 minutes (approx. 60 feet) unless I have good reason to believe my coordinates are significantly better and the hide is in a "target rich" area (ie. the cache is a micro and there are a hundered spots to hide it in a fifty foot radius).
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RJ
Past MnGCA Board


Joined: 14 Mar 2003

Posts: 326
Location: St Louis Park

PostPosted: Mon Jan 19, 2004 9:39 pm    Post subject: Reply with quote

I rarely change the coords on caches I've posted unless I get many posts on the inaccuracy of my coords. 20 feet or less is pretty good if you ask me. I used to think that 30 feet was about average.
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Marsha and Silent Bob
Past MnGCA President


Joined: 02 Sep 2003

Posts: 6261

PostPosted: Mon Jan 19, 2004 9:44 pm    Post subject: accuracy. Reply with quote

R.J. wrote:
I rarely change the coords on caches I've posted unless I get many posts on the inaccuracy of my coords. 20 feet or less is pretty good if you ask me. I used to think that 30 feet was about average.

Yup, best advice for a new cacher is to put the GPS away when you are in a 30ft. radius of the cache... The most recent one I put new coords on was off by about 135 ft.

Silent Bob
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SirPoonga
Geocacher


Joined: 07 Feb 2003

Posts: 144
Location: Marshfield, WI :(

PostPosted: Mon Jan 19, 2004 9:45 pm    Post subject: Reply with quote

Yep, mathematically you could be a ways off.

Ok, lets say your gps is getting good coverage and you are gettign an EPE of 15ft. You hide your cache. The GPS marks it down, but it's coordinates are actually 15ft north of the cache. Now, the cacher's gps with the same EPE will be led 15ft north of the cache. Actually, with his 15ft EPE he could be led 15ft further north, a total of 30ft north away from the cache.

This most likely won't happen, but it is possible.
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15Tango
Past MnGCA Chair


Joined: 17 Dec 2002

Posts: 841
Location: St. Paul

PostPosted: Tue Jan 20, 2004 3:48 am    Post subject: Reply with quote

My "Down in the Valley" cache is down in a valley, with nearby high-tension wires, and I always get complaints about the coordinates--every time I've gone to do maintenance, my GPSr has been with 20-30 feet. My hiding technique helps out my accuracy--I'll scout out a location, mark the coords in my GPSr, then go home, write my cache page, assemble the cache, then go hide it with a different sat. constellation. If I'm within 10-20 feet, I'll consider everything good to go, if not, I'll come back a few more times and change the coordinates to whatever average I get.
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Marsha and Silent Bob
Past MnGCA President


Joined: 02 Sep 2003

Posts: 6261

PostPosted: Tue Jan 20, 2004 6:47 am    Post subject: Reply with quote

15Tango wrote:
My "Down in the Valley" cache is down in a valley, with nearby high-tension wires, and I always get complaints about the coordinates--every time I've gone to do maintenance, my GPSr has been with 20-30 feet. My hiding technique helps out my accuracy--I'll scout out a location, mark the coords in my GPSr, then go home, write my cache page, assemble the cache, then go hide it with a different sat. constellation. If I'm within 10-20 feet, I'll consider everything good to go, if not, I'll come back a few more times and change the coordinates to whatever average I get.

I usually take 10 to 12 readings (approaching the cache from different angles) and average them. That seems to work ok for the most part.

Silent Bob
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Arcticabn
Past MnGCA Board


Joined: 30 Nov 2003

Posts: 1846
Location: Lakeville, MN

PostPosted: Tue Jan 20, 2004 9:46 am    Post subject: Reply with quote

Yea SB, that's why you were using all that foul language when you were looking for the GPS Knowledge Challenge Cache right. You did good. I guess I only heard 15 or 20 swear words per point on the course. Laughing

We need to do it again.
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Marsha and Silent Bob
Past MnGCA President


Joined: 02 Sep 2003

Posts: 6261

PostPosted: Tue Jan 20, 2004 9:50 am    Post subject: Reply with quote

Arcticabn wrote:
Yea SB, that's why you were using all that foul language when you were looking for the GPS Knowledge Challenge Cache right. You did good. I guess I only heard 15 or 20 swear words per point on the course. Laughing

I was cursing YOUR coordinates only on one of the micros. I think I found all the others fairly quickly. The one that was buried in the snow was difficult even for YOU to find Wink

Silent Bob
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murph
Geocacher


Joined: 23 Jan 2004

Posts: 51
Location: Rochester, MN

PostPosted: Sat Jan 24, 2004 4:34 pm    Post subject: Reply with quote

As a previous poster said I pretty much turn mine off when I get to the area. Or I sometimes will find a open spot from trees and lay it on the ground and wait a bit.

murph
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