Accuracy
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Accuracy

 
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Scotsbob
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Joined: 27 Sep 2005
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Location: Hamilton, Scotland

PostPosted: Mon Oct 02, 2006 10:02 pm    Post subject: Accuracy Reply with quote

While up at DLXXVIII I could only get an accuracy of 25 ft yet on my way back down I got 7ft, I would have thought the higher one was with no obstructions the better the signal would have been, anyone got an explanation?
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ghiribizzo
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Joined: 22 Nov 2005
Posts: 510
Location: Ferryport-on-Craig. The Kingdom of Fife.

PostPosted: Mon Oct 02, 2006 10:36 pm    Post subject: Reply with quote

By the time you get back and notice the difference, the satellites in view + their signal strength will have changed from the values found when at the cache.

Or, alien influence. Twisted Evil
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yob kulcha
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Joined: 26 Jan 2006
Posts: 359

PostPosted: Mon Oct 02, 2006 11:06 pm    Post subject: Re: Accuracy Reply with quote

Scotsbob wrote:
While up at DLXXVIII I could only get an accuracy of 25 ft yet on my way back down I got 7ft, I would have thought the higher one was with no obstructions the better the signal would have been, anyone got an explanation?


The highest I've ever been with my GPSr was up An Stuc and Ben Lawers a couple of weeks ago. Reception was so appalling that we couldn't even begin to search for a cache. I noticed the same flaky reception on Dumgoyne while looking for "Whisky Galore". The track logs of those particular expeditions show that position data fluctuates most wildly at the peaks.

Perhaps this has something to do with signals being reflected by the shape of the hills?
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Wildlifewriter
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Joined: 04 Aug 2005
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Location: Norn Iron

PostPosted: Tue Oct 03, 2006 12:11 am    Post subject: Reply with quote

This phenomenon doesn't happen all the time with handheld GPSrs, but I've sometimes observed it myself - on the rare occasions when I climb a mountain without quitting halfway up.

The explanation is extremely technical and tedious. If you are of a nervous disposition or hate long words, please look away now...

The variation in the displayed 'accuracy' number can result from inadequate corrections/estimates of tropospheric delays on the incoming signals.

Anybody want a moment to read that again? OK, so...

Signals from satellites observed at narrow angles to the horizon are most affected by tropo effects, which occur in the low levels of the atmosphere. Local effects (such as temperature inversions) make matters even worse.

Q: Why does this happen on mountain tops?
A: Because that's where your GPSr gets a good view of these low-angle satellites. When you're down in the valleys, it doesn't 'see' them and isn't confused by the variances.

Q: I had WAAS mode on. Why didn't that fix the problem?
A: Because WAAS corrections are mainly for ionosphere (higher altitude) signal displacements.Tropo effects are very variable and often localised, so WAAS/EGNOS can't really be expected to correct for them.

Q: Do these modern SIRF chipsets help?
A: Short answer- yes, a bit. Long answer- it's a very long answer.

Q: So you're saying that I can't trust my GPS on this summit we're on now?
A: Nothing of the sort. Go check that trigpoint over there, you'll find that the GPS is almost spot-on. Remember that the 'accuracy' number isn't a statement of how the unit is performing.

Q: How do you know all this technical shit?
A: I helped to write the book...


-Wlw
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yob kulcha
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Joined: 26 Jan 2006
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PostPosted: Tue Oct 03, 2006 5:18 pm    Post subject: Reply with quote

Thanks for that Wlw. The worst of my readings were, in fact, on peaks with cloud inversions. However...

Wildlifewriter wrote:

Q: So you're saying that I can't trust my GPS on this summit we're on now?
A: Nothing of the sort. Go check that trigpoint over there, you'll find that the GPS is almost spot-on. Remember that the 'accuracy' number isn't a statement of how the unit is performing.

-Wlw


...I wasn't looking at any "accuracy" number (I don't even know if I have one on my Legend C). I was completely unable to get a reliable fix whilst on the summits and, if the tracklog is to be believed, I was throwing myself all around the mountaintop with psychotic urgency and inhuman strength.

Oh, wait a mo, I've just remembered I had tied my spacehopper onto my pogo stick for safekeeping only moments before I lost my footing Razz
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Wildlifewriter
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Location: Norn Iron

PostPosted: Tue Oct 03, 2006 6:52 pm    Post subject: Reply with quote

yob kulcha wrote:
...I wasn't looking at any "accuracy" number (I don't even know if I have one on my Legend C).

It's in there, somewhere.


There IS another aspect to all this, which has nothing to do with tropo paths or temperature inversions...

Some hills (even a few mountains) have features that can affect GPS in another way. I remember going for the FtF on a new multi in County Louth, and not finding it. Why? Because the co-ords were more than 40 metres adrift. GPS reception at the location was more than a bit dodgy - and the reason was that the cache was originally hidden less than 80m from the region's main television transmitter mast.

In the car, on its active antenna, my IQue3600 suffered from complete out-of-band front end block - with Betty announcing "Lost satellite reception" at regular intervals.

We always have to remember that what we're carrying around is, in essence, a small, battery powered and not-very-well-designed radio receiver... Smile

-Wlw
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trilobite
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Joined: 30 Jan 2006
Posts: 26
Location: Menstrie

PostPosted: Mon Dec 25, 2006 5:30 pm    Post subject: Accuracy Reply with quote

Had a wonderful walk yesterday (well, once I wheezed up onto the final stages) with a cloud sea below. Doing Beinn Donachain off Glen Orchy. At the summit my Legend C reported an accuracy figure of 1.9m, best I've ever seen. Checked satellites and found 11 strong signals. Despite poorer accuracy re altitude, this allowed me to do a rough height check on the three possible summits of this hill.

Looks as if temp inversion is still on today. If it's still on tomorrow I'm up a hill again! Sunshine deters SAD you know. Razz

Merry Xmas to all

Trilobite
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hengis
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Joined: 02 Jan 2006
Posts: 2

PostPosted: Tue Jan 02, 2007 6:02 pm    Post subject: Accuracy Reply with quote

Have a look at
http://www.gpspassion.com/fr/articles.asp?id=185&page=1

There are some very interesting reports and articles in the site.
My reading of the situation is that the equipment we have works pretty well except when I cant find a cache.

Hengis
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Wildlifewriter
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Joined: 04 Aug 2005
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Location: Norn Iron

PostPosted: Tue Jan 02, 2007 7:40 pm    Post subject: Re: Accuracy Reply with quote

hengis wrote:
My reading of the situation is that the equipment we have works pretty well except when I cant find a cache.


Laughing Laughing Applause


-Wlw
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hengis
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Joined: 02 Jan 2006
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PostPosted: Tue Jan 02, 2007 11:06 pm    Post subject: Accuracy Reply with quote

A second interesting point. Well interesting to me. Recently, on holiday in Spain, I had trouble finding some caches. After checking and discarding various possible errors in the data on the geocaching .com site (We all know that foreigners cant be as good as we are) I began to doubt my GPS system. I thought perhaps the heat had affected the accuracy of the system.

I left the system on and watched in disbelief the variation in my location which was motionless. The variation was a max of 75ft I think.

Eventually I solved the problem of not finding local caches.

Our apartment vas very close to the meridian and I had missed the W or E when noting the location. Normally I use programmes to transfer the data to my GPS system but couldn’t in an internet cafe.

Finally I can confirm there is no line on the ground and we didn’t need to alter our watches as we crossed the meridian.
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