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CONFLICTING DISTANCES ON TWO SATMAPS

 
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sam8
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Joined: Oct 02, 2009
Posts: 6

PostPosted: Sat Oct 24, 2009 6:23 pm    Post subject: CONFLICTING DISTANCES ON TWO SATMAPS Reply with quote

Can any one explain why on a walk last week, two of us with Satmaps got different distances at the end?
We both pressed start at the beginning but during the walk and at the end mine was showing a greater distance; mine read about 9.2 miles and hers was about 8.5. I was expecting the walk to be about 8 miles.
I thought that she might have smaller (and prettier legs) than me but discounted this!
The only thing I can think of and I hope someone will correct me or confirm that this could be the answer, is that she had her Satmap clipped to her belt for the duration of the walk. I was leading the walk, so I held my Satmap throughout and I wondered if the swinging of my arms on each step could have added an extra distance to the walk? Is this possible or is there a more mundane reason (gravitational effect on males in Dorset is less than on females?)
Any useful ideas gratefully appreciated!

Sam
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guestlinger
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Joined: Jun 17, 2006
Posts: 12

PostPosted: Sat Oct 24, 2009 8:46 pm    Post subject: Reply with quote

how much stationary time did you take and did you turn "stop" the satmap data collection when you stopped? The reason I ask is because I believe the satmap gives arbitrary distances moved when stationary that maybe could account for the difference?
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jonesbach
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Joined: Jun 07, 2008
Posts: 185

PostPosted: Sat Oct 24, 2009 9:47 pm    Post subject: Reply with quote

I'm more inclined to blame sensitivity differences between the two machines. If the signal weakened at any stage(trees) one unit might deviate more than the other.
A way to check this would be to drop both routes(his and hers) into Google Earth and compare at a fairly high zoom level.
EDIT: If you are going to do this use the raw files not the exported files.
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lucevans
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Joined: Mar 21, 2007
Posts: 261
Location: Cambridgeshire, UK

PostPosted: Sat Oct 24, 2009 10:11 pm    Post subject: Reply with quote

My guess would be that this phenomenon is caused by differences in the gps accuracy of the two units throughout the walk due to their different positions (yours in your hand, hers clipped to her belt).
In theory, the worse the accuracy, the greater the cross-track error, and this would be cumulative over several hours of the walk, leading to a greater recorded distance.
This reinforces my belief that the positioning of the unit in use is important - a high position with clear sky-view (I clip mine to my rucksac shoulder strap) will give the antenna a better view of more satellites than putting it in the bottom of a rucksac, or even on a waist belt, where it's sky-view will be partially obscured by the wearer's upper body (especially when leaning forwards when going up hill).
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FrequentFlyer
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Joined: Jun 12, 2006
Posts: 959
Location: London

PostPosted: Sun Oct 25, 2009 12:58 pm    Post subject: Reply with quote

Does this mean then, that none of my saved walks are correct and accurate in their length ? (+/- 10/15% ?)
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lucevans
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Joined: Mar 21, 2007
Posts: 261
Location: Cambridgeshire, UK

PostPosted: Sun Oct 25, 2009 1:24 pm    Post subject: Reply with quote

FrequentFlyer wrote:
Does this mean then, that none of my saved walks are correct and accurate in their length ? (+/- 10/15% ?)


To a greater or lesser degree, no GPS track is ever 100% accurate. The best we can do is try to minimise the error by optimising the receiving conditions. I think given identical conditions, the error will be different for different hardware manufacturers too, as each will have their own algorithms to try to correct errors.
I'm afraid I've never tried to actually record the distance of a walk by an alternative method (only measured it on a paper map with a wheel-type map measurer - which itself will be subject to errors from several sources) - so the 10-15% figure may or may not be reasonable. I think consistency (precision) is more important than absolute accuracy.
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sam8
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Joined: Oct 02, 2009
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PostPosted: Sun Oct 25, 2009 1:28 pm    Post subject: Reply with quote

Thanks for the replies so far.
When I planned the walk, I first did a physical recce a few days before, I then saved my actual red dotted trail as a track and the distance shown on the Satmap was 8.12 miles. Then for the walk in question with my fellow walker with her Satmap, I just followed the exact recorded track and was surprised to note the extra half mile or so compared to her shorter distance on her Satmap and an extra mile compared to what I had done the previous week. (On the recce the previous week, I was carrying the Satmap in my pouch on my belt with frequent checks, not holding it throughout.)
We did have a short 10 minute coffee stop and a 25 minute lunch break in which we all gathered in one place with no moving about; I had no breaks on my recce.
I still wonder whether the actual swinging of the arm holding the Satmap might induce a foot or so discrepancy with every stride. Could this be possible or would the +/- effect cancel it out?
I'm still baffled!

Sam
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LostMike
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Joined: Jan 17, 2008
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Location: Monmouthshire

PostPosted: Sun Oct 25, 2009 7:11 pm    Post subject: Reply with quote

Two further thoughts you might like to have.

If you were holding the A10 in your hand you possibly turned the aerial towards the ground as you walked this would decrease the accuracy and increase the apparent distance.

When I finish a walk I look at the distance on the Log screen and then convert track to trail. The resulting trail distance is always shorter - presumably as it has taken out minor fluctuations. So if one of you quoted the trail length and the other the log length that would be another reason for a difference.

Hope that makes sense.
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LostMike
Satmap A10. Platform 21
Software version 1.5.9193
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FrequentFlyer
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Joined: Jun 12, 2006
Posts: 959
Location: London

PostPosted: Sun Oct 25, 2009 7:52 pm    Post subject: Reply with quote

LostMike wrote:
Two further thoughts you might like to have.

If you were holding the A10 in your hand you possibly turned the aerial towards the ground as you walked this would decrease the accuracy and increase the apparent distance.

When I finish a walk I look at the distance on the Log screen and then convert track to trail. The resulting trail distance is always shorter - presumably as it has taken out minor fluctuations. So if one of you quoted the trail length and the other the log length that would be another reason for a difference.

Hope that makes sense.


So that's why I always get a shorter distance...I always assumed that the software was somehow checking against the map used to determine the correct distance.
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4ndynorfolk
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Joined: Jul 02, 2008
Posts: 237
Location: Norfolk

PostPosted: Mon Oct 26, 2009 4:32 pm    Post subject: Reply with quote

In addition to other comments made, I think the you were leading is significant as well.

Presumably you were stopping to wait for peoole to catch up, maybe turning round to check where everyone is. This additional movement could mount up, particularly if handheld.

In therory, switching to advanced power save with the 4 second update interval, should reduce error.

Hopefully the next software update will improve this.
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sam8
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Joined: Oct 02, 2009
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PostPosted: Tue Oct 27, 2009 10:20 am    Post subject: Reply with quote

I'm going for a walk tomorrow (not leading) and if my colleague with her Satmap is there, I'll synchronise both of them before the walk and try to use them identically to see if any of the useful suggestions bring down the deviation.
The only other thing I could think of is that I believe her Satmap is over a year old and I doubt if she has upgraded it to the latest software; I will check this tomorrow as well.
Thanks.
Sam
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goon525
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Joined: Aug 25, 2005
Posts: 75

PostPosted: Wed Nov 04, 2009 10:12 am    Post subject: Reply with quote

I had a very siilar experience back in the Spring and reported it here. A 12% discrepancy between two Satmaps. Turned out I was over-reading by about 7%, my friend under-reading by 5%. However, when we both converted track to trail, the difference virtually disappeared, his increased, mine decreased so there was only about 0.2 mile difference on a 12 mile walk. I reported this strange behaviour to Satmap, but got no response.
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