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Apr 2016
1:39pm, 29 Apr 2016
3,576 posts
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Ceratonia
The trigonometry type effects are generally tiny (easy for me to say, living in East Anglia) certainly in comparison to GPS error, unless you run up or down long, exceptionally steep hills.
Your GPS watch/phone does record an altitude as well as the longitude, latitude position, but it's not particularly accurate - the usual few metres precision of the GPS is not as good in the vertical direction as the horizontal and is more prone to distortion by buildings and trees. Also the reference ellipsoid used for the calculation may not correspond exactly with local reality depending on where you are. These errors mean that the altitude can potentially flick up or down 10 metres or so on each datapoint, even when you run on the level, so the total ascent/descent shown by your watch is not very reliable.
A better method is to read off the GPS co-ords, but ignore the altitude information from the watch and correct it with real altitude data (e.g. from Google maps) which is what Fetch does AFAIK. It's an option on the Garmin website, too. Even here, you're limited by how well the map data matches reality and the GPS accuracy. If you run at the foot of a cliff and the GPS error of a few metres shows you at the top of the cliff on a couple of datapoints, you might still see a lot of extra ascent/descent.
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Apr 2016
2:05pm, 29 Apr 2016
841 posts
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jdarun
Icelandic trigirl, if you really were averaging a 10% gradient in your run, then as you say, there's potentially a small error (only 0.5% in your example). For mortal runners, it's a even more trivial issue. GPS measurements are not that accurate anyway - note all the debate over marathon distances...
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Apr 2016
2:28pm, 29 Apr 2016
5,554 posts
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paul the builder
If I understand the fell runner rule-of-thumb correctly, then your 35km with 3500m elevation change (so 1750m ascent) is 'worth' 35km plus 8*1750m. Which is 49km. If you did that in 3h17m, then I'd say you're looking at about 2h30m for Edinburgh Marathon. Make sure you check the Icelandic Olympic qualification criteria before you go, you'd hate to just miss out 
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Apr 2016
3:28pm, 29 Apr 2016
7,194 posts
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Rosehip
I've notice that my Garmin goes into auto-pause when I walk up the steep steps of the footbridge over the A1, so it thinks I'm covering horizontal distance very slowly - and at the end of a run it's right :). If I go up the long cycle slope it knows I'm still moving. So very steep hills - you will be missing out on some distance.
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Apr 2016
3:30pm, 29 Apr 2016
5,556 posts
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paul the builder
...if you have auto-pause on. I never use it.
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Apr 2016
3:34pm, 29 Apr 2016
7,195 posts
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Rosehip
I don't now I've worked out how to turn it off but I meant that in using autopause it "thought" I'm not moving forwards - because I'm going steeply up.
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Apr 2016
3:51pm, 29 Apr 2016
706 posts
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larkim
Not wishing to cast doubt, but a 10% gradient on a 35k route - that's like running up Alpe D'huez three times (and descending!).
I did just trawl to see if anyone had compared their cycling GPS with a traditional cycle computer (wheel rotations) to see what difference transpired, but couldn't find (quickly) anyone that had done that experiment. But in general for running the margin of error would be small (although consistent) as most gradients are significantly shallower than that over the length of a normal days running or cycling.
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Apr 2016
4:00pm, 29 Apr 2016
7,305 posts
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Badger
It depends on exactly which Garmin you have. All of them just use 2D distance by default. Some of them (definitely the Fenix watches, possibly the 920xt as well) can do 3D calculations, but this seems to make accuracy worse rather than better unless you're actually skiing at speed down a mountain side, because GPS elevation is noisy to say the least, and as others have pointed out, the error unless you have really large gradients throughout is smaller than the error in GPS measurement in general.
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Apr 2016
4:07pm, 29 Apr 2016
11,748 posts
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Wriggling Snake
Ceratonia had he answer, the average watch is inaccurate at any particular time, but it can b + or -, so overall, it ain't going to out by that much. I would not worry. 10% is indeed huge though.....
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Apr 2016
4:33pm, 29 Apr 2016
11,695 posts
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Dvorak
To go back some actual calculations: lets say you have long steep sections of a km (measured flat). If the gradient was 12%, you would cover 1007m; were it 20% (which I think would challenge even the most goat-like) it would still only be 1020m.
Say you had a 10K road race over pretty variedly hilly terrain: slopes varying from 3-13%. So, average 8% which I reckon would actually be a good bit over the level of runnability for most runners. That would still only have another 32m or so.
Though to take on ptb's point: it would feel an awful lot harder than just running that other 32m ;-).
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