 Poster: A snowHead
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I'm looking for people who have created or skied iconic pistes to post the segment, either Strava or otherwise. The segment should start at the start and end at the end and follow the official run.
For example here is the Sarenne in alpe d'Huez
https://www.strava.com/segments/14206834?filter=overall
On Paper: 16km, 1730 meters drop
From GPS (barometric): 10km, 1750 meters drop
I'm looking to compare the resort claim with what is measured on the ground. In this case the Sarenne is 6km less than the resort claims. I'm going to look closely at how the segment distance is calculated to see if things like drop are taken into account.
This is just a bit of fun, I'm not looking to publish this anywhere else.
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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Maybe put for Strava geeks in the title - I think I'd need a rather big JCB to create an iconic piste
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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@davidof, maybe the resort expect you to put some turns in on the way down? 
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You need to Login to know who's really who.
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Don't use Strava, but Ski Tracks gave 14.4km for total distance on Sarenne, & that was most definitely starting at the start & taking it all the way to the chair at the bottom Alpauris. I read recently that there was a report carried out comparing actual run distances with those claimed by resorts. Surprise, surprise they didn't find a single resort that actually had the Km of piste it claimed - typically under by around 10 - 15% with some being even worse!
It is interesting that Strava sees it as even shorter (by a long way!) than Ski Tracks made it, however my starting altitude reported as 3364m as opposed to Strava's 3252m - I think it all needs to be taken with a pinch of salt - a bit like max speed! 
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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I suspect that GPS accuracy can cause significant errors depending on the ability to lock sufficient satellites. Horizontal accuracy is generally specified to be around +/-15m and vertical more like +/-23m. Depending on the view of the sky you may get a greater or lesser degree of accuracy. Altitudes on commercial GPS devices are often up to 50m out at sea level so I wouldn't rely on one to land a plane
The net result of this is that every time the GPS reads your position it could be several meters out in both the vertical and horizontal plane and hence the distance travelled e.g. a slope actually dropping 21m and 28m horizontal you will travel 35m in a straight line. However the drop might measure zero and the horizontal as little as 15m and your distance travelled then is little as the 15m. The errors over a run of that length might either even out or be significantly inaccurate.
Take any of these measurement devices with a large pinch or salt or at least do the run several times and see how they differ.
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You'll need to Register first of course.
You'll need to Register first of course.
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@tarrantd, GPS units are remarkably accurate at measuring speed, as any errors tend to be similar at both the start and the finish. The GPS unit will average the start and stop locations and an error of 15m (DoP is normally less than 5m) won't make much difference over a 1500m distance covered. To understand how error creeps in, you need to understand what the unit is receiving. Each satellite has a very accurate clock. It is permanently broadcasting the time from that clock, to 8 decimal places. By knowing the position of the broadcasting satellite, from a fixed log, knowing the time stamp received and knowing what time you received it, you know how long the signal took to get to you and how far away you are from the satellite as the signal should travel at the speed of light. If you do this with 3 satellites, you can fix your position. A fourth satellite (the minimum required for a GPS to give you any info) acts as a check. The more satellites you can 'see' the more accurate the averaged position.
Why do they need to be averaged? Because things can affect how long it takes for the signal to reach us, that cannot be factored into the correction algorithms. Things such as solar storms, air density/humidity and local gravity fluctuations all minutely affect the time it takes for the signal carrying the time stamp to arrive.
Height is measured very poorly by GPS and the GPS model needs to assume the earth to be a flat ball, which it isn't. As more satellite signals are received it can be deduced more accurately but height change is covered much more accurately using a micro baromter. Your bike's Garmin and our phones have these built in for this purpose. They are normally accurate to less than 10% and often less than 5% but if the pressure changes much during a day, error can creep in.
The only way of making the service more accurate is to put the satellites lower (not possible in a geostationary orbit) or have more of them so you can get a better average.
If the signal was always received perfectly, 4 satellites could give you a position accurate to just a few millimetres. The whole thing is much more accurate than people would have you believe and modern car GPS units will know what lane you are in as you approach a junction.
I'm sure you know that aircraft land determining their height using ground radar.
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The net result of this is that every time the GPS reads your position it could be several meters out in both the vertical and horizontal plane and hence the distance travelled e.g. a slope actually dropping 21m and 28m horizontal you will travel 35m in a straight line. However the drop might measure zero and the horizontal as little as 15m and your distance travelled then is little as the 15m. The errors over a run of that length might either even out or be significantly inaccurate.
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You will have to be travelling very slowly for the error to be this large. The error in GPS is caused differences in the ionosphere and atmosphere as @bar shaker pointed out. So the error at the start and finish is exactly the same. For these to change significantly as you travel a mere 28m you would need to take a few hours to do it. Once you apply the atmospheric corrections GPS gives sub metre accuracy. I beleive the WAAS satelites broadcast the correction data live and even hand held GPS use it.
Incidently meters are what we use to measure electricity consumption, metres are units of distance.
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