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GPS speeds (SkiTracks)

 Poster: A snowHead
Poster: A snowHead
My experiences of ski tracks are that it often throws up anomalies that are obviously not correct, and also it will only ever be as accurate as the gps device it is used through, BUT, and while agreeing it is no where near perfect, when it does work, it's not so far away. I base this not on ski racing, but most of my life I have ridden push bikes and motorbikes, both on and off road, and I reckon I am reasonably good at judging speed. I mention on and off road because anyone who has done it knows that on push or motor bikes, 40mph on road feels much slower than 40mph off road. It all helps with experience.
I know the fastest two runs I have ever made, one at Tignes and one in Yllas, both straight lining perfect and icy empty piste, with a good wind up my jacksy. On only one of them did I have ski tracks on, and it showed 66mph. While I think that was a touch high, I reckon it was about 60mph. As others have pointed out, aerodynamics is everything. Without a good wind behind, even in a skin tight suit, getting over 45mph just gets harder and harder for every 1mph gained. Just try straight lining an empty piste to your top speed while in a tuck, and then stand up and feel how much you slow. Aero dynamics is the biggest leveller in ski speed racing and the main reason the times are always so close. If anyone is trying to beat their mates for the highest speed on ski tracks one day, just wait until you have the wind behind you. I wouldn't try it though unless you have both skill and an empty piste. And by empty, I mean EMPTY.
Just out of interest, I have left my ski tracks on in Åre, which included a bus trip to Tegefjall and back, and it was showing the bus peaking at about 48mph, which, given the conditions and my driving experience (not just cars and bikes) I reckon was about right. Looking back at the ski tracks record, my max ski speed on one of those days was 39mph, which, given the conditions that day (flat light, light snow, soft to medium piste) I reckon was about right.
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 Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
Having done some digging into this, it seems that while every GPS internally calculates the 3D velocity (that is, the true velocity going down a slope, etc) using doppler, the value is not made available via either serial NMEA or via the android or ios API; these give you just the 2D (horizontal component of) velocity.

So somebody writing e.g. a ski app has to generate the vertical velocity vector themselves, and the only data they have to work with is the GPS altitude. This has significant errors which - depending on the constellation at the time - are generally bigger than the horizontal errors. Determining the vertical velocity vector from changes in GPS altitude is therefore subject to large errors.

One could reduce the problem substantially by using the phone with a separate GPS (bluetooth connected) which is WAAS/EGNOS compatible. There are some out there. A google on e.g.
waas compatible bluetooth gps
finds several. I have used some in the past and found none picked up the European EGNOS signal, however. This one might work
https://bad-elf.com/pages/be-gps-2200-detail
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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?
Just going back to this thread to add a bit of data that I thought was interesting. For the first four seasons I used SkiTracks my max speeds have been been 102 and 115kmh. This season my maximum speed credible to me (i.e. a fast speed recorded when I thought I was actually going fast) has dropped to 86kmh. I don't feel like I'm going slower, the only significant change is I've moved from an iPhone 5 to an iPhone 7. I've also got a Garmin Fenix 5 watch which I used skiing this year to compare to the iPhone 7 and there was a pretty good correlation between the two. My conclusions are that GPS recorded speeds do depend on the quality of the device and I'm not as fast as I thought I was! Embarassed
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 You need to Login to know who's really who.
You need to Login to know who's really who.
My ski tracks recorded similar figures throughout the week, for the same runs, at similar times, when conditions were good.

My highest this year was 86km/h, but never really pushed it. Other snowHead recorded very similar speeds at the same time using this app, so am quite happy to accept yours as accurate enough.

The thing to always bear in mind that these speeds are usually only reached for a very short period of time, usually seconds, on clear runs.
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