 Poster: A snowHead
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colinstone, your Inferno speed sounds about right I would think, I'd be very happy if I was in a similar speed range.
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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| CathS wrote: |
dredgey, if you want to go super-fast downhill, why don't you take up the luge as a winter sport?
Apparently you can reach speeds of up to 140 kph, with the bonus that the only person you risk killing or injuring is yourself... |
Don't worry, I wasn't putting anyone else at risk.
This is proving to be an interesting discussion...
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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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The trouble with getting your kicks out of skiing fast is that as you get better, the consequences of a mistake become more serious. Also, as you get older, accidents become more serious and it becomes increasingly difficult to make a full recovery.
For that reason, I consciously chose to avoid skiing fast several years ago and now I only have a couple of blasts in a typical skiing trip. Instead, I get my kicks by attempting to ski increasingly difficult terrain with greater fluidity. I get just as much adrenaline with much less risk.
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I second the above, steep technical terrain is excellent fun. I also only blip the speed now and again; the pistes don't allow for more in my opinion. I think these apps are sure to encourage the speeding out control skiers to have a nasty!
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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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| lampbus wrote: |
| The thing with GPS is...it is not recording 'speed' it is recording a longitude+latitude + timestamp, about once a second. |
Interesting thread...I use the app mentioned, but didn't know the sample frequency - the bit above led me to do a bit of finger-counting (with eyes crossed and tongue firmly in cheek..).
Quite used to using Garmin GPS for ballooning competitions and had found the most accurate position reading on the app was '+/-5m' - poor when compared with a dedicated hand-held GPS unit.
70mph was my randomly-selected starting point. This is 112.6km/h which works out as 31.3m/s.
Taking the stated sample frequency and the worst-case scenario in accuracy the skier could have actually have travelled only 21.3m in that second, so a 30% error margin! 49mph just doesn't sound quite so heroic, somehow. (They could also have travelled 41m in the same time...91mph....?).
Have fun with the app, but I'm no longer quite so impressed by my max speeds as stated/claimed by the app 
Will have to update when I've had the chance to calibrate the app vs 2 hand-held GPS units in a car (or would that be a bit anal??)
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I've run a speedo app on my android phone while driving. This one https://play.google.com/store/apps/details?id=com.binarytoys.speedometer&hl=en
It has always been accurate to within about 4% or so of the car speedo. Does this not suggest that the skiing app might be close to the same % accuracy?
Having said that, my skiing app (Alpine Replay) has had a couple of pretty fast speeds. .  Empty blacks with long run outs.
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Any accuracy like that is for a sustained speed, not a quick few seconds long schuss on a speed run. Refer back to my stats. Best part of 20% variation from the exact same GPS data. Over 1km or something, yep, probably reasonably accurate to within a few %
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| andy wrote: |
| Any accuracy like that is for a sustained speed, not a quick few seconds long schuss on a speed run. Refer back to my stats. Best part of 20% variation from the exact same GPS data. Over 1km or something, yep, probably reasonably accurate to within a few % |
Ah, see what you mean. Cheers.
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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| HossDoc wrote: |
| Will have to update when I've had the chance to calibrate the app vs 2 hand-held GPS units in a car (or would that be a bit anal??) |
I don't think that calibration is likely to be the issue - it's about margins of error. If you look closely at the GPS trace from a typical mobile phone, you'll find that it's often wobbly compared with the trace from a dedicated GPS unit. The problem is particularly acute if the phone doesn't have a clear view of the satellites - perhaps because one part of the sky is hidden by a mountain and another by your body, or because the cloud cover is particularly dense. Having the phone hidden in your pocket doesn't help, too. If you're actually moving at 20m/s and the phone reads your position every 0.5s, a measurement error of just 1m could give a final speed calculation error of 20%.
A further problem is that your phone needs to know your rate of descent to calculate your true skiing speed over the snow. Unfortunately, GPS is pretty inaccurate at measuring elevation - that's why many dedicated GPS units calculate altitude by measuring air pressure. When you add the vertical error to the horizontal error, you can wind up with some pretty strange results.
The proper solution is to use a radar gun from a fixed point on the ground. A more realistic option is to use a high quality dedicated GPS, preferably wrist mounted so it's less likely to be in your shadow, that's been properly calibrated for skiing and adjusts for your rate of descent. I've never tried using a skiing GPS unit, but I know that, on a bike ride, my phone can give a measurement error of 5-10% (both horizontal and vertical) under adverse weather conditions, but my Garmin 800 is always accurate to much less than 1% on a regular trip. Having said that, my phone's GPS is particularly poor. My wife's phone is much less affected by aerial obstructions like trees, buildings and clouds.
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Vertical error is pretty dire. My phone seems to think the altitude of the start point of my MTB rides is anything from 195m to 218m, and all but 1 ride this year show the altitude drop as I start riding uphill. The map claims 155m altitude. I did some "investigation" before, over on STW trying to work out the algorithms they use. Some interpolate/average out, some just add up every +/- change in altitude between each data point, and at least 1 popular app (the one that does encourage people to race) basically invented data that was not even in the GPS data file!
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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| skimastaaah wrote: |
| RENDL..................... 110.87kph. I think I got those numbers in the right order!!!!!! |
How on earth did you manage that? We've just got back from St. Anton and, unless the Rendl speed trap has moved since you did it (just below the park off the side of the blue run) 110kph was out of the question. We did it a few times, and even getting in to the tuck position right from the top there just wasn't enough of a slope or distance to get much above 85kph.
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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129.21km/h measured on a closed black run with a 2 photocell speed trap (using FIS timers / photocells) over a measured distance.
In case you are wondering... we are running in speed skis for the U16 CanAm champs in Sugarloaf next week and getting data on wax/grinds/skis....
The ski app is near useless for short sections, especially in a mtn area where GPS sats pop in and out of view very frequently due to the terrain. Good for boasting in the bar though..
GPS is not designed to tell you how fast you are going. It is designed to tell you where you are with pretty good accuracy. The GPS chips in cellphones update either once a second or 10 times a second. All with a precision error of 10+m
I will be repeat running later this week. I can load some various apps on a few phones and go them out against a real timing base if you want a proper answer.
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| paulhothersall wrote: |
| I will be repeat running later this week. I can load some various apps on a few phones and go them out against a real timing base if you want a proper answer. |
Good idea, it'd be interesting if you could do that.
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You know it makes sense.
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Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
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So on these apps the max speed is possibly dangly bits. What about the "sustained speed" as presumably they are recorded over a period of time rather than an "instant speed grab" ?
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Poster: A snowHead
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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mad_mike, 
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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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Someone told me these apps are that accurate because of assisted GPS. I have a suunto ambit and got it for other sports and to plot tracks. I used it for three weeks in the alps over Christmas and whilst I wasn't trying to break any records the fastest it recorded was 51.3 mph, I also used the ski tracks on an iPhone 5 and whilst most days they were close in terms of distance and speed, every now and then the ski tracks would throw up a dodgy track and speed. If I was using that as my evidence I could say I've skied at 90 mph.
Having said that I think the ski tracks app is generally good and useful to give you a idea of the ground covered.
The thing that surprised me most was how slow we generally ski. According to the suunto 30 to 40mph was a usual daily max and when you add in the stops and lifts the daily average was shocking. Some days it was lowish single figures.
Less stopping and faster lifts is the way forward.
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You need to Login to know who's really who.
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| Quote: |
Someone told me these apps are that accurate because of assisted GPS
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They told you wrong. What A-GPS does is (potentially) help in getting a first fix on start up.
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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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So regarding accuracy, the piste I recorded 60.4mph on using the ski tracks app, I had actually recorded 58.7mph two days before on the same slope with same weather conditions (and yes, both times I made sure the slope was empty of other skiers!  ) . Would this not suggest that it's possibly accurate due to two samples being taken? Or are both samples innacurate?
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dredgey, could simply mean that a systematic error is the same in both cases?
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Phones are nowadays pretty accurate, but you need to filter things a bit. I actually did few runs this winter (during super-g training of one WC team), and I got to try their GPS stuff, while I also put my phone (with Sports tracker) in "backpack" in which all the processing power for their custom GPS equipment is. It was pretty open course so no much problems for normal phone GPS to work, and result of those few runs were very similar... of course phone with 1sec sampling didn't produce track anywhere near similar to system with 10cm accuracy and 1ms sampling interval, but speed weren't all that far off (max was 136km/h, with sg skis and skinsuit, but between the gates).
But then again, when terrain is not so open (or sometimes even if it is), I did already clocked 3000km/h based on phone gps  So if you depend on handheld gps units (specialized or phones/watches), you should do some filtering first before believing numbers which they show 
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primoz, that’s a very useful point to know- having compared both options together. So is it plausible to say in an open mountain range, without weather interference they may be loosely accurate, but in areas blocked by mountain ranges etc they become completely inaccurate?
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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I wouldn't say completely inaccurate. In general phones work pretty well even in forest (i use them a lot for xc skiing, running and mountain biking), and on slopes surrounded by mountains. But as I wrote, you can't really say for sure that max speed is 100% accurate, no matter if its open or not.
Most surprising thing for me is, that I had most of problems with them on Molltal glacier, which is really open, with extremely wide slope and without high mountains around (that's place where I got most of weird reading, and with several different devices), while on the other side I never had problems on Soelden glacier on tight, dark higher peaks surrounded Icebox.
I would assume GPS would work better on open space, but Molltal glacier doesn't really prove this theory
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| Mosha Marc wrote: |
| paulhothersall wrote: |
| I will be repeat running later this week. I can load some various apps on a few phones and go them out against a real timing base if you want a proper answer. |
Good idea, it'd be interesting if you could do that. |
That would be very useful!! 
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