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How to find fast and safe pistes?

 Poster: A snowHead
Poster: A snowHead
meh wrote:
foxtrotzulu, both of those things are false, GPS does do elevation...

Elevation is pretty inaccurate if measured by GPS. That's why devices at the posher end of Garmin's range use air pressure measurements for much more precision. Unfortunately, air pressure is great for relative altitude but rubbish for absolute altitude, so the Garmin solution is to use a known point (eg your home) as for calibration at the start of your trip.

I'd be gobsmacked if an iPhone app managed to do any of these things. Do iPhones feature an altimeter? I very much doubt it. If you don't have accurate altitude measurements, you can't make the triangulation adjustments that you assert are made. I agree with foxtrotzulu.

meh wrote:
and modern devices update at a reasonable frequency. They do typically smooth the results so if you had a very momentary peak speed it might be averaged out a tiny bit but there are other larger problems for very accurate speed reporting by GPS in the mountains.

The killer for GPS on phones is battery life. In my experience in the Android world, phone apps vary considerably in their frequency of polling the GPS system in an attempt to reach a compromise between accuracy and battery life. Some apps will therefore miss much of the zig-zagging that skiers naturally do, although, a faster skier will do much less traversing than a slower skier.

Other factors to consider are the inaccuracy introduced by satellites disappearing behind mountains, low quality GPS chipsets in a typical phone, and signal attenuation caused by thick clothing if the phone lives inside a jacket pocket. All of these will tend to introduce wobbliness to the GPS trace and might serve to overstate true speed.

If you want to know whether the recorded speed is real, the best approach is to load the course into a web app like Strava or Garmin Connect. Examine the traced course closely to see if it reflects the real path that was taken. Also look at the claimed peak speed: was it sustained for more than a second or two? If not, it's a spurious reading.

Finally, I'd suggest that the speed reported by web sites like Strava could fairly be increased to reflect vertical travel by the judicious application of a little Pythagoras.
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 Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
Jonny Jones, how inaccurate is the elevation measure by GPS? Usually it's a little more inaccurate than horizontal position. Air pressure measurements can also be imprecise as the weather changes. I've had them off by 200m vertical before and rather than just calibrating them once you should be calibrating them regularly at known elevations particularly if you want to rely on them as an aid to navigation.

How are you measuring the frequency of polling by any one app on any one phone?

BTW I'm not arguing that phone apps or GPS are any good but both facts put forward by the previous poster were spurious. You seem to be under the mistaken impression that I'm saying something otherwise.
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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?
meh, I think that both facts put forward by foxtrotzulu have some substance according to the phone and app you're using.

Some apps have an option to change the frequency of polling. You can also tell polling frequency by looking at the gps trace in detail. That might influence the zigzagging issue.

As you say, air pressure measurements are imprecise in absolute terms, but they're fantastically accurate over the course of a few hours when air pressure doesn't change.

I completely agree that gps measurements should be taken with a pinch of salt, but they can under read as well as over read.
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meh, I'm not yet convinced my points were spurious. Of course, I am well aware that GOS calculates altitude, but I am not aware that it factors this into the equation when calculating your speed. Having done a v quick google of the subject it seems as though my view is correct in that the speed calculation is only done using lateral data. This seems a sensible approach given the known in accuracy of vertical data. Do you have any info to the contrary?

My second point related to the gap between polling and even if the gap is as small as 1-2 seconds I still maintain that there will be an inherent bias towards 'cutting the corners' and the GPS will calculate distance between data points which will inevitably be shorter, even if only minutely, than the actual distance travelled by the skis over the snow. I strongly suspect that a typical GPS would underestimate the distance (and therefore speed) travelled by a professional slalom skier showing his route as an almost straight line. Please show me where I have gone wrong on this one.
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 Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
primoz wrote:
I was trying to say, that number itself doesn't mean anything.


It does if something goes wrong. Kinetic energy at 100kph is 6.25 kinetic energy at 40kph. That applies whether you expend that energy by hitting a solid obstacle or another slower-moving skier.

At least one British skier has killed themselves while, according to some accounts, racing against a speed recording app. He did so on what, at the time of day he died, would certainly have been a crowded piste. I'm sorry he died; he could very easily have killed someone else too.

If you believe you can ski safely on a public piste at 100kph, that doesn't leave a lot of allowance for another skier doing something you don't expect. I imagine the skier who crashed into me from behind at high speed was also blissfully convinced he was under control up to the point both of us were tumbling down the piste.

I am all for freedom of adults to exercise choice as to the risk they take. I'm not so enthusiastic when one person's risk taking puts others at risk and that's the behaviour being encouraged by some in this thread.
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foxtrotzulu, meh is talking nonsense. Off road mtb folk regularly and accurately complain that the cheaper Garmin units miss the apex of corners due to a 5s polling interval. As a result, average speed is understated on courses with a large number of switchbacks. Expensive units solve the problem by using a built in compass to poll more frequently when direction changes are detected. 5s is ample time for the problem you mention to manifest itself.

I would be very, very surprised if any iphone app corrected for the vertical component of motion. A dedicated skiing app, perhaps. Anything else; no chance. It's just not a big enough figure to be worth calculating in running, cycling and walking, the core market for these apps.
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 Then you can post your own questions or snow reports...
Then you can post your own questions or snow reports...
dogwatch, the other disconcerting fact in my experience is that people who crash into other people more often than not seem to blame it on the downhill skier they crashed into. They won't admit to being outnof control but I've seen them skiing and they're terrible. The lack of insight is clearly as dangerous as their speed.
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 After all it is free Go on u know u want to!
After all it is free Go on u know u want to!
Jonny Jones, thank you. I do love it when I'm right, or at least someone else thinks I am!
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primoz, I would be happy to ski behind you. A few minutes behind you though, in case of accidents.
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foxtrotzulu, Jonny Jones, of course I'm not talking nonsense, for one point you guys seem to be blind that we are in fierce agreement that polling and averaging data lowers the overall reported speed. Even my aging GPS polls at 1Hz unless you put it into a special battery saving mode.

Modern GPS units use the Doppler shift in the frequencies of the satellites to calculate speed which is inherently three dimensional. You can tell if one is doing this or using track points because the inaccuracies of fixing a position cause the unit to 'walk' even when stationary in the latter case so it will misreport motion when the unit is just sitting still.
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 snowHeads are a friendly bunch.
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I know when we tested my mate's sons iphone ap it consistently gave a series of max speeds within 3mph of each other when repeatedly tested on a straight run with two of us skiing together in a tuck down a red.
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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.
There seems to be a lot of moralising about speeding on the this thread, but the OP was actually asking for advice on where to speed safely. Personally, I'm not a lover of fast skiing; I prefer to challenge myself with technical terrain such as steeps, powder, chutes, bumps and glades. I can certainly see the appeal of speed, though.

To safely achieve high speeds, the best approach is to find a very wide piste with clear visibility and no other skiers. You also need a bluebird day, fresh grooming and fresh legs that have properly warmed up. You need to be familiar with the run and you need a suitable pair of skis - cheap hire skis are likely to be much too flexible. You'll probably reach your highest speed on a red run, so make your choice, do a few practice runs and gradually build your speed up.

Unfortunately, runs and days like that are all too infrequent. Don't be tempted to let your frustration with less than perfect conditions persuade you to attempt to ski too fast where there are other skiers on the hill, where visibility is less than perfect, or where pistes merge. And accept that skiing fast will always be dangerous even in perfect conditions: my wife once tripped in a moment of carelessness and face-palmed her pole. Her face bent the pole through more than 90 degrees, and she was very lucky to have not lost an eye or at least a few teeth. You have been warned.

Some resorts offer much more opportunity for speed than others. Try somewhere quiet in low season. Or, for real solitude on the pistes, go to Big Sky in Montana. You'll be able to ski as fast as you like without threatening anyone else at all.
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 So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much
So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much
Jonny Jones, sounds like good advice to me.
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 You know it makes sense.
You know it makes sense.
meh, I'm happy to discuss your arguments, and I never said you were talking nonsense. I simply disagreed with you. I'm not sure where the 'of course' bit comes from in your opening line. I'm not sure there is any 'of course' about it.

Anyway, while I will happily bow to your superior knowledge of GPS I would still question a couple of things:

1. If we accept that the GPS in a smartphone, which is obviously what we are discussing rather than a high end unit, is polling at 1Hz, then that implies that should I drop my phone out of a second floor window, and repeat a few times to increase accuracy, then it should show a speed of c.70km/h.


2. On the basis that I have never seen the GPS on my smartphone to 'walk' then I presume that they are not using Doppler. So, in consequence, I think it unlikely that this is how SkiTracks etc. are using it to plot the track and route.

I'm not arguing that GPS is not capable of calculating vertical velocity, just that it doesn't normally do so in these applications.
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 Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
foxtrotzulu, let's do an experiment and drop you smartphone out of the window a few times and see what it records. Toofy Grin
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 Poster: A snowHead
Poster: A snowHead
Avalanche Poodle wrote:
mnbv, If you want to find safe pistes - purely trial and error. This run was lunch time at VT.


http://youtube.com/v/kTTWVBUygyo


What run in VT is this?
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 Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
Handy Turnip, It was up off the Funitel, the Medaille red run down to the junction of the Fond and Rhodos (minute chance it may have been Variente but I doubt it). The places to take care are on the corners and then where the junctions cross. You can see that I checked the uphill run joining on.
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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?
mnbv, if you really want to establish how fast you are in a real world setting against some rather good skiers, www.muerren-inferno.ch would be my suggestion. Or any of the other volksabfahrts...
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under a new name, +1

Wasn't a season worker killed last year on a normal piste trying to beat his highest speed using an iPhone app? I imagine the bone crunching thunk against the pylon was quite something. But there again it could just as easily been against another person.
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 Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
Avalanche Poodle, cheers! I thought it might be Medaille - I'll definitely be giving that a go in March Smile
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Handy Turnip, When I skied it I was working on really rapid short verging on medium turns, it was pure luck that we hit an empty piste. But heading up high at lunch time is often a great way to find empty runs Smile
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 Then you can post your own questions or snow reports...
Then you can post your own questions or snow reports...
The lesson with Claude was really paying off Smile
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 After all it is free Go on u know u want to!
After all it is free Go on u know u want to!
I remember my dry slope coach, 25 years ago, telling me that on a pre season course (tignes?) which the Bell Bros. were on, a skier was injured. Despite skiing at full tilt, my instructor couldn't keep up with Martin who was carrying the skier's skis and poles !
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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.
Well thanks for the debate. Not sure I like the casual slander from those suggesting that I've been skiing fast when other skiers are around but anyway...

It would be cool to have safety gradings as it would be faster to identify appropriate runs just by looking at the piste map. Of course yes then there would be gentle initial runs...
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Quote:

It would be cool to have safety gradings


And just how would that work? "Blue=safe for idiots, black=safe for those who have a reasonable idea of what they're up to"

And can you imagine the lawsuits??

rolling eyes
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 snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
mnbv, It depends to a great extent on the snow conditions on the day. If you are focused mainly on avoiding runs with close surrounding trees and rocks then you are on the wrong tack. People tend to stand at the sides of slopes when they stop, hitting one of those has been known to cause a fatality many times. If you are not in control enough to avoid hitting a tree then IMHO you should also keep the speed down on pistes with flat run offs, unless you can see 100% that the way ahead and to the sides are clear.

I know where you are coming from, that you would maybe like a danger indicator to mark pistes with lots of hazards close to the edges, but in crashes people don't often leave the piste, they mostly cannon down them like bowling balls.
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