 Poster: A snowHead
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Am sure this has been explored before but have done forum search and googled but can't find out how the various ski speed and distance apps work using gps.
Can't work out how 1 level gps can translate to 3d speed and distance.
Just know answer is going to be stupidly simple.
Thanks
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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they do some maths in the app (or in the website), throw away some data, smooth out some data points here and there, and come up with a number that is probably reasonably close to reality.
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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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GPS gives a three-dimensional location.
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You need to Login to know who's really who.
You need to Login to know who's really who.
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| finestgreen wrote: |
| GPS gives a three-dimensional location. |
Agreed but this is what I don't understand. GPS gives very accurate lat/long cords but elevation is very hit and miss so what happens in the twisty turning real downhill world to sort out speed and distance, understand the basic trig but how accurate is it.
Hey ho not really important as I will believe the biggest numbers the app says
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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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In theory, it could use the accelerometer in combination with the GPS - in practice I don't think it does though!
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You'll need to Register first of course.
You'll need to Register first of course.
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Didn't think of that.
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It could, but I'd be surprised if it did.
Elevation/altitude is the biggest guess. I've seen some apps come up with 2x or half the total altitude variation of others, and one other actualy making up extra data from somewhere (everyone's favourite one for racing against each other).
Distance is quite straightforwards to deduce. Speed by definition is derived from that, and is where inaccuracies can creep in. I go by the biggest number too, although I have managed mach 1.4 on my bike. Uphill ! 
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Actually that's not correct. Modern systems use Doppler for speed and get significantly better results than you'd expect from the suggestions here because of it.
Speed measurement does appear to produce erroneous peaks - I assume that's something to do with the patchy reception on cheap receivers coupled with algorithms which are tweaked for cars not skiers. I haven't worked on current generation systems, but earlier systems were very much "optimized", so they took advantage of typical car behaviour to smooth out unexpected variations.
It's fun to see kiddies on their park boards boasting of speed-course velocities, and interesting to see how people are happier to believe their machines rather than all the other information around them.
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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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Modern smartphones are equipped with G-sensors. So knowing horizontal speed from GPS and an angle to normal it's pretty easy to derive the actual speed
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I'll be using either Galaxy S4 or Nexus so hopefully up to date, but not really about bragging really about accuracy and how the apps use the info, just curiosity
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