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Ski Speedometer



 
 
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  #11  
Old January 18th 06, 02:11 PM
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"fat_boy" wrote in message
ups.com...
I saw this on half baked, and it spiraled into a discussion on the
granularity of the surface of the piste.

What will really work though?


OK, my hands are in the air!!! I did get the idea from the halfbakery.... I
thought it could work though. For many years I have wanted to find my skiing
speed and allways thought about a device with a wheel attachment until
portable GPS became available and cheaper. I did use a Garmin Legend for a
few years but for this year I will be using my Windows smartphone (SPV C500)
with a bluetooth GPS unit and the software called Skido
http://www.skido.info/ . The only problem with this is that they have not
done any calculations to incorporate the vertical as you say. The problem
with the GPS altitude is that there is a degree of error. They are aware of
this problem and I have suggested they could sample the data and give two
speeds which are the non-altitude related speed and then an estimated speed
based on the actual based on distance along with altitude. With some clever
calculation im sure you could get within 5-10% of the actual speed, this is
no good for any serious record attempt measurements but it is good enough
for the recreational skiier.

What about one of these?
http://www.rei.com/online/store/Prod...ry_rn=40001903
It uses radar, no data given on accuracy though

Paul


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  #12  
Old January 18th 06, 02:43 PM
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davidof above rekoned all GPSs look at altitude too. But I gues not.

If it didnt, you could do the cos slope angle to get thre ratio of true
speed to measured speed.

  #13  
Old January 18th 06, 02:43 PM
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"Alun Evans" wrote in message
...


On Wed 18 Jan '06 at 10:51 "fat_boy" wrote:

OK all, here is the challenge, come up with a viable way of measuring
your speed on skis.

I have heard tell of using GPS, but since it will only get your
horizontal speed component, and not the verticla, it will always read
low.

What else can you think of?


Agree it does 3-d, but besides, your max speed will likely be on the
horizontal, i.e. when you bottom out of the run...

They claim to measure speed pretty well:
http://www.garmin.com/products/etrexVista/spec.html
Velocity: 0.05 meter/sec steady state
(that's about +/- 1.1mph)
Remember, it has the big error on your exact location, but velocity is
measured between two locations with the same error, thus that error is
removed.
My friend clocked 91.5mph last week. I'm not sure if that's good or

not...
A.


91 is fast, I haven't been over 70 yet and that feels really fast. I usually
get 55-65 without to much problem but that is on steeper slopes and I think
there must be something to do with the vertical factor. When I did about 70
it was on a really steep start but it flattened toward the end of the run so
I was traveling really fast on a flatish run.


  #14  
Old January 18th 06, 02:48 PM
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Why?

Think of a flat, grid defined plane. If an object is programmed to get
to xx,yy the satelite(s) can take care of that. Altitude is more
easilly taken care of by an altimiter.

  #15  
Old January 18th 06, 03:10 PM
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"fat_boy" wrote in message
oups.com...
Why?

Think of a flat, grid defined plane. If an object is programmed to get
to xx,yy the satelite(s) can take care of that. Altitude is more
easilly taken care of by an altimiter.


Well the world isn't a flat grid.

Usually the positioning is calculated from several (5-8) satellites, and
will calculate the exact location including altitude. Though, due to the
spread of the satellites the accuracy of the altitude can be less than the
that for the horizontal plane it is far more accurate than altimeters, which
as you pointed out in a previous post have calibration
issues and are not nearly accurate enough. In the first gulf war John
Simpson reported about the flight paths of the cruise missiles following the
main thoroughfares in Baghdad at building height!

--
Paul Schofield



  #16  
Old January 18th 06, 03:26 PM
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Altitude can be calculated with a GPS when it can see more than 3
satellites.. and it is not calculated using Barometric pressure..

"fat_boy" wrote in message
oups.com...
And not only that, because air is compressible its density with respect
to altitude is not constant, so the device would need calibrating (at
set altitude) to get an accurate change in altitude.



  #17  
Old January 18th 06, 03:53 PM
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OK all, here is the challenge, come up with a viable way of measuring
your speed on skis.


In about any Austrian ski resort there is a run where you can measure your
speed like in a ski race.

Look for a sign like WISBI (wie schnell bin ich - how fast am I?)

Turan

  #18  
Old January 18th 06, 04:37 PM
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On Wed 18 Jan '06 at 13:51 "fat_boy" wrote:

91? Thats extraordinary. Does he often get that speed with GPS or is
it a freak result?


nah, that was peak, and a schuss down a pisted black.


Though we'd regularly hit around 50mph carving reds / blacks.

You can test the GPS in the car, it's quite accurate.


Actually, what was a good test was that it would hit chair lift speeds dead
on, and they do change altitude significantly.




A.
  #19  
Old January 18th 06, 04:41 PM
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"fat_boy" wrote in message
oups.com...
davidof above rekoned all GPSs look at altitude too. But I gues not.

If it didnt, you could do the cos slope angle to get thre ratio of true
speed to measured speed.


oh, I didn't mean that really, my gps DOES measure altitude and the Skido
program does report it but they do not calculate the speed with altitude in
mind.

I can see why too, if I sit in my living room and get a fix on some
satalites and I can watch the altitude go up and down, sometimes quite a big
difference too. This is why I said a my previous post that they could
probably throw out the odd results and get the average and then work out a
more accurate speed.

Paul


  #20  
Old January 18th 06, 05:12 PM
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fat_boy wrote:
: I have heard tell of using GPS, but since it will only get your
: horizontal speed component, and not the verticla, it will always read
: low.

Last time this came up someone did the maths and came to the conclusion
that ski slopes are nowhere near steep enough to make a major impact on the
speed - even if only the horizontal speed is measured (despite what skiers
might like to think!). As others have pointed out GPS receivers account
for height anyway - some (the barometric sensor types) better than others!
 




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