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Speed while skiing


rwolf
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it could be... in which case it would be very good i was wearing a helmet and working (ski patrol) so no one could yell at me for doing 62 (or 70) by the slow sign.

 

So you are the bastard that passes me in the slow zone when I'm trying to be considerate and slow down to the mid 40s :huh: Btw, how do you ski in Tucson?!

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Haven't skied with the GPS (in fact haven't skied in a couple of years, hope to finally do so next winter as the kids are getting older and may be able to finally enjoy it), but my experience using it in the car is that it is VERY accurate.

 

The one time I have trouble (which may explain the person who has the 300 mph reading) is that some GPS devices (like my Lowrance IFinder GO), can start to show speeds when you're standing still if the GPS reception and/or satellite geometry is poor.

 

Btw, how do you ski in Tucson?!

 

Poorly.

 

I imagine like other "southern" West Coast areas there are nearby mountains high enough that they can get snow. While I haven't been that way in years, I know there is some skiing just outside of LA and Las Vegas for this reason as well.

Edited by HaLiJuSaPa
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Btw, how do you ski in Tucson?!

 

Poorly.

 

Haha, northeast Arizona is where I work as ski patrol and then during winter break I'll go to colorado or utah for some really good snow. Amazingly I'm actually going skiing today.

 

I suspect the accuracy of reported speeds when skiing is dependent on how fast you are going - likely pretty accurate at "high" speeds and less so at slower speeds.

 

This **may** be the case due to the accuracy of the unit. When tearing down the mountain you are moving in pretty much a straight line and you cover the distance of the unit's meaurement uncertainty very quickly so it is very probable that the next point it measures is a new position.

 

At slower, carving speeds you are making a lot more directional changes and it takes longer to cover the area of uncertainty. It takes longer to cover the area of measurement uncertainty and the positional values recorded may not reflect your **true** path and hence distance covered.

 

This is further amplified in the recorded tracks, so looking back at your recorded tracks is really very cool, but they are only representative of your speed since they record value every so often. You won't see your carving in the recorded tracks.

 

We took our 60CSx skiing in Colorado in February for the first time. Zipped it in the chest pocket of my ski jacket. We have a very cool electronic diary of our skiing for each day, trails, lifts, distances covered. It is very cool to pull this up on a good topographic map.

 

I hope you weren't looking at the GPS when you were going 70mph! We did a geocache at the ski resort and tried this for a few hundred yards at 20 mph. This was even harder and weirder than skiing with a video camera.

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Hmmm, if GPSrs measure only horizontal distance/position and not distance from the previous point, then you may be receiving inaccurate measurements. For example, if the slope is 30 degrees, where the vertical distance you traveled is X, the full distance is 2X, but all you're showing on your GPSr is the horizontal, which is Xrad3... So really, you're going a bit faster. 1.15470054x faster, actually.

When you drop this to 45 degrees, it's exaggerated a bit more.

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if i remember correctly from trig, for a 30 degree slope you would want to divide your indicated speed by cos(30)

 

which does in fact = 1.1547... as the previous post states.

 

1 / cos(30) = 1.1547

 

or 62mph / cos(30) = 71.59mph

 

so figure out the angle of the slope and your golden.

 

if you dont know the angle of the slope but you do know the height of the slope, you could figure out the angle by measuring the distance from top to bottom with your gps (this would still be horizontal distance or the base of the triangle), then calculate tan(height/distance) to give the angle.

Edited by BlackBeard Pirate
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Hmmm, if GPSrs measure only horizontal distance/position and not distance from the previous point, then you may be receiving inaccurate measurements. For example, if the slope is 30 degrees, where the vertical distance you traveled is X, the full distance is 2X, but all you're showing on your GPSr is the horizontal, which is Xrad3... So really, you're going a bit faster. 1.15470054x faster, actually.

When you drop this to 45 degrees, it's exaggerated a bit more.

I'm pretty sure that the speeds you see in an "active track" when you look at it in MapSource are derived from the track point to track point changes in position, and are horizontal only. But I've also recently heard from reliable sources that the speed calculations in the GPSr are based on doppler shifts; those easily could be 3D. I was thinking about this just today; what we really need is for someone who is a skydiver to conduct a definitive test of this. Jumping off a high diving board with your GPSr might also work, but I'm not sure I ready to do this any more than I'm ready to skydive :anitongue:

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if i remember correctly from trig, for a 30 degree slope you would want to divide your indicated speed by cos(30)

 

which does in fact = 1.1547... as the previous post states.

 

1 / cos(30) = 1.1547

 

or 62mph / cos(30) = 71.59mph

 

so figure out the angle of the slope and your golden.

 

if you dont know the angle of the slope but you do know the height of the slope, you could figure out the angle by measuring the distance from top to bottom with your gps (this would still be horizontal distance or the base of the triangle), then calculate tan(height/distance) to give the angle.

 

The GPS calculates vertical changes as well - why the unit needs at least 4 satelite signals to complete its trilateration. The only problem is the vertical "accuracy" is lower than the horizontal.

 

There are a number of theads on the forum from hikers questioning the acent calculations in the units. I'm not sure of the source of the uncertainty.

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Thanks all, that is what we have been thinking. I have the Garmin 60csx and use Tpopfusion to watch my tracks. It is really cool to get the days log and watch where we went and the distance covered. We never realized we covered so much distance before in a day.

The slope angle and distance traveled is what we still have to figure out. But still seeing 64.3 mph top speed was fun. That was going down Go Devil at Keystone. It is the slope the ski racers train...

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While using the GPS we were looking at our top speeds while skiing. We were wondering about the accuracy depending on the steepness of the slope. One of my friends thinks he actually went faster than the GPS registered. Any thoughts ???

 

I've been quite surprised while biking (pedal, not motor) with my GPSr to find that my top speed has been close to 100 mph at some times. Knowing that I would be scared you-know-what hitting those speeds, tried to figure out why. Came to the conclusion that while I was off the bike and carrying the unit, I probably swung my hand around quite a bit trying to get a reading at a high rate of speed. Seems to me that you can probably move your hand at a very high speed, and that is what was being registered.

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I've been quite surprised while biking (pedal, not motor) with my GPSr to find that my top speed has been close to 100 mph at some times. Knowing that I would be scared you-know-what hitting those speeds, tried to figure out why. Came to the conclusion that while I was off the bike and carrying the unit, I probably swung my hand around quite a bit trying to get a reading at a high rate of speed. Seems to me that you can probably move your hand at a very high speed, and that is what was being registered.
The GPS gets a Position FIX each second, and if the previous fix is 88 feet off from the next position fix, then that equates to a speed of 60mph, so if the GPS is getting a poor fix, you may see that as a jagged tracklog on the screen. The Max speed is calculated, based on the difference between position fixes that happen once per second, and some people have had Max speeds in excess of 1000mph, because of this.
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