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Ski apps mph section

post #1 of 21
Thread Starter 

I have been around the same person for 3 trips this winter, "thinks" that he is a fast skier. Will show you his recorded top speed for hills in W NY or Snowshoe WV, 70-80mph. Claiming that the app matches what his car speedo is reading, hence it MUST be right. I can't tell if he is mentally challenged or just refuses to consider the science involved. I'm fairly certain that 60mph =80 fps, feet per second, obviously a higher speed will increase that to the high 90's to low 100's fps at the speeds he is talking. Aerodynamics alone would not allow such speed let alone the lack of available descent/ grade/distance.

 

How do you explain the hole in this app? Garbage in garbage out?

post #2 of 21

http://gpssystems.net/accurate-gpsbased-speedometer/

http://www.thenakedscientists.com/HTML/questions/question/2840/

 

All smartphone speed apps works by GPS to locate your position, then comparing it over time.

 

Everybody knows GPS in your phone is not perfectly accurate, and especially in the mountains the signal reflects off the rock formations so there is a lot of additional error as compared to the open road.

 

If your position is changing and the phone only has a couple of data point for it to calculate that you were in one place 1 second, then over somewhere else a second later to calculate a speed.  So measuring max speed is especially inaccurate if it is only comparing data points.

 

I've had a garmin automotive GPS indicate in the stats section that my top speed is 241 mph in a car.    Also when driving up to tahoe, in one section the GPS will consistently tells me I'm over in a creek a couple hundred feet over from the road, as it's getting a reflected signal off the mountainside.


Bring a radar gun with you if you want to prove him wrong so that it doesn't rely on getting a signal to atomic clocks located in satellites 26,200miles above you, then test again.

Or, alternatively mark out or plot a known distance in the run, and then use a stopwatch to do straightforward velocity=distance/time (aka miles(a distance) per hour(a time)).

 

 

You can also show him a video of ski races (downhill)  with a speed trap where you see skiers actually go max out less than 80mph, but also note they hit the tiniest lip and they catch air for awhile, so your friend should definitely try out for 2014 olympics as he'll get a gold for sure.  

 

But in any event, just have fun, there's little point in getting into an argument over this.  Who cares if someone is a bit of a braggart, you know the truth, that's good enough.  

Edited by raytseng - 2/25/13 at 1:48am
post #3 of 21

GPS is notorious for incorrect "Highest" speed; all it takes is one misread data point to mess it up. track logs are more accurate.  60 mph on a typical 700 foot vertical hard snow hill is perfectly possible.

 

Here you go, enjoy  http://www.epicski.com/t/55337/how-fast-do-you-ski/240

post #4 of 21
MPH is not accurate on ski apps
post #5 of 21
I know for a fact it's not accurate...I use SkiTrax which has had me clocked at about 50 MPH when in fact I'm going a wimpy 25MPH or so.

But heck, I'm the first guy at lunch to pull out my SkiTrax app and say dang, only went 50MPH today. People are like NFW??!! Wow!
post #6 of 21
Thread Starter 

What I think I'll point out to him, a few things actually is that 60mph = 80 fps, (feet per second) and 80mph=117fps and then point out to him that he is trying to convince us that he is traveling the length of roughly 2 semi trucks nose to tail in 1 second. I'll also point out to him weather channel doods hanging out in hurricane force winds need tethered to something solid in 50 mph sustained winds or they get knocked down, perhaps the light bulb will come on. One person pointed out that the ambulance goes 117fps!

post #7 of 21
post #8 of 21
Easiest way to shut him up is beat him to the bottom of the run.
post #9 of 21
Quote:
Originally Posted by bulletbill View Post

What I think I'll point out to him, a few things actually is that 60mph = 80 fps, (feet per second) and 80mph=117fps and then point out to him that he is trying to convince us that he is traveling the length of roughly 2 semi trucks nose to tail in 1 second. I'll also point out to him weather channel doods hanging out in hurricane force winds need tethered to something solid in 50 mph sustained winds or they get knocked down, perhaps the light bulb will come on. One person pointed out that the ambulance goes 117fps!

I'm not sure you read the above posts. While it is extremly unlikely that your friend is skiing at 60-80mph those speeds are by no means impossible. Ski racers do it almost every race. The fact that news reporters get knocked over in wind has less to do with the speeds you achieve while moving and more to do with balance, strength and traction.

 

Just so you know the speed skiing record (going down a really steep hill in a straightline) is currently 156 mph   

post #10 of 21

aerodynamics will determine the speed, and standing upright in baggy clothes will not produce 80mph. don't even need a device to know that. there was a thread on this a while ago, and one of the guys showed a pic of a claimed 69mph. doubted that accuracy at the time.

post #11 of 21
Quote:
Originally Posted by bulletbill View Post

I have been around the same person for 3 trips this winter, "thinks" that he is a fast skier. Will show you his recorded top speed for hills in W NY or Snowshoe WV, 70-80mph. Claiming that the app matches what his car speedo is reading, hence it MUST be right. I can't tell if he is mentally challenged or just refuses to consider the science involved. I'm fairly certain that 60mph =80 fps, feet per second, obviously a higher speed will increase that to the high 90's to low 100's fps at the speeds he is talking. Aerodynamics alone would not allow such speed let alone the lack of available descent/ grade/distance.

 

How do you explain the hole in this app? Garbage in garbage out?

 

GOS tracking will have inaccurate maximum speeds all the time. All it takes is for the tracking device to lose contact from a satellite or two for one data plot, and thus your position at that instant is more inaccurate.

 

The next plot picks up the satellites again, and gets closer accuracy to your actual position. If the more inaccurate plot had you further down the hill than you actually were, your speed will be calculated as less than it actually was, as the tracker sees you moving less distance over that amount of time. If the plot was further up the hill, your speed will be calculated faster.

 

I've seen maximum speeds on my tracker of 100-120mph.

 

These errors usually stick out like a sore thumb, and a seriously doubt your friend can actually believe the one reading he got indicating he was going that fast was legit.

post #12 of 21

Just for the record. 60 mph =  88 ft/sec.

post #13 of 21
Alpine replay has some sort of build in correction for these errors, they have an explanation for how it works on their website.
post #14 of 21
Thread Starter 
Quote:
Originally Posted by slider View Post

Just for the record. 60 mph =  88 ft/sec.

Right, it's been probably 10 years or so since a driving instructor used the figure to demonstrate speed and braking distances, I vaguely remember the 80ish fps figure. The fps really sort of puts things in perspective for a lot of people, sort of like $16 trillion dollars that is thrown about these days as a national debt, you really can't wrap your head around the figure until you put into a "your dollars" perspective.

 

I have absolutely no desire whatsoever to try to see how fast I need to get down a hill/mountain. The hair better be standing up on my arms before i'll try to do that, if you know what I mean.

post #15 of 21
Quote:
Originally Posted by bulletbill View Post

Right, it's been probably 10 years or so since a driving instructor used the figure to demonstrate speed and braking distances, I vaguely remember the 80ish fps figure. The fps really sort of puts things in perspective for a lot of people, sort of like $16 trillion dollars that is thrown about these days as a national debt, you really can't wrap your head around the figure until you put into a "your dollars" perspective.

 

I have absolutely no desire whatsoever to try to see how fast I need to get down a hill/mountain. The hair better be standing up on my arms before i'll try to do that, if you know what I mean.

Well, knowing does help put things in perspective.  If you start to feel too comfortable cruising around at high speed without a number you may begin to feel invincible and the laws of physics don't apply on you on the slopes.   

Once you get a report that you are going say 25-30 mph or higher you get a reality check and mental comparison with cars.  A car hitting a tree at that speed ends up suffering serious injury.  Without the car your are pretty much likely to die.  Without the speed readout, you may never make the link back to cars and potential for injury should a collision occur.

post #16 of 21
Quote:
Originally Posted by bulletbill View Post

Quote:
Originally Posted by slider View Post

Just for the record. 60 mph =  88 ft/sec.

Right, it's been probably 10 years or so since a driving instructor used the figure to demonstrate speed and braking distances, I vaguely remember the 80ish fps figure. The fps really sort of puts things in perspective for a lot of people, sort of like $16 trillion dollars that is thrown about these days as a national debt, you really can't wrap your head around the figure until you put into a "your dollars" perspective.

 

I have absolutely no desire whatsoever to try to see how fast I need to get down a hill/mountain. The hair better be standing up on my arms before i'll try to do that, if you know what I mean.


Big deal 5.5 mph. diff. But then again on skis it prolly is.biggrin.gif  GPS the chair lift speed. High speed quad-10mph=880 ft/min. Anyho, skiing is not how fast you go but how much fun you have on the way down. It is painful to discuss politics..........Catch my drift.icon14.gif

post #17 of 21

I get the feeling that some GPS devices under-report speed in many cases. When I zoom in on my ski tracks on any of these apps, I see a series of zigzags. In reality I'm moving from arc to arc. So if a GPS is only updating every second or two, it's recording the shortest distance between those two points rather than the longer route your centre of mass may be taking. 

post #18 of 21
Quote:
Originally Posted by Metaphor_ View Post

I get the feeling that some GPS devices under-report speed in many cases. When I zoom in on my ski tracks on any of these apps, I see a series of zigzags. In reality I'm moving from arc to arc. So if a GPS is only updating every second or two, it's recording the shortest distance between those two points rather than the longer route your centre of mass may be taking. 

 

Every measurement of speed needs to take into account the fidelity level you wish to measure.

If you put a string around your phone and whizz it aorund like a lasso at 50mph the GPS is not going to measure any speed even though the moving quite fast.

post #19 of 21

i think alpine replay is pretty accurate i got it up to 40mph the other day a few times and actually thought i was going faster
 

post #20 of 21
Quote:
Originally Posted by raytseng View Post

 

Every measurement of speed needs to take into account the fidelity level you wish to measure.

If you put a string around your phone and whizz it aorund like a lasso at 50mph the GPS is not going to measure any speed even though the moving quite fast.

Have you tried this - you may find you are wrong - depending on how long the string is of course.smile.gif

In addition to ski tracks I have the walk/run equivelant app to track mileage.  If the phone is in my pocket, the mileage is fairly accurate compared to odometer and otehr measurements.  If the phone is held in my hand and swinging as I walk, it shows a fair amount of extra distance.  This is just from the swinging motion of my arms while walking - even if not walking fast and not swinging arms much.

Cheers

J

post #21 of 21
If a ski app misses a couple if data points (easy to do in the mountains), the calculated instantaneous speed when readings resume becomes very high. If your ski app gives a speed curve run (Ski Tracks does) check it at the high speed part to be sure it is a smooth curve and not a jagged line with a lot of missing points.

Phresheez uses a 30 second running average which tends to understate your speed unless it is a long run. Their web analysis allows you to look at it with 15 and 5 second averaging, and I've found the 5 second gives much too high instant readings.

Using a dedicated hand held GPS and analyzing the curves, I've found it hard to get much above 55 mph without being in a tuck on well waxed GS skis (in normal loose ski clothes)
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