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Thread: Speed board/ski apps inaccurate? why? Reply to Thread
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04-06-2013 11:55 AM
timmytard [QUOTE=poutanen;896954]I tracked myself with apps for a couple years, various apps and various phones. My top speeds (after correction) were consistently anywhere from 100-130 kph each day.

Ah, shitty, I read that & was thinkin'....

That was last year?

Fuck, we must be pushin' @ least 150 this year?

Not bad almost 95mph



I've never used one of those things, but I'm really curious.
Done 100kmh on my home made longboard, didn't seem near as fast as I can go on my snowboard.


TT
04-05-2013 08:20 PM
redlude97 minute elevation changes on the mountain are very inaccurate. Simple geometry tells you that those inaccuracies get amplified when calculating speed. Hence, completely inaccurate readings even with the best GPS signal.
04-05-2013 05:37 PM
Lamps
Quote:
Originally Posted by chomps1211 View Post
Ok, I get all that. As I mentioned in op, I never took any reading of my max. speed to b 100% spot on. I always assumed that any logged speed would likely b +\- a few mph. So I suppose the argument being made by detractors of these apps. Are the flat out claims of top speed without acknowledging THAT discrepancy?

Or are people claiming that these apps. are unreliable and inconsitantly inaccurate enough that ANY given speed is questionable?
Additionally, that still doesn't address my observation that similar apps used when I ride my MTB seem to B reasonably close to the milage & mph's logged with my bike mounted odometer! Again, usually anywhere within 3-5 mph. Any time I've encountered larger variences than that, they can usually b traced to signal loss or interference.

That level of accuracy is good enough for me to be reasonably confidant that I'm not making wildly inflated, bullshit claims for my snowboarding speeds. Are we arguing about split hairs here?
Average speeds will be accurate but top speeds can be off by a lot. If you look st a series of instantaneous speed indicators from GPS data it can be quite spiky.
04-05-2013 05:32 PM
AcroPhile
Quote:
Originally Posted by The Deacon View Post
I guess I mostly just use it to compare one day to the next, not one rider to the next. And for that, it works. Plus it scares my wife, so that's fun!
I think even then it may be inaccurate. I think that at times it looses gps reception and that's when some of the stat keeping goes haywire. Sometimes I look over my GPS tracking it shows that I rode a straight line down a trail when I know I was deep in the trees in between two trails.

Accuracy probably depends on the terrain that you are riding.
04-05-2013 05:05 PM
Deacon I guess I mostly just use it to compare one day to the next, not one rider to the next. And for that, it works. Plus it scares my wife, so that's fun!
04-05-2013 04:55 PM
AcroPhile Me, a few buddies, and the gf all have have the same Ski Tracks app. I have an iPhone 5. One of my friends has an android phone and the gf has an iPhone 4. Whenever we all start our apps at the same time and ride together all day and then check our stats at the end of the day we all get different results.

The gf's iPhone 4 is the worst offender. It consistently reads faster and steeper than anyone else's stats. There are days when we all cruise at the same speed all day and ride the same runs then when we check the stats, the gf's phone usually records speeds 5-15 mph faster than all the other phones. One day she had a maximum slope angle of 54 degrees when I'm sure we didn't exceed 30.

I take these ski tracker app stats with a grain of salt. Whenever I achieve a new top speed I'm always skeptical.
04-05-2013 04:25 PM
poutanen
Quote:
Originally Posted by chomps1211 View Post
Poutanen, just curious which GPS watch you are using and do you know how it is logging coordinates? 1 sec. or 1/2 sec. intervals. Or do dedicated GPS receivers handle things entirely different than phone app GPS does?

Looking to school my ignorance here,..
It's a Garmin Forerunner 410, and it doesn't have a set interval for logging. They claim it has a "smart interval" that decides when to log based on calculations.

From what I've read, the intervals can be anywhere from 1/2 a second to 5 seconds or so.

For what it's worth, the apps I used recorded raw GPS data from the phone at intervals that I told it to, and I uploaded that data to Garmins website, so I'm comparing apples to apples as far as how the data is interpreted after it's recorded.
04-05-2013 04:13 PM
chomps1211 Poutanen, just curious which GPS watch you are using and do you know how it is logging coordinates? 1 sec. or 1/2 sec. intervals. Or do dedicated GPS receivers handle things entirely different than phone app GPS does?

Looking to school my ignorance here,..
04-05-2013 03:57 PM
poutanen I tracked myself with apps for a couple years, various apps and various phones. My top speeds (after correction) were consistently anywhere from 100-130 kph each day.

I bought a Garmin GPS watch this year (after losing a phone on the hill) and was expecting similar results. My boarding hasn't changed, in fact I'm likely going FASTER as I'm used to the mountains more this year, but the top speeds have gone down to the 70 kph range at Fernie and Lake Louise (more off piste), and the mid-80 kph range at Nakiska (groomer bombing/hard carving).

The other thing is, my max speeds used to fluctuate by 20 kph a day at the same hills. Now it's very consistent. All my max speeds since changing to the watch (about 30 on hill days ago) have been within approx 15 kph over ALL the hills.

Next time I'm out, I'll take a smartphone with the app and run the GPS watch at the same time.

Overall distance will be reasonably close, although the apps don't pick up carving. Max speed for the day is just so random that looking at an app and saying things like "my max speed is higher than yours because _____" just doesn't make sense.

The apps are fun, it's a great way to push yourself for longer distance and vert each day, just ignore the max speed readout. It shouldn't even be there...
04-05-2013 03:43 PM
kungfulu
Quote:
Originally Posted by chomps1211 View Post
Ok, I get all that. As I mentioned in op, I never took any reading of my max. speed to b 100% spot on. I always assumed that any logged speed would likely b +\- a few mph. So I suppose the argument being made by detractors of these apps. Are the flat out claims of top speed without acknowledging THAT discrepancy?

Or are people claiming that these apps. are unreliable and inconsitantly inaccurate enough that ANY given speed is questionable?
Additionally, that still doesn't address my observation that similar apps used when I ride my MTB seem to B reasonably close to the milage & mph's logged with my bike mounted odometer! Again, usually anywhere within 3-5 mph. Any time I've encountered larger variences than that, they can usually b traced to signal loss or interference.

That level of accuracy is good enough for me to be reasonably confidant that I'm not making wildly inflated, bullshit claims for my snowboarding speeds. Are we arguing about split hairs here?
I don't think they are inconsistently inaccurate. I think they are consistently accurate for how it is being logged. To your point of signal drop...that could be a huge factor in the results, solar flares can alter GPS readings…the list goes on. Even a half a second drop of not logging would throw the math off of how these track and would change the +/- big time. Each app creator uses a different algorithm based off of how they are tracking and how they built the app. You could take 3 different apps, load them on 3 iPhones and throw all three in a backpack, bomb down the mountain and when you pulled them out, they would probably all have a different number. The closest you could get to the real number is using the same app on the same phone over and over and that is because it would use the same way of collecting over and over.
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