General Discussion Triathlon Talk » GPS Accuracy Rss Feed  
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2014-04-16 6:45 PM

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Seattle
Subject: GPS Accuracy

This guy pulled together an impressive amount  of data on various GPS devices.

 

His description: I evaluated the real world accuracy of GPS watches while running over 3,500 miles/5,600Km and recording over 14,000 data points as part of my evaluation of the Best Running Watches. Under good conditions most of the watches are remarkably good, but when things get a little tough the differences become more apparent.

http://fellrnr.com/wiki/GPS_Accuracy

 



2014-04-16 6:47 PM
in reply to: Asalzwed

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Subject: RE: GPS Accuracy

Interesting.
Tests the 310 with and without footpod, but 910 only with....   Wonder where 910 without falls.

2014-04-16 10:11 PM
in reply to: ChrisM

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Subject: RE: GPS Accuracy
I can't really follow those numbers.

All Garmins are good enough for training-- nothing is completely accurate. It's going to depend on too many things. When I use my Garmins in road races the measured distance is usually within 1-2%. Same when I use them and compare to a known measured distance. Good enough for me.
2014-04-17 12:15 PM
in reply to: jennifer_runs

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Seattle
Subject: RE: GPS Accuracy

Originally posted by jennifer_runs I can't really follow those numbers. All Garmins are good enough for training-- nothing is completely accurate. It's going to depend on too many things. When I use my Garmins in road races the measured distance is usually within 1-2%. Same when I use them and compare to a known measured distance. Good enough for me.

Well, essentially the data proves something I hope people already know: that these devices can be used for a estimate, and in most cases a pretty good one but can't be taken as 100% accurate. Especially in "non-ideal" conditions such as bridges, skyscrapers, trees, turns etc. And in my experience many courses or routes have at least one of those things if not more. More often than not I see people in their RRs claiming that their garmin read xx:xx. I'd trust the certified course/Jones counter.

The things that I thought to be particularly interesting were

1. How high the iphone rated in accuracy. That's quite impressive for something that most of us already have. I wonder how other phones/operating systems compare?

2. The trueness is not only affected while running under a bridge but for some time after.

3. GPS watches have bad days

2014-04-17 12:15 PM
in reply to: ChrisM

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Seattle
Subject: RE: GPS Accuracy

Originally posted by ChrisM

Interesting.
Tests the 310 with and without footpod, but 910 only with....   Wonder where 910 without falls.

yeah, I wonder if he will add that.

It seems that no matter what device you have, the footpod should be added. Which really is no surprise. 

2014-04-17 1:16 PM
in reply to: Asalzwed

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Subject: RE: GPS Accuracy
I was disappointed that all the garmins were on 'smart recording'.

He does note that it didn't make a difference vs. 'every second', so why not show that data?


2014-04-17 1:43 PM
in reply to: dfroelich

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Seattle
Subject: RE: GPS Accuracy

Originally posted by dfroelich I was disappointed that all the garmins were on 'smart recording'. He does note that it didn't make a difference vs. 'every second', so why not show that data?

What is 'smart recording?' 

Sorry, I am not all that familiar with GPS watches heh...

2014-04-17 2:02 PM
in reply to: Asalzwed

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Portland, Oregon
Subject: RE: GPS Accuracy
It reduces the position writing rate to decrease the resulting file size and to slow battery drain. Since the GPS is still always on, the battery benefit would be minimal. And who cares if the watch files are 700kb or 3mb? As long as you sync your watch every few weeks, you'll have plenty of space either way.
So...in essence, there is no real reason to prefer 'smart' over 'every second'.
Downsides? As long as you run in a straight line, it doesn't matter how often the watch records your position, but once you hit a curve, you want as many points as possible so that your resulting path doesn't cut that corner. 'Smart' recording attempts to fill in the gaps with a smoothed line, which may or may not be anything close to the actual line.

Therefore, there is no reason to use 'smart' recording, but there is at least one big reason to use 'every second'. The only reason to use 'smart', is because it is selected by default.

That was pretty hilarious that watches can have 'bad days'.
2014-04-17 2:05 PM
in reply to: 0

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Subject: RE: GPS Accuracy
"Smart recording" has a way of recording data only when one of the parameters changes-- it supposedly saves on battery life and memory, but not necessarily compromising accuracy.

Strangely I have found repeatedly that my iPhone 4s (with various running apps) does NOT give as reliable data as my Garmins (I've used 305, 110, 610, and now 910xt). If I relied on my iPhone I'd be pretty upset when I race. (It gives me up to 5% more distance.) So I think it depends a lot also on where you use the devices.

eta: sorry I was replying at the same time as above..

Edited by jennifer_runs 2014-04-17 2:06 PM
2014-04-17 2:27 PM
in reply to: jennifer_runs

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Portland, Oregon
Subject: RE: GPS Accuracy
Yeah, I was quite surprised that the phone did so well! I used to run with my old phone (galaxy S), and it was all over the place! I'd also be interested to know which app he used, as that may have influence as well.
2014-04-17 2:30 PM
in reply to: dfroelich

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Seattle
Subject: RE: GPS Accuracy

I think maybe I'll just 'run' with a Jones Counter. I'll bet I'd PR every distance. 



2014-04-17 2:42 PM
in reply to: dfroelich

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Ridgeland, Mississippi
Subject: RE: GPS Accuracy

Originally posted by dfroelich It reduces the position writing rate to decrease the resulting file size and to slow battery drain. Since the GPS is still always on, the battery benefit would be minimal. And who cares if the watch files are 700kb or 3mb? As long as you sync your watch every few weeks, you'll have plenty of space either way. So...in essence, there is no real reason to prefer 'smart' over 'every second'. Downsides? As long as you run in a straight line, it doesn't matter how often the watch records your position, but once you hit a curve, you want as many points as possible so that your resulting path doesn't cut that corner. 'Smart' recording attempts to fill in the gaps with a smoothed line, which may or may not be anything close to the actual line. Therefore, there is no reason to use 'smart' recording, but there is at least one big reason to use 'every second'. The only reason to use 'smart', is because it is selected by default. That was pretty hilarious that watches can have 'bad days'.

The 3MB file takes a LOT longer to sync. 

2014-04-17 8:19 PM
in reply to: msteiner

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Subject: RE: GPS Accuracy
yeah, they are a guesstimate, but a reasonably good one and fairly reliable

I've been running, biking the same loops for years now and have used the Garmin 201/301 and 910. I'm pretty sure the terrain doesn't change but the 'mileage' listed by the device does vary...but not much.

Of course, I have no real way to check for accuracy in swimming.

I can say, that I slung caution to the wind once and rode my bike with all my devices on at the track to document the accuracy of my odometer and try to understand the variances of the GPS.

I'm such a nerd.
2014-04-17 9:19 PM
in reply to: msteiner

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Portland, Oregon
Subject: RE: GPS Accuracy
Meh...
I come in from a run, drop the watch on my desk and it starts syncing. By the time I'm out of the shower, it is done. 30 seconds or 5 minutes...doesn't really matter to me. I have now been uploading most of my runs through my phone which seems to take a little longer, but never so long that I would risk crappy data from 'smart' recording.
2014-04-17 9:57 PM
in reply to: dfroelich

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Subject: RE: GPS Accuracy
I wish I could load directly to my phone, but I'd need that wahoo connector thingy.

I'm hoping the rumored Apple smart watch will be good enough to replace my Garmin for running, but I sincerely doubt it.
2014-04-17 11:41 PM
in reply to: Asalzwed

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Subject: RE: GPS Accuracy

Impressive data collection and reduction.  I think.  I haven't really figured out exactly how he did his tests- and am frankly surprised that the footpoded unit came out higher than without.  Footpods are inherently inaccurate when you change your pace significantly, or go up or down steep inclines.  ???

I'm also surprised that some of the newer units, like the 620 did poorer.  over 8% off?  that's crazy bad for a modern GPS.  the newer devices have newer/better GPS chips that lock onto more sats and use GLONASS as well as GPS I think.  Did the old 205 do all that?  So= despite his incredible amount of data- the answer he gets doesn't seem right.  

Wish he tested the Ambit 2.  He disses the watch as irrelevant because in order to get the 50hr life you need 60s GPS mode, but loves the Fenix2 which only lasts 11hrs by his measure (Ambit will go ~16hrs in 1s high quality GPS mode).  Pretty similar feature set between the two.  Fenix has a few things that the Ambit doesn't. but the Ambit2 has custom apps.  The Suunto is also much more stable by most accounts. 

But- I don't want to criticize.  He did an huge amount of work.  I didn't know there was anyone else out there crazier on this stuff than Ray Maker (DCRainmaker) (whose reviews I live by)



2014-04-17 11:54 PM
in reply to: morey000

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Subject: RE: GPS Accuracy
Originally posted by morey000
Impressive data collection and reduction.  I think.  I haven't really figured out exactly how he did his tests- and am frankly surprised that the footpoded unit came out higher than without.  Footpods are inherently inaccurate when you change your pace significantly, or go up or down steep inclines.  ???

A footpod used WITH a GPS device will be iteratively more accurate than either one used alone. A footpod used alone is more accurate than a simple pedometer and will account for differences in stride length with different speeds. When used with the GPS, the footpod will "learn" the user's distances vs cadence and speed, and then use this data when the GPS signal drops out. Of course, this means that the combination effect is single-user dependent and gets better the more it's used.
2014-04-18 10:04 AM
in reply to: jennifer_runs

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Portland, Oregon
Subject: RE: GPS Accuracy
Yeah...I would have only guessed that the footpod/watch combo benefit when the watch loses GPS contact, but the data actually shows the opposite!
Under the bridge, the 310+FP had 3X the error of the 310 alone!

This makes me wish Garmin would share more about how the watch integrates the FP data with the GPS.
2014-04-18 12:47 PM
in reply to: dfroelich

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Subject: RE: GPS Accuracy
Originally posted by dfroelich

Yeah...I would have only guessed that the footpod/watch combo benefit when the watch loses GPS contact, but the data actually shows the opposite!
Under the bridge, the 310+FP had 3X the error of the 310 alone!

This makes me wish Garmin would share more about how the watch integrates the FP data with the GPS.


Yeah, that's weird.

I wouldn't want to bother with a footpod anyway-- one extra thing to worry about. Which is why I want the apple smart watch.
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