Hey guys, I have a question about running watches. My friend has a Garmin 645, another friend has a Garmin 245 music, and I also have a Garmin 245 music. But everytime we run, we get totally different numbers. For example, on a run where I got 4.22 miles, I had run about 200 meters more than my friend with the 645, and he had 4.30 miles. I also checked on my Garmin 245 that it was on Glonass+GPS. I am just wondering why there are always these huge inconsistencies and if there is a way to recalibrate it so one of our watches isn't so far off?
GPS inconsistencies between Garmins?
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It's definitely a thing, my vivoactive 3 is consistently .10 miles behind when compared to friends' Forerunner 235s.
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nike shill wrote:
It's definitely a thing, my vivoactive 3 is consistently .10 miles behind when compared to friends' Forerunner 235s.
hmmm. Were you able to ever recalibrate gps or something. I still haven't figured that out. -
You're not going to get an accurate view of discrepancies from 3 different runners. You're all starting and stopping your watch at different times, and some might have different features turned on (Auto Pause, for example). The only way to tell is with 3 watches, same settings, one wrist.
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If you look at a race on Strava with Flybys you can see different distances that many people got. It varies but really not that much, someone 2% different than most of the people there would be a real outlier.
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It's because one of you has 1-second recording turned on in settings and the other has left it in the default of "smart recording" which is like every six seconds or worse.
Some old/cheap Garmins only have smart-recording available but most newer ones can turn on 1-second (or "every second") buring in the settings, check your manual or poke around.
The reason why this matters is called the "coastline paradox" where the finer the detail of increments that you measure something, you get more distance. If you reduce the sampling rate, it reduces the distance counted.
The picture in wikipedia is pretty good explainer, the one on the left would be like "smart recording" and the one of the right would be like 1-second recording:
https://en.wikipedia.org/wiki/Coastline_paradox -
It’s not even that, the Garmin is working out a distance from the satellite signals it receives, regardless of recording modes the signal we receive even from someone running next to or slightly in front/behind will be slightly different.
Therefore unless a completely straight route with no interference, those tiny differences add up. Now over 4 miles shouldn’t be that much. But did you get gps sync before starting, did the pace actually read 0:00 or as soon as it was green you started running? -
I run the same route every day starting my watch at the same spot +-5'. The one mile mark varies by over a hundred feet.
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FormerAverageRunner wrote:
I run the same route every day starting my watch at the same spot +-5'. The one mile mark varies by over a hundred feet.
Then there is either something wrong with the watch, the reception your watch receives likely need the start, the pace you run or something you actively do.
Modern watches regardless of brand are fairly reliable and are consistent even if consistently wrong -
FormerAverageRunner wrote:
I run the same route every day starting my watch at the same spot +-5'. The one mile mark varies by over a hundred feet.
Exactly.
(Check out the link below)
The GPS device is a "fairy" accurate device, considering the purposes and limitations that are defined by how it collects and computes position.
There are 24 satellites circling the globe at various altitudes and speeds and traveling at different directions. Your device needs to find three satellites to determine your location to within 15 meters 95% of the time (two standard deviations). 95% of the time it is accurate within 15m, 5% of the time the error is greater than 15m. With a fourth satellite your elevation can be determined.
I haven't found any information as to the frequency of calculations your GPS runs per minute, but if you take a look at some of the maps on the link I am supplying, you'll get the whole idea how this is all done.
If you go on a 100m run, your gps is not going to be very accurate. At the start of your run your gps will pinpoint you anywhere within a 15m radius 95% of the time (and greater than 15m 5% of the time) It could position you at your starting point (not very likely) or in front of it, behind it, to the left, right or in your neighbors garage. At each positioning it does the same. At the end of your run it does some sort of post-hoc interpolation, and hopes that it's close, As you can see, the longer your course is the more accurate the GPS is likely to be. But that's not because of pinpoint accuracy of the individually recorded positions, it's because of the smoothing algorithm that guesses where you are most likely to be at any given positioning measurement. The beginning and ending positions will amount to a smaller percentage of the total distance traveled, and the deviations from the true path SHOULD average out to a virtual representation of your actual path,
Take a look at the pathways that were measure by the same device on the same course recorded on different trials (specifically images 6-8):
http://fellrnr.com/wiki/GPS_Accuracy -
I noticed the same thing between a forerunner 35 and 45. There was a 0.1 mile difference, which over 5k could be as much as 40 seconds.
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You just run 200 meters more if you eat Tofu.
It's the good food that propels you forward. -
mysteries revealed wrote:
It's because one of you has 1-second recording turned on in settings and the other has left it in the default of "smart recording" which is like every six seconds or worse.
Some old/cheap Garmins only have smart-recording available but most newer ones can turn on 1-second (or "every second") buring in the settings, check your manual or poke around.
The reason why this matters is called the "coastline paradox" where the finer the detail of increments that you measure something, you get more distance. If you reduce the sampling rate, it reduces the distance counted.
The picture in wikipedia is pretty good explainer, the one on the left would be like "smart recording" and the one of the right would be like 1-second recording:
https://en.wikipedia.org/wiki/Coastline_paradox
This is a common misconception with Garmins. One-second vs. smart affects the datapoints that get recorded in the activity file, but it has no effect on the actual distance recorded by the watch. The only thing it affects is the size of the file--accuracy will always be the same because the distance calculated is happening in real-time, not after the fact from the datapoints. -
I’ve done a bunch of controlled testing of GPS watches. The TL;DR is that they are generally off by up to +/-2% with a pretty wide variance....even in open sky conditions. If you are running in downtowns or canyons, the error can jump to 10% or more. I have a YouTube series on the testing and how GPS works if you’re interested.
