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Discussion: Two GPS watches showing different values

in: Orienteering; General

Jul 18, 2017 8:06 PM # 
Left arm vs Right arm

Any science to explain this difference ?

Jul 18, 2017 8:23 PM # 
Because space-based triangulation-on-your-wrist is hard? :) Is this consistent when you do the run over and over, with the same watches on the same wrists? Etc?

< 2.4% error isn't terrible, particularly if there's any canopy, buildings, weather, mountains, etc. I get that much error with the same watch on the same wrist on the same course. I try not to worry about it ... and for my local running courses, I use a wheel and measure them 'definitively'.

That said, my Fenix 3 asked me which arm I put my watch on during setup (I don't think my FR305 did), so apparently there is some science. Likely antenna-based algorithmic fiddling. Why not write, and let us know what they say?
Jul 18, 2017 9:00 PM # 
That seems like more of a discrepancy than I'd expect, but as hughmac gets at, there are a lot of variables that you didn't give. Was this on open roads, or in a wooded area, or somewhere in between? Did both watches have ample time to acquire satellites? If one acquired satellites a lot faster and better than the other, it could be that the slower one was inaccurate for the initial part of the run.

The percentage disagreement would be the equivalent of 400 meters in a 10K race, which seems rather large, if you indeed were on open roads. If under a tree canopy, though, I wouldn't be surprised at that much difference.

And it would, again like hughmac says, be interesting to know if this is reproducible in terms of a given watch on a given arm always being different in the same direction than the other.
Jul 18, 2017 9:07 PM # 
mostly wooded
both GPS had the same time to acquire satellite signal
one watch had 26.070 km
the other had 25,460 km

will have both watches on the handlebar of a bicycle and go for a bike ride in the open, to test if there is an equipment issue
Jul 18, 2017 9:29 PM # 
Mr Wonderful:
Both are in the same recording mode (1 sec or smart)?
Jul 18, 2017 10:21 PM # 
They are different models so may use different algorithms to process the data which can affect results. An algorithm that does more smoothing of the route might produce a shorter distance. Can you download the routes somewhere and see if they look any different.
Jul 18, 2017 10:31 PM # 
Mr Wonderful's question about sampling rate seems like a potential answer. Like the coast of Britain, measuring a route at different frequency of samples can lead to different results.

Another possibility is that the watches, which are different models, were able to receive different sets of satellites. If the 205 saw fewer satellites, it might report a greater uncertainty about position, and conceivably that factors into calculation of distance. (A watch that believes that its data represents +/-4m accuracy might include a 10m "lateral" movement (relative to the recent direction) in its length calculation, while a watch that believes that its accuracy is +/-12m might not. Or something like that. I'm wildly guessing here. An engineer or developer who works with GPS is more likely to have a good answer.)
Jul 18, 2017 10:34 PM # 
Where's Wyatt?
Jul 19, 2017 12:57 AM # 
My partner and I both use the same model garmin. When we go on a run together she always runs further according to the garmins.
Jul 19, 2017 1:08 AM # 
Like robplow wrote, have you and your partner downloaded and compared your tracks on a map? This might reveal the source of the difference.
Jul 19, 2017 1:51 AM # 
Just now went for a 15.7 km bike ride on flat open surface with 3 GPS watches tied on handle bar. As you can see from the result, margin of error was about 30 m, or about 0.2%.

... and btw, two of them had a sampling rate of "smart recording" and one had it as "every second"

Jul 19, 2017 2:13 AM # 
Stop showing off with your three Garmins. I can't even afford to eat at the moment.
Jul 19, 2017 2:13 AM # 
So, perhaps the difference between the watches when running through woods is due to reception differences. Downloading and plotting the tracks of the watches from a woods run in which they have different lengths might be interesting.
Jul 19, 2017 2:15 AM # 
Yeah, maybe you should list a couple for sale on eBay or AttackPoint. Perhaps you can advertise them based on whether they read high or low when tracking through woods... Which would people prefer? ;-)
Jul 19, 2017 2:18 AM # 
High when trying to beat a target pace but low when orienteering to show less distance over the straight line.
Jul 19, 2017 2:25 AM # 
Jim Baker: interestingly, downloading the data via QuickRoute (for example) reveals a value of distance covered that is quite different from the value on the watch display, and the downloaded value is more homogeneous and consistent among all the watches. So the discrepancy seems accentuated on the displayed value more than anywhere else.

to the Aussie out there who, (poor thing), is starving at the moment: 'em Garmins showed off were cheap scores at flea market and thrift stores, nowadays that 'em ritzy people use iPhones and thus got rid off 'em watches (sooo past decade... )
Jul 19, 2017 2:29 AM # 
Since the topic is wrist-strapped devices, taking the opportunity to place some humor here ...

Jul 19, 2017 2:33 AM # 
I've also noticed that the distance on the Garmin usually varies to what I upload to GC but tends to match AP.
Jul 19, 2017 2:45 AM # 
electrical devices in close proximity will interfere with each other
Jul 19, 2017 2:51 AM # 
Worse, they'll conspire and form a global network of devices dedicated to eradicating humans by sending robots back to the eighties.
Jul 19, 2017 2:54 AM # 
... and they will communicate among themselves telepathically, form societies, develop culture, literature, poetry, beauty pageants such as "Miss Garmin GPS", elect a Trump, and who knows what more will happen ...
Jul 19, 2017 4:25 AM # 
Smart vs. 1 sec mode makes no difference to the distance value you see on screen (FR 205/305). In both cases the watch calculates position every second and figures distance by them the same way. Smart mode just does not store every point as 1 sec does. What it comes to the distance calculated by points as a post analysis, then every second recording usually gets larger value than the screen value, because some small zigzag gets calculated in, something smart mode does not record.

Left vs right hand issue. When you run loops clockwise left hand records larger value. That's because left hand units struggles to pick up direct signal from satellites behind your body at right, instead it sometimes uses reflections. That makes unit think distance to those satellites is longer and that pushes your position to the left. Same thing pushes gps at right hand to the right. And now when you go around clockwise you get the result you get. On handlebars you don't see same behaviour because left and right are equal.

Some models are better and some worse, some try compensating that with correction algorithms and asking the hand device is, with varying result.
Jul 19, 2017 8:10 AM # 
Left and right are not equal. My mum used to be forced by my grandfather to write with her right hand because he said writing with her left hand was a sign of the devil.
Jul 19, 2017 8:23 AM # 
Writing my real name with the left hand (backwards) would be like worshiping ahSatan.
Jul 19, 2017 8:40 AM # 
But if you just let people do what they want their writing ends up rickty.
Jul 19, 2017 6:48 PM # 
People still use a Forerunner 205?
Jul 19, 2017 10:11 PM # 
Sherpes: maybe there are some directors in common between VW and Garmin?

This discussion thread is closed.