We are currently running 2 garmin’s between Mrs diz and myself. On the same 52 miles ride today, riding side by side (on cycle tracks) the edge 500 recorded 51.8 miles and the edge 810 recorded 52 miles. Can anyone explain the difference? We have noticed they record different distances each time we’ve used them this holiday.

0.4%

maybe one’s better than the other at ups/downs?

Tandem or it didn’t happen.

We took our tandem to a LBS for a service and some other work – my wife’s Strava showed 7 miles and mine 6.8 miles. Both on iPhones. This is normal – she always rides further than I do whether I use my Garmin Edge 25, my previous (Android) phone or iPhone

0.2 miles is 321 meters so one is off by about 6 meters per mile…

Or both are off, one reading over, the other under?

Is one using glonas and the other not?
Both have barometric Altimeters right?

Different Sample rates or applying trace smoothing perhaps?

Maybe one of you is just taking all the racing lines, and straight-lining for those KOM/QOMs on the sly… 😉

I was gonna say if you rode a loop, or someone was on the outside more often, that would be a good explanation. The real explanation is they are not 100% accurate.

Went to CYB recently and my Garmin 810 clocked up 21 miles on Dragon’s Back whilst my 2 riding companions clocked up 23 each on their Lezyne watches. I’d like to think the Garmin is more accurate, but it all seems a bit hit and miss to me.

As Cookeaa. Check GPS settings for Glonas or just GPS. Also ensure that they are both set to 1s recording.

Garmin’s what?

🙂

You’re worrying about a 0.2 mile discrepancy? Over 51.8/52 miles? Really?

That could just be different line choice in a few corners. It could be weaving a bit on climbs. Or, just maybe, it could be utterly irrelevant and not worth worrying about. I’ll go with the latter.

One of you is rider 1613

Did Mrs Diz beat you on the climbs and then do a total of ~300m riding round in circles as she waited? 😉 😀

More seriously though, probably just wheel size discrepancies. A very quick (and probably wrong) calculation suggests that an error of well under 1cm in diameter either way might account for a difference of c.300m over that distance…

My wife consistently beats me on Strava segments when we are on the tandem – using identical Lezyne GPS units.

More seriously though, probably just wheel size discrepancies. A very quick (and probably wrong) calculation suggests that an error of well under 1cm in diameter either way might account for a difference of c.300m over that distance…

Er.

GPS..

Nevermind.

Different sampling rates would do it, or even just luck about sampling points in corners – it wouldn’t take many cut-off hairpins to add up to that distance.

This is a travesty. Burn one of them and send the blackened mass back to Garmin’s office demanding your 0.2 miles back.

Seriously, it’s just sampling noise. Don’t worry about it.

Er. GPS.. Nevermind.

Ah, but a larger wheel will have a greater gravitational pull on the earth, moving the planet slightly towards the nearest satellites and thus affecting the result. Come on, think it through.

GPS is a very accurate system of measurement, only if the antenna are large, you have a correction system in use, and you are using all the satellites available. With no correction most systems will be +/- 5m, especially if your moving, trees, line of site to horizon etc. So its likely, even with identical equipment you will get some deviations. My phone suggested my last ride went through peoples gardens, over houses etc.

Er.

GPS..

Nevermind.

Am I being dim then? I’ve never really given it much thought (and it was very early) but I presumed that when you set the wheel size on the garmin if you are using a speed sensor it measured distance from that rather than rely on 100% GPS reliability? Every day IS a school day after all if I’m wrong… Must’ve been my first theory then 😉

Er.

GPS..

Nevermind.

Some GPS units use wheel sensors too.

Anyway – I’d be pretty pleased with that level of correlation. GPS isn’t perfectly accurate, of course. So each time it takes a reading, you might be a few metres away from where it thinks you are. This tends to result in your trace being more of a zigzag than you are actually riding.

molgrips – Member 
Some GPS units use wheel sensors too.

You’d know if you had one as you’d have had to fit the sensor to the bike.

That said, wheel sensors are more accurate on rolling distance, though need accurate wheel size calibration on the day. Typically they’ll be quite different to GPS distance. I found distance and speed with a sensor was generally lower than GPS, so was coming out worse than friends. Stopped using the sensor now, though mostly because it kept getting knocked out of position.

superfli – Member 
Check GPS settings for Glonas or just GPS. Also ensure that they are both set to 1s recording.

Neither 500 or 810 have GLONASS.

Reality is it’s complicated and many variables can cause difference. Even with seemingly identical models, it can be down to timing when acquiring satellites so it gets different fixes to the other, or just being a few metres apart or off course, one happens to lose satellite sight because of tree cover but bike next to it is getting clearer view, etc.

How long you wait for a GPS fix or after getting a fix can be important too. GLONASS for example is mainly of benefit to get a quicker fix by adding a second group of satellites, and likewise phones use A-GPS by phoning home via mobile data to get satellite positions quicker. These speed up getting a fix, but it’s an adequate fix, not necessarily all the satellites possible. Enough to track, but maybe not perfect. Once moving it may or may not pick up the rest.

https://www.dcrainmaker.com/2011/06/2011-sport-device-gps-accuracy-in-depth.html
https://www.dcrainmaker.com/2011/06/2011-sport-device-gps-accuracy-in-depth_09.html

You’d be well grumpy with my Garmin, it chops 200ft of of every 1000ft climbed (according to other Garmins on same ride), either that or mine is the only accurate one and everyone else’s over-reads!

It’s never 100 % accurate OP, just use it as a (normally very good) guide

GPS altitude readings are always terrible. That’s why the more expensive ones have barometric altimeters, and it’s advertised as a benefit. Check your mates’ models.

Look on the bright side, you can sell it to the USAF. They’d love the capability of being able to drop a bomb as close as 0.2 miles away from where it was supposed to go.

Disparities between Garmin’s

What is it that belongs to the Garmin that has the disparity?

Your discrepancy is not much. My wifes and my garmin 200’s are usually c 5% different with mine recording under. Compared to my mates iPhone, mine is reliably 10% under for distance and height, or his is reliably 10% over.

GPS altitude readings are always terrible….. Check your mates’ models.

Compared to both exact same model (at least x3 of*) and more expensive ones, and phones, it’s not hat mine is just crap, it’s that it’s consistently 20% lower than any other device.

It’s not really a problem, just a good example of how devices can vary and to only use as a guide.

* and once just for kicks I borrowed one (same model) and ran both on my bike, same discrepancy, and I refuse to believe it’s because the right side of my bars is significantly more bouncy than the left 😀

Thank you for all the replies, it’s given me a great read. I’m not that bothered but it’s Mrs diz’s 500 that was reading a little short and she is pi$$ed. lol

Swap garmins then. Job jobbed 🙂

amedias – Member 
it’s that it’s consistently 20% lower than any other device.

Does yours have a barometer in it?

If so, could be port blocked or it’s just iffy barometer.

Can calibrate them, sort of. It’s basic but you can set a known altitude at known locations. Though even then I find if I do a long loop from home with it calibrated, when I return home I’m 100ft above or below where I started (often below sea level). Pressure and weather differences during the ride basically skew it. If I was a pilot I’d be adjusting it often for conditions I guess.

If no barometer GPS altitude is overwritten by Strava’s topographical data, and I think Garmin Connect does the same if you use their web site (can turn it on and off and see the difference).

In general, they’re all inaccurate and don’t compare well with each other. I just compare my own rides with the same device and generally they’re consistent.