Is Garmin/GPS distance measurement inaccurate?

[User6179]

Do you even understand how GPS works? The reason we set a lowest speed is to remove the deviations in reported position that is inherent in all GPS position calculations as a result of calculation errors and signal fluctuation. If I want journey average speed I will turn off auto pause. Because I want moving average speed (I don't want to count the time I spend behind a bush having a pee or in a cafe stuffing my face with cake!) I select auto pause. The fact I have auto pause switched on means I am not counting the irregular position discrepancies that are inherent in the system as distance travelled when I am actually motionless. The problem I am experiencing is an under reporting of the distance travelled, and seeing as I probably spend less than 30m of a typical 10 mile ride moving at less than 2mph that 2.5% is being lost elsewhere.

I stopped using auto pause on Endomondo or strava , cant quite remember which one, because it sometimes came on when moving , I had the app set on voice notification so I would hear it speaking from my saddle bag , worth checking .

 

[User6179]

Blimey ... I thought that perhaps I was exaggerating, but I have just looked it up and the difference between high and low tide at Fleetwood/Knott End today is nearly 5.5 metres. Next week it will be nearly 9 metres - LINK.

There is a wee tidal river near me that is about 3 feet wide at low tide, sometimes almost running dry in the summer , it is 4 or 5 metres below the road and at some high tides it floods the road .

 

[Dogtrousers]

I've still got one of these somewhere

 

[User6179]

I've still got one of these somewhere
View attachment 336294

Pizza cutter?

 

[Salty seadog]

Is it really that important for a few meters?

 

[psmiffy]

Is it really that important for a few meters?

thats 0.37miles per day - could be 2.59miles per week - 135miles per year - 217,295.5m

 

[Aravis]

I've still got one of these somewhere
View attachment 336294

Wouldn't it take a long time to measure out a 100-miler with one of those? Or do you use it on a map?

I can remember trying to use roads with mileposts wherever possible. And keeping a dossier of regularly used sections of road as measured in my car.

 

[nickyboy]

Skolly, you and I cycle in similar areas. I use a GPS device like you (a Garmin 810). Assuming your other measurement methods are more accurate, the only possible explanation I can come up with are intermittent GPS outages which, when your devices locks back on, produces a straight line from the last lock.

I certainly get some of those. They are easy to spot when you upload the track to somewhere like Strava or RidewithGPS. GPS outages are not uncommon where we ride; steep valleys, heavily wooded areas. Would it also be the case that the more remote areas of Britain (like the Peak District) have fewer positional satellites available and thus outages are more likely?

Anyway, it's obviously not the effect of gradient and the autopause has almost not effect either (it only kicks in when you've almost stopped and restarts when you start cycling again and the device constructs a GPS line between these two points)

 

[psmiffy]

GPS outages are not uncommon where we ride; steep valleys, heavily wooded areas.

Yes

Would it also be the case that the more remote areas of Britain (like the Peak District) have fewer positional satellites available and thus outages are more likely?

No


knowing the computer is calibrated properly is the key - new computers I always calibrate against a 100m that I have measured outside - always get a different number than that quoted in the "standard! tables" - new number I'm normally within 3% - I then calibrate it against a 10km stretch of road and/or the GPS over at least 60km - new number is then normally within 1% - check/calibrate again the next day - repeat until it is less than 1% - job done

 

[DaveReading]

GPS outages are not uncommon where we ride; steep valleys, heavily wooded areas.

Have you stopped to consider what effect you're having on the rest of the world, that depends on GPS, when you cause a system outage every time you cycle through a valley ?

 

[I like Skol]

I did my calibration ride earlier in the week and I think I have identified the issue (but not the solution).
The ride is exactly 16 miles each way. On the way there the Garmin recorded 15.93 and once home the total was 31.90. Not bad and certainly better than other rides I have done. One thing I did notice when climbing a hill with trees on my left and overhead was that the speed was erratic, bouncing around from 2-6mph and occasionally flicking to zero due to the autopause function. Once out of the trees and still moving at the same speed the display settled to a steady and believable 7-8mph.
On the way back past this point I was travelling downhill at 30-40mph and on the other side of the road so much less tree cover. I have no idea how steady the speed reading was as I wasn't looking but the recorded distance for the return leg is better than 99.8% accurate against the mapping data.
Now I just need to play with the Garmin settings. Either I turn off autopause, as has already been suggested, or I find how to make it fill in any gaps. I obviously haven't teleported between two point in space while the unit has had no signal so I just need to make it fill in the gaps (a straight line between known points will be close enough).

 

[Crackle]

Speed sensor. You never listen, you.

Bontrager do one as well as Garmin and it's a fair bit cheaper. Especially if you get one 2nd hand on ebay as per me

 

[si_c]

Speed sensor. You never listen, you.

Bontrager do one as well as Garmin and it's a fair bit cheaper. Especially if you get one 2nd hand on ebay as per me

Rose bikes do a sigma one for under £20.

 

[I like Skol]

Speed sensor. You never listen, you.

Bontrager do one as well as Garmin and it's a fair bit cheaper. Especially if you get one 2nd hand on ebay as per me

OK say I was too lazy to hit Google and find this out for myself (about to start cooking....) What, if any, speed sensor set-up can I use with my Garmin edge touring?

 

[the_mikey]

GPS has really poor height resolution, the system is designed to provide really good lat/long accuracy, and while it can take account of height, it's quite poor. Some GPS units use air pressure as a more reliable gauge of height but even this will go awry if the weather is interesting or if you haven't given the unit a reference location and height to calibrate itself against.


Some solutions:

Enjoy the ride
Ignore the GPS