Is Garmin/GPS distance measurement inaccurate?

[ColinJ]

I'm not certain as to where the assumption of GPS not recording elevation comes from as calculations from GPS do provide an elevation component. The issue is in part that it can be inaccurate especially when changing rapidly, this comes more to the forefront in applications such as flight or skydiving. In such circumstances it is important to have barometric readings to provide elevation, which of course some (I'm not sure if yours is included) GPS devices also include.

As @potsy says though, the greater likelihood is that either you are setting off before the GPS has acquired a proper lock or possibly the recording interval is set too high and you are "missing" small sections of the ride. For example when recording on the Strava app I often find that roundabouts are recorded as a straight line and some corners are recorded as a straight line, for GPS applications this will result in an under recording of distance (See Reimann sum for how this works, although that evens out over time as a result of directionality not applied in this case).

The only times that my GPS has been seriously wrong were when I set off before acquiring lock on enough satellites.

As for elevation ... My ancient Garmin Etrex actually does pretty well on elevation. Most of the time it is within about 2% at places that I know the elevation of. For example, Cock Hill at the top of the A6033 between Hebden Bridge and Oxenhope tops out at 432 m. My old GPS usually shows it in the range 429 - 435 m.

Funnily enough and annoyingly, the elevation readings on my newer Garmin Edge 500 (which has barometric assistance) can be hopelessly bad. I calibrate it automatically every time I set off from home but it can be out by 60-80 m by the time I get back.

 

[I like Skol]

Daft question but are you making sure the gps has 'locked on' before you set off?

Mine can take up to a 1/4 mile sometimes, and if you've already pressed start it will not count that distance.

Also a good answer, but unfortunately not 'the' answer. I always put the GPS bike outside and switch on while I faff about locking up and fitting earphones/donning gloves etc. By this time the display usually reads 'GPS fix acquired' so I hit the start button and set off. I always wait for the satellite fix before setting off as it doesn't take long but I suppose at 15 mph, if it took just 1 minutes to get the fix then that would be 1/4 of a mile.

 

[Sharky]

Do you have any of the auto pause set?

 

[I like Skol]

Do you have any of the auto pause set?

Yes, auto pause below 2mph.

 

[Sharky]

Yes, auto pause below 2mph.

Does this explain it then? I have mine set to auto pause when stopped.

 

[lazybloke]

from memory, even a static gps measurement will generally have very poor vertical accuracy compared to long/lat accuracy. It's called dilution of precision.
I'd find a link but I knew how to cut and paste on my phone!

 

[I like Skol]

[QUOTE 4667375, member: 45"]Does a GPS not synchronise its route with whichever programme you upload it to?[/QUOTE]
I don't upload to anything. I only went to a garmin for the navigation capability. I can create a route online then load this to the GPS. Apart from that I just use it as a dumb unit and only want a [EDIT - accurate] distance and ave speed.

 

[Hacienda71]

[QUOTE 4667375, member: 45"]Does a GPS not synchronise its route with whichever programme you upload it to?[/QUOTE]
Strava and Garmin interpret data differently giving slightly variances in distance.

 

[huwsparky]

The only times that my GPS has been seriously wrong were when I set off before acquiring lock on enough satellites.

As for elevation ... My ancient Garmin Etrex actually does pretty well on elevation. Most of the time it is within about 2% at places that I know the elevation of. For example, Cock Hill at the top of the A6033 between Hebden Bridge and Oxenhope tops out at 432 m. My old GPS usually shows it in the range 429 - 435 m.

Funnily enough and annoyingly, the elevation readings on my newer Garmin Edge 500 (which has barometric assistance) can be hopelessly bad. I calibrate it automatically every time I set off from home but it can be out by 60-80 m by the time I get back.

What makes you think that the 500 is the one that's wrong provided that is the only way you have of measuring through barometric sensor?

 

[robgul]

Have to say I gave up with cycle computers about 5 years ago - both wired and wireless - and am "all GPS" (Garmin Edge 200 and Edge Touring)

When I was comparing the two (both mounted on the same bike) there were very slight variations over short distances of <20 miles but in the 50 - 80 range the difference was minuscule.

The other upside of GPS is of course the ability to swap from bike to bike and be confident that the results will be accurate/consistent (yes - I know you can get old style computers with 2 bike settings) - and above all convenient - with no risk of knocking the spoke sensor out of alignment.

Rob

 

[HLaB]

If you're relying on just GPS and not an additional speed sensor and there's a lot of trees/ tunnels etc and your GPS may drop out. IIRC for Ride London with a lot of tunnels the Garmin recorded 99miles (its supposed to be 100). Strava filled in the blanks but with straight lines and up revised it to 99.8miles and taking account of bends takes it to 100miles.

 

[ufkacbln]

They all have different algorithms and measuring characteristics which mean that compared to each other, accuracy will never be perfect

The easiest way forward is to use 1 system and treat it as a base line


Then if you want to increase effort, do so, measuring the increase from the base line

 

[ColinJ]

What makes you think that the 500 is the one that's wrong provided that is the only way you have of measuring through barometric sensor?

(1) I look up the actual elevations on OS maps.

(2) My house definitely didn't move 30 metres nearer to the Earth's core while I was out on today's 37 km loop but according to the 500 it had! Sometimes I switch if off at cafe stops and when I switch it back on it thinks the cafe is 70-80 metres higher/lower than it was 30 minutes earlier!

My old Garmin Etrex is pretty consistent - if I do a loop then the elevation it showed when I set off is the same (within a couple of metres) that it shows when I get back.

 

[User6179]

I notice with the Garmins there is a lot of folk cycling below sea level .

 

[Dogtrousers]

Yes. All measurements are inaccurate.

As @Cunobelin says pick one method and stick to it.