Distance totally wrong

Thanks, patsdiner. Your fix worked perfectly. It also correct elevation gain and average speed. 

Hi patsdiner, 

Thank you for proposed solution. Of course, I did exactly this even before I wrote the initial post. By exporting Gpx and importing it again into Connect I knew that FIT file was wrong (and gpx was correct). 

As you already suggested this is very unfortunate solution. First you buy a 400 euro equipment that you expect that correctly measures distance and automatically transfers the data to Connect (and Strava), but later on you find out that distances are wrong and that automatic transfer is of no use (in the case of wrong distances). 

The transfer that should be automatic should have the following manual steps:

1) login to Connect

2) export data as Gpx

3) delete the original data in Connect

4) import the (previously saved) gpx

5) manually update the basic info about the activity

If you use Strava in addition, you should also delete the wrong info in strava and upload the correct gpx. 

And after all this, you still have wrong distance on Garmin Edge! 

In short: really too much manual work to correct something that garmin should do correctly. 

Regards, Viljem

This has nothing to do with tree cover or buildings or anything else you're blaming to avoid having to fix your software/data file formatting. Anytime a ride is at a slower speed (including hikes/runs/backcountry adventure mtn bike rides) the fit and tcx files underestimate the distance (despite disabling autopause and more than sufficient data point density). As soon as you export to gpx the distance is correct (as in, it matches everyone you did the activity with - using older Garmin equipment or Wahoos). You need to make your software stop assuming that any activity done at a slower speed is not correct. There are plenty of mountain bike trails with 20% uphill gradients that people either hike a bike, or pedal up at 2mph. It's inexcusable that you can't get this right.

2 day old new owner of Garmin Edge 830 MTB bundle and same thing happening here. Just ridden a 6 mile route with the 830, Forerunner 245 and Locus Maps Pro on Galaxy S10 and the phone and 245 were within 0.01 miles of each other. The 830 was 0.49 miles behind. 

This is extremely worrying, Distance is crucial to me

I'm not sure if data recording interval might be relevant to GPS distance calc, but perhaps if the 830 is capturing data at a different rate than the other devices there might be a discrepancy.   If you have a power meter sensor active then the rate is 1/sec but otherwise you need to check in Home > System > Data Recording.

If you have a wheel sensor then that will take precedence over GPS (as others have mentioned).  In that case your 830 is not measuring the same thing as your other devices.

I use the 530 for Road biking, Mountain biking and hiking. My road bike has a speed and cadence sensor. When I follow a saved route the distance matches. My mountain bike does not have a speed and cadence sensor. When I follow a route the distance is often shorter than the route. Usually it's off by about a mile over a 15 mile route. The slower the speed the worse it is. When hiking the distance is off by 30% to 50%. If you go into Garmin Connect and save the GPX file to your computer then import it back into Garmin Connect, the distance will be correct. I think what's happening here is that the software in the 530 is filtering out GPS drift and it's including any low speed movement in this process. I believe a speed and cadence sensor on the mountain bike will correct this problem. For hiking there is no solution other than to do the above gymnastics with the GPX files. That will fix the distance after the fact but is no help while you are actually  doing the activity. Garmin is a stupid company. I have owned several Garmin products. There is always something that doesn't work correctly. Always.

I just went out on a 2 mile hike with the 530 and an old Legend HCX that I had laying around. The Legend had the hike at 2.03 miles and the 530 was at 1.6 miles. Then I exported the GPX file and uploaded it and then the distance was 2.09 miles. This is unacceptable. The GPS is fine. It's the FIT files that is messed up. Oh and BTW, on my road bike with a speed and cadence sensor (bontrager duo-trap) the 530 is right on. It matches the saved route I was following almost exactly. 

I cycled 29km yesterday in the Lake District with an Edge 830 mtb bundle using the speed sensor supplied with the bundle. Auto pause disabled etc and still it only recorded 12km. So the speed sensor makes no difference.

I've basically wasted my money on a useless piece of kit that doesn't do what it says on the tin. And I'll have to revert to using my watch as a device or recording rides.

I need a solution to this issue or be looking to Garmin for a refund for selling crap that that is not fit for purpose.

Was your speed sensor paired and communicating with the 830?  Was it set to manual or auto-calibration?