UKH

Went for a run this afternoon. My Tomtom Runner watch told me it was 7.8km. Synced with Strava and the reported distance didn't change.

But then I measured the distance on Google Maps and it told me I'd covered 8.2km - it was a simple there-and-back along a pretty straight road and I could select start and turnaround points very accurately.

The difference is ~5%, which is not insignificant. My question is: which figure will be more accurate?

Different mapping systems use different ways of measuring. Strava reduces the number of data points from most watches (recording at 1 s intervals) and may remove a little distance occasionally as a result. I have no idea about Google Maps, but other mapping software I have loaded to has increased the distance and changed the height gained without a good explanation that I can see. 

Thanks Steve. I can see how height gain could be very inaccurate, as even recording altitude itself is often fraught with difficulty, but I would have expected distances along a well-mapped road to be pretty accurate, certainly to within less than 5%. Don't suppose it's really important one way or the other, but it would be good to explain the difference.

Can you examine the recorded route on Strava? Do the start, turn and end all line up with your measurements on Google Maps? 100m out on each of those would be enough to account for the difference.

I've looked at it as closely as I can, and to the best of my knowledge I don't think I can be more than about 20m out, which would be 40m in total.

Do you have the local OS map? You could measure it the old fashioned way on there and see which electronic version is closest

Great idea, except that ...

I'm currently in Oman.

Ok, not quite so useful! Lol

I'd be inclined to go with the watch and strava. My tomtom watch shows if I chage lane on a running track, which is a distance change of about 1m or so.

Google on the other hand, are mapping the world. They are also gluing satellite images over street maps of varying quality. This is then backed up with google cars driving around making yet another measurement. Considering all of those variables and the scale of the achievement, they do a pretty good job. However, a dedicated hand held gps is probably going to be more accurate.

I have the Runner 3 and it underestimates my distances - for example local parkrun which I have confirmed is a dead 5k is always 4.75 on my watch. I thought the reason for this is that it is a very curvy, undulating and tree lined route. You say your route is straight so wouldn't have expected it to be much out. Done an sync just before starting?

I tend to do all my training using the same route so I am just hoping that all this underestimation will translate to better times when I next do an official race elsewhere.

That's curious because it's exactly the same proportional difference as I was observing. Like you, I would have sought to explain it by gps-trace corner-cutting, but there really was precious little scope for that on my route, and if dread-i is right in it being so accurate as to record lane changes, I doubt the cuts would add up to much anyway.

Maybe I'll just add 5% to all my recorded speeds to make me feel better

If you were in the UK I'd suggest the OS website as that should be accurate.

As for speed, do a race on a measured course of the appropriate distance to see how fast you are. Or find a proper running track and time yourself (if you can cope with the boredom of doing loads of laps).

Otherwise chill. You have very little chance of being as fast as when you were young

In my examples it is because all curves are inside the line of where I actually ran - its quite consistent that way, and I see it as normal/expected for my routes.

Additional to this I do have an occasional issue (maybe 1 run in 20) where the distance is marginally short, and it sounds more like what you describe. For instance the first part of my run is pretty straight and no trees etc, so I always hit the same 1km marker pretty much withing a couple of metres. Sometimes it falls about 30 yards later. Next time out it is back to normal. I always do a GPS update before running, so maybe sometimes the update file is inaccurate/out of date.