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An embarrassment of GPS

Decided to do a long slow run today, ended up getting 15K. I have known for a while that my GPS watch, the Forerunner 220 runs a bit short on distance - and therefore pace - parkrun on my phone records 5k pretty much bob on, my Garmin is always 4.89k, so I ran 15K according to the 220 and came out as 15.25K on my phone. I used to be OK with it as I felt that at least its underestimating, so come to parkrun or a race and I go faster than I expect, and I prefer to be an underestimate as it makes me want to go faster. But now its just annoying as I always feel like checking my pace but I know its pointless.. Going to try a hard reset to see if anything improves, then might start leaving the watch. Today I found out about Strava, so can try that out on my phone. What I do get extra from the watch is cadence, and that is pretty accurate I ASSUME as it is measuring time accurately just not distance. Not sure if strava gives cadence too, my default tracking app on my phone doesn't (even though it counts steps).

Will report back regarding the watch reset in-case anyone is interested!

Have been looking at new watches too but would want to wait till the new updated wrist based HR stuff on the FR935 is put on cheaper models, I saw a prediction for such watches to be released around August. Would also have to track distance better than my current 220 obviously!

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Phenomenal - well done!

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Cheers Katie :) my knees are telling me to use the crosstrainer today! Hope you are OK!


Some background for you. All these watches and phones have 'GPS' support, and we use this term colloquially. Everyone knows what we mean when we say 'GPS', right?

But there are two standards of GPS... the old fashioned 'GPS' flavour, and the newer GLONASS standard. GLONASS is actually owned by the Russian Federation, whereas GPS is owned by the US government and operated by US Air Force.

Your older FR 220 uses the GPS system, and is a bit slower to locate the GPS satellite constellation. I bet that your phone is a 2015 model smartphone (or newer) and will use both the GPS and GLONASS system. It'll be much quicker to detect a lock, and (with more satellites) is more accurate in tracking your position. I vaguely remember something about GPS not coping well with leap-seconds, and this would have an impact on absolute accuracy.

That's just advancing technology for you. You may as well say "the battery life on this 5 year old laptop is not as good as the battery life on my iPad".

Personally I would not stress too much about the differences. Just pick one device to trust. Maybe that's your FR 220, since it shows your pace on your wrist. That is more helpful on a run than knowing that you did 4.89km when you finished.

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Cheers for that MarkyD, I've never considered the change between using one to both systems. Still though, my watch is consistently short on distance, It's difficult to see how that would come from inaccurate positioning. Infact if the sampling was at a reasnable rate I would expect distance estimates to consistantly longer due to noise in each position (if the positioning was inaccurate). I looked on-line also and found people with 220s that were constantly short, long and reasonably accurate. The only thing I can think would make it constantly short would be under-sampling the route, I cant realy see that when I look at the route however. Ive also tried elevation corrections on & off to no avail.

The other thing was the pace it gives, I cant trust that either. I did parkrun in 24:11 last time, my average pace from the garmin was 25:05, I did fail to turn it on at the start. My average moving pace for 4.35k was 5:00, I feel like I paced myself fairly well but for the missing 0.65/0.54k I must have completed in 2:26 (2.433min) my average pace would have therefore needed to be ~3:45/4:11 per k. or 16/14.3 kph

Like I said I think I paced myself well :p

Which translates to me thinking on the day, I know my watch is slow so I think im just under 25minutes. Then at 4K the 24min pacer passes me and I was actually a minute under what I thought.


I also noticed that with my Garmin F10. My parkrun on strava (which my garmin links to) was recorded as 4.89k whereas people who were presumably running using the strava phone app on it's own had a distance of 5k exactly. I only have the garmin as I don't have a smartphone. Does the phone app use GPS (or similar) like the garmin does or does it triangulate off the phone masts? I would guess if it was the latter then it is less accurate than a garmin. But I may be wrong in that assumption

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Thats a good question , I'm not sure how it will works exactly but I assume its using the GPS from the watch and storing the details on strava on your phone. The Phone app will very likely use the phones GPS or a combination like MarkyD mentioned.


Just for info Kally, and in terms of only Garmins, the FR35, FR225, FR235, Vivoactive HR and FR735XT all have the wrist based optical HR like the new FR935XT.

I have the FR225 which has the same GPS hardware as your FR220 and based on my experience the distances measured are fairly accurate (5.02km was registered for my last tour of my local Parkrun route, for instance).

As Marky D mentions, the FR220 does not have GLONASS capability but I would still be genuinely surprised if it was less accurate than even the most modern smartphone.


Cheers Dunder, the optical HR on the FR935 has been improved from the info I have seen (chiefly dcrainmaker.com/2017/03/gar... )

"Optical HR Sensor: Revamped tech, now records 24×7 data every 1-2 seconds

Optical HR Sensor: Flattened out even more, virtually flush with back of unit"

According to the review the HR information is much more reliable than previous models (apart from the fenix 5 wich has the same improvements) I'm not paying that much for it though..

Yep from what I've seen people have had variable accuracy with the Garmin 220, implying some are OK. My 220 is always short (please don't use that as a quote :P).


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