GPS Accuracy

1. Train by numbers race by feel!
2. Align your expectations of handheld, consumer GPS, with reality.

GPS+GLONASS; I will do a comparison between the two different options tomorrow on the same route.

I can't remember where I read it but there was a suggestion that after getting a GPS lock wait until your pace shows as zero before setting off....

1. Train by numbers race by feel!

I did this last weekend and had an epic fail on my half marathon. I didn't watch the F3 at all because it was a hilly course I raced many times before. But last weekend I burned out totally before km15.... No watch on earth could have saved me because I started to fast like stupid... :rolleyes:


I prefer GPS only, because you would need at least one additional sattelite for clock synchronization only and from my experience with the F3, Glonass didn't give any accuracy improvements.
The additional batterie consumption can be neglected on "normal" 1-2hr activities. The F3 and F5 watches have plenty of battery life.

Obviously (and there are even CIQ/DFs that can auto-correct the distance and time ahead/behind) but you're missing the point, if you've trained for weeks at 4'30"/km on the open roads with your F5 and that translates to 4'33"/km during the race using actual/"mile marker" pace you have the two options I mentioned above, neither satisfactory.

Not missing the point, but if a difference of 3 seconds per kilometer is going to ruin your marathon then something is wrong with your race plan.

What happens when the 1st mile is a net elevation gain and then 2nd mile is a net elevation loss? Are you going to run those EXACTLY at your determined pace? Now THAT is a recipe for disaster

Sorry to hear about your HM but at least now you'll have to experiment with the "Auxiliary Heart Rate" DF ;-)

Right! That's what I'm about to do this evening!! Heartrate-testing, GPS-track-testing and so on. A lot of testing with two devices, hf belts, etc.... Maybe I should focus more on training to prevent another halfmarathon breakdown instead of testing overpriced equipment... :rolleyes:


I think, you can use the additional Glonass satellites minus one (for the sync). That means, if you have 4 GPS-Satellites and 3 Glonass satellites in view, you can use 4+2 in total. I expected a better signal and a faster returning of the track to the path after running under bridges or similiar but I never noticed any benefit. (Fenix 3 experience only)

I'm not sure, but I think there is a limitation of satellites anyway. So it makes no sense to have more than a certain number of satellites since they aren't part of calculation any more. I think I read it somewhere in a GPS documentation in the past (the times as I was frustrated about the F3-GPS-performance :) )

Yes it's hard to know what's going on GPS wise, it's too bad they removed the satellite screens of the old FR205 (even the FR405 had them I think) where you could see a "target" with the location of the satellites and a graph with the signal level of each satellite.

Have fun with AHR !

Found this on Facebook
The English might not be the best but you can get the point
A lot of detail on GPS
https://www.facebook.com/notes/andrew-nugged/gps-quality-is-changing-every-minute-consider-this-in-comparing-testing-and-comp/1177670918956558?hc_location=ufi

I did this last weekend and had an epic fail on my half marathon. I didn't watch the F3 at all because it was a hilly course I raced many times before. But last weekend I burned out totally before km15.... No watch on earth could have saved me because I started to fast like stupid... :rolleyes:

There are so many reasons why I coach my athletes to train by numbers and race by feel but here's some:

1. Running by pace is only of any use on a truly flat course where you can consistently hold the numbers.
2. Running by instant pace on GPS doesn't work, because instant pace on a handheld GPS doesn't work, hasn't worked, and is very unlikely to work.
3. Running to heart rate has so many limitations because it is affected by heat, humidity, fatigue, hydration levels etc.
4. Running by GPS distance or course markers is always going to lead to uncertainty as to which one is correct. Rarely, do they ever line up. And if they do, are you sure they are correct?

The only true measure when you race is RPE - rating of perceived effort. When training, relate how you feel to heart rate and pace and develop the ability to run without numbers.

Why should I do this when I'e just spent hundreds of dollars on an expensive watch to tell me all this? You do this because, as good as these watches are, they are not able to tell you how to run your race to your best advantage. The information is good enough for you to review and analyse progress, evaluate what's working and not working in training, but not good enough to entrust the outcome of any important race. That outcome should be under your control. Not delegated to an instrument.