Fenix 6x GPS Accurate - But Records Incorrect Distance

And since your run is pretty simple, it's very easy to measure the actual distance of the race. I would say it's closer to 21.5km, looking at your start & end points. So the watches GPS measure a bit too much, and the correction is definitely too strong. I wish there was some kind of "correction factor" that you could modify in the watch advanced settings, to adjust by yourself. 

TobiasLj said:

That is quite common for many GPS-tracking devices.

I'm not sure I agree with your explanation - for example the device plots two points a and b one second apart with a distance of x.

Any more granular data that exists between a and b, call it c would only make the value of x larger, it could never make it smaller since by definition a line between a and b is the shortest possible distance between the two.

Therefore if there is more data, the only thing that could happen is that a longer distance would be recorded, right?

RISBAC said:

They must be using of a mix of accelerometer data + data correction for the GPS points to "improve" the distance.

This explanation makes a bit more sense to me, its just unfortunate that its never correct for me so now whenever I am running I have to subtract 10s/mile to get a pace that makes sense to me :/

D3v01d said:

'm not sure I agree with your explanation - for example the device plots two points a and b one second apart with a distance of x.

Any more granular data that exists between a and b, call it c would only make the value of x larger, it could never make it smaller since by definition a line between a and b is the shortest possible distance between the two.

Therefore if there is more data, the only thing that could happen is that a longer distance would be recorded, right?

The problem is the deviation (or what we call errors) that will make a straight line not so straight. Image that you run straight from a to b, which is about 200m. The watch will record your position every second and every second you will (according to the GPS) deviate from the straight line. When using the coordinates saved it will result in a longer distance than the one you actually ran. It is just like you said in your first post, 20.60km vs 21.7km. If the absolute truth for that run was 21.0975 (half marathon) 20.60 is much closer to that than 21.7km. 

We can always argue about which of them that is the most accurate but I find the F6 (and F3) more accurate than many others. There are so many threads about the Fenix being far off but everyone compares to something that is not proven to be more accurate. It is just an opinion, not based on facts and data.

From my point of view +- 2% is actually accurate for a consumer device that uses GPS for distance measurement, 20.60 is 2.4% from a true half marathon and 21.7 is 2.9% above.

D3v01d said:

I have to subtract 10s/mile to get a pace that makes sense to me

If pace is what you use to plan and measure your training and competitions I (and probably everyone else that uses pace during a race) would recommend that you invest in a foot pod. There is no GPS watch on the market that will give you the same accuracy.

If you use pace during training it doesn't really matter as long as distance is consistent on the same runs. You only measure yourself and the pace will always tell if you are faster or slower than what you decided to go for.

Personally I do use pace but just as my personal indicator during training and afterwards to compare the same runs. On races I use HR because that's what I'm used to, I know which HR I can stay on for a short time and for a long time and pace can vary depending on weather, elevation and terrain.

TobiasLj said:

From my point of view +- 2% is actually accurate for a consumer device that uses GPS for distance measurement, 20.60 is 2.4% from a true half marathon and 21.7 is 2.9% above.

Thanks for your replies, I will admit that I think 2% is a fairly reasonable number for accuracy

TobiasLj said:

would recommend that you invest in a foot pod.

This is interesting, I wasn't aware that the watch could use another source for distance when running. Is it not possible that my HRM Tri+ is giving the watch enough data to estimate how far I am running? I can't find much data on what the difference is between run dynamics and a foot pod.

I agree; If precise distance and pace are important (and for a road race or interval training, they are), either you manually do your laps, or you buy a pace pod. You can find tons of data here : https://fellrnr.com/wiki/GPS_Accuracy

It's interesting to see that :

-pace pods beat the GPS, any time. The Stryd seems really nice, never tried it though.

-the Mediatek GPS chips of most Garmin watch is not great (let's say that it sucks...)

-the newest watches don't really beat the older ones. The good old SiRFstar chips are hard to beat (my beloved Ambit 3 :-)

Read through this thread and I think that OP question isn't about the GPS accuracy. It's about why the distance measured in watch is so much shorter than the GPS track from watch.

Since my FR602, the accelerometer in the watch helps GPS to smooth out the data before calculating distance. Maybe to compensate for GPS errors.

I always run with a Polar V800 together with the Garmin 6x and 6x is always shorter and from time to time they differ considerably and I get a lower avg pace on the Garmin. Not so good for my ego:) or my more structured side of my brain. 

I then export the file and run it through a couple of tools, like fitfilerepairtool, plotaroute, mygpsfiles to compare the measured distance.

I havent done any scientific research, but I see a bigger diff between measured dist and GPS track when running trail than road.

phoglund_ said:

It's about why the distance measured in watch is so much shorter than the GPS track from watch.

Exactly right.

For anyone interested I did a run with a different heart rate monitor (not the HRM Tri+) and from one run on a notoriously GPS unfriendly (trees) run it was already better.

Makes me think that the watch is using the HRM as some sort of accelerometer like a foot pod does. I'll keep playing with it on "known" routes.

Also as a side note the reason I bring all of this up is that even if the measurements are _wrong_ I don't really care as long as they are _consistent_ so that I can keep track of my progress relative to _myself_. Absolute values are not really relevant to me, and since I had a watch that was giving me consistently slightly higher distances I'd like this one to do the same.