About Ambit distance calculation

[ambit_cracker]

Hi Ambit afficionados,

I saw your thread Abmit Route and Distance Anomalies where some wonder why the distance obtained on the Ambit differs from the one if the track is reimported to SportTracks/RubiTrack/.... (The Ambit being always shorter than SportTracks/RubiTrack/....). Did you guys get over that small issue, or are you still interested to know what explains this difference?

[srwilson]

Hi Ambit afficionados,

I saw your thread Abmit Route and Distance Anomalies where some wonder why the distance obtained on the Ambit differs from the one if the track is reimported to SportTracks/RubiTrack/.... (The Ambit being always shorter than SportTracks/RubiTrack/....). Did you guys get over that small issue, or are you still interested to know what explains this difference?

Still interested.

[ambit_cracker]

Still interested.

Ok, good. Before revealing how I did understand the issue, I propose that you send me via PM an Ambit log file in xml format (zipped) of one (or few) of your track. I will send you back a gpx version of it (thanks to joelc for it's python script), and you tell me what you think about what SportTrack/RubiTrack/... says about it. Ready to play?

[srwilson]

Ok, good. Before revealing how I did understand the issue, I propose that you send me via PM an Ambit log file in xml format (zipped) of one (or few) of your track. I will send you back a gpx version of it (thanks to joelc for it's python script), and you tell me what you think about what SportTrack/RubiTrack/... says about it. Ready to play?

OK you got it, or a link to it. I'm not sure you can attach files to a PM. Anyone know if that's possible?

Anyway, you have a link to a log.
Here is the results from Ambit vs Sporttracks

Ambit Watch: 10.0
Movescount: 10.07
Ambit Track imported into SportTrack: 10.24


Oh and I've use joelc python script too.

Update:
oops... I see the attachment button now. sorry...

Update #2:
Nope, nope, I only see the attachment button in the advanced forum post window, don't see it in the PM window. So question still stands.

[ambit_cracker]

OK you got it, or a link to it.

Thanks, I did send you in PM my filtered gpx file. Let's see what SportTracks tells about it

[srwilson]

Thanks, I did send you in PM my filtered gpx file. Let's see what SportTracks tells about it

Imported distance of 10.11 in SportTracks. That is very close to what the Garmin posted.

So what's the secret? Looks like you have it figured out!

[ambit_cracker]

So what's the secret? Looks like you have it figured out!

For sure, I would not have chosen my pseudo if I would not have
If you read French, you can read the detail of my investigations here, written episode wise (I have also a life !):

Du calcul de la distance sur la Suunto Ambit - Kikourou

[srwilson]

For sure, I would not have chosen my pseudo if I would not have
If you read French, you can read the detail of my investigations here, written episode wise (I have also a life !):

Du calcul de la distance sur la Suunto Ambit - Kikourou

Thanks so much for sharing your investigation! I don't read French but thank goodness Google does and I think it did a good enough job for me follow your work (I hope).

I had noticed that the distance numbers reported in the logs did not change with each second of log data. I even thought there must be a key to when the number jumped, but I didn't spend the time to investigate it further. Did you figure that out? If you did I must have missed it.

So, does this boil down to a simple case of rounding of the meters? Seems rounding could go either way making the distance longer or shorter. I've always thought the distances to be short. Maybe I missed the part where you explained that and if so please forgive.

Nice work and if you'd like to explain further, please do!

[ambit_cracker]

Thanks so much for sharing your investigation!
Nice work and if you'd like to explain further, please do!

Yes, I will make a short summary in english in this post, just give me some more days to write that up.
In very short, Suunto's algorithm to compute the distance is to retain only a small fraction of all logged data, the distance between two retained GPS fixes being > 2*EHPE (this is the key you missed, but I did not yet come to this point in the french write up, Watchuseek get's the scoop first!).
In that way, it approximates a smoothing of the track, to "cure" the notoriously bad GPS accuracy of the Ambit.
When you import the original track to SportTracks, SportTracks computes the distance as the sum of distance between all logs (every second).
And because the original track resembles that of a drunken driver, it gets the distance too long (this is best sen for tracks in dense forest).
So the distance computed by the Ambit is finally closer to reality, except when in dense forest and with lot of turns, where the "filtered" track will cut corners.

[or_watching]

For convenience, here is the link with translation to English:
Google Translate

Indeed, the logged distance in the xml file is updated every few seconds, and not with every track point. Looks like it could be every 3,4 or 5 in this quick check I did. Apparently it can be even less often.

(See if I get all this right, ambit_cracker)

So, effectively the Ambit distance is a "smoothed" (or "filtered") value. SportTracks, etc, would use every point as logged.

You can see the "quantized" steps in the this chart of xml distance vs xml time.
As ambit_cracker says, the Ambit distance is then computed only when the distance is updated. He shows the calculation at the link. And so any in the tracks points between these distance updates are not captured in the distance (whether they be accurate positions or not). Upon upload to Movescount, the Summary and in-watch distance is used, and not recomputed from the raw track points.

This choice works well when there is more GPS track point error and the true track is straighter. It works less well when there is less GPS track error and the more zig-zaggy the true path is within the 3-to-5-7 second time frame.

Since the pace and distance in the Ambit are intertwined with both the GPS data and Fused Speed, I wonder if this choice is a consequence of that algorithm.
If anyone has a log with 1sec and FusedSpeed Off, does that show any different distance logging behavior? (increment every second vs every few seconds)