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open water GPS distance accuracy
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Hi All,

when you guys do triathlon in open water, what GPS distance readings do you get?

For me in my last races it was the following:

1.5k race => read: 1.7 k
1.9k race => read: 2.255 k
2k race => read: 2.4k
1.9k race => 2.16 k
1.5k race => 2.01 k

the watch I am using is a Suunto Ambit 3 peak.

But I don't want to embark on a discussion about open water GPS accuracy. I would just like to get a feel for the numbers good swimmers get...I have the impression that I am zigzagging a lot and maybe my GPS readings account for that. But if other swimmers also constantly get higher distances then maybe it is not so bad....

Thanks a lot for any kind of input.

Take care and enjoy your off season.
U
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Re: open water GPS distance accuracy [uw234] [ In reply to ]
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General rule of thumb I use is +/- 10% for most devices, assuming the track itself at least looks plausible (no totally wonky sections).

That does however assume a course is measured correctly, which many are not. It's more of a swag for buoy placements.


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My tiny little slice of the internets: dcrainmaker.com
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Re: open water GPS distance accuracy [dcrainmaker] [ In reply to ]
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The accuracy on my 910 was poor. Sometimes I didn't get any usable data, other times the distance was 300m short or long, so any data was pretty much disregarded. My 920 and Fenix5 seem very accurate. A couple of times I've had it read at the very most 100m long/short, but typically it's been pretty accurate.
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Re: open water GPS distance accuracy [dcrainmaker] [ In reply to ]
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OK, that makes sense. My tracks are in the margin of error but then again the precision seems to be pretty high in terms of that I always score more to the +10% side and never to the -10% side. Could that then be device specific?
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Re: open water GPS distance accuracy [uw234] [ In reply to ]
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You are correct, generally speaking you'll see things more towards the +10% than the -10%. This is because each time the wrist goes under the water it loses GPS signal. Ideally, when/if it re-acquires it when your wrist is briefly above the water, the accuracy is +/- a couple meters. But sometimes that doesn't happen, and the filters don't throw out the data. So it ends up being something like 20m or 30m away. It's why you tend to get so many wobbles in a track. All of which almost always result in the addition of more data.

A scenario where you get less distance is if you make a sharp turn and it's unable to plot points in both the lead-up to the turn and after the turn, in which case it would show you cutting a corner.

In general, historically Suunto has had the best OW GPS tracks with their Ambit series. With their Spartan series I haven't seen as strong of tracks as the Ambit ones, and I tend to find Garmin/Suunto/Polar pretty similar these days for their latest gen products.


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My tiny little slice of the internets: dcrainmaker.com
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Re: open water GPS distance accuracy [dcrainmaker] [ In reply to ]
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Am I the only one who straps his watch to the back of his goggle straps. I always do it for open water training and sometimes in races too. I'm B/MOP so might lose a few seconds but seems to be more accurate.

My race site: https://racesandplaces.wixsite.com/racesandplaces
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Re: open water GPS distance accuracy [uw234] [ In reply to ]
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For training putting your watch on one of those towed swim buoys works great if you're not worried about stroke info. That said I always train in the same lake with the same course marked out so a stopwatch is perfectly adequate.
Last edited by: InvictaScoop: Oct 17, 17 1:45
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Re: open water GPS distance accuracy [dcrainmaker] [ In reply to ]
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thanks a lot for the explanation....unfortunately you have just destroyed my illusion of not being that slow, if only I could swim straight ;-)...but hey, knowledge is supposed to be power.
U
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Re: open water GPS distance accuracy [uw234] [ In reply to ]
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Cool question. Here are mine...

1.2 miles → read 1.24 miles 3.3%
600M → read 625M 4.2%
1.2 miles → read 1.13 miles -5.8%
600M → read 667M 11.2%
0.8 miles → read 0.79 miles -1.3%
3.2 miles → read 3.36 miles 5% (I am pretty sure this course was long, so the watch may have been more correct)
.93 miles → read 0.88 miles -5.4%
1.2 miles → read 1.17 miles -2.5%
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Re: open water GPS distance accuracy [uw234] [ In reply to ]
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There have been so few courses that I've done that have been set up accurately that it's difficult to say whether my 920 is accurate.

I do practice swim in a local lake and I get .4 miles across it every time I swim it. Other people get the same thing, we measure it on Google Maps and get .4 miles as well.
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Re: open water GPS distance accuracy [uw234] [ In reply to ]
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As soon as you stick your watch under the surface of the water, it loses GPS signal. So the only time it can log GPS is when you're in the recovery phase of the stroke. GPS watches are much better these days at guessing where it was when the signal was lost, but there's a pretty good error margin. I've found that whenever I want a really accurate GPS distance, I put my watch just below the crown of my head, under my swim cap.
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Re: open water GPS distance accuracy [exxxviii] [ In reply to ]
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...interesting. your numbers are closer than mine. Are you a good swimmer?
U
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Re: open water GPS distance accuracy [Jigsy] [ In reply to ]
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Jigsy wrote:
Am I the only one who straps his watch to the back of his goggle straps. I always do it for open water training and sometimes in races too.

I used to do that with my old Timex GPS watch, that didn't have any swim stroke data.


Member of the Litespeed Factory Team
www.litespeed.com
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Re: open water GPS distance accuracy [uw234] [ In reply to ]
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I'm using a 920 and find it to be pretty accurate. My swim stroke is pretty strong and consistent. I swim straight and most of my open water swimming is done on a Lake where I can also use measured markers that the rowing/canoe club have put out. It is a one kilometer stretch marked every 50 meters and it pretty much always matches up with my watch readings.
I did a few swims this year for fun where I didn't hold a normal consistent stroke pattern/rhythm and threw in a lot of turns and the distance wasn't accurate. The watch over read by 152m over a 750m distance, so 902m.
In my experience the straighter and more consistent my stroke is the more accurate the watch is.


Member of the Litespeed Factory Team
www.litespeed.com
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Re: open water GPS distance accuracy [uw234] [ In reply to ]
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I consider myself a middle-of-the-pack swimmer. And, I still struggle to swim straight. However, I have also improved a great deal since last year. My first race was the New Orleans HIM last year. That was a 1:55/100yd pace (according to my watch) in a wetsuit that included some treading water an a lot of breast stroke. My swim rank was 364 in a field of 1,600 (23%). Earlier this year, I did a 5K OWS (no wetsuit) for giggles. I think I was one of the last few out of the water with a 1:57/100yd pace. In sprints and Olympics, I swim around 1:40 - 1:45 without a wetsuit.

Edit: I should also have added that this is with a Garmin 910XT (NOLA HIM swim) and Garmin 735XT (everything since NOLA). Some GPS watches may be better than others in the water, and that is something worth considering.
Last edited by: exxxviii: Oct 17, 17 7:33
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Re: open water GPS distance accuracy [uw234] [ In reply to ]
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I got 4,444 yds @ IMLOU on Sunday w/ my 935. I would assume the course is pretty accurate (11th running). I know I swam wide at the first buoy (on purpose).

4,224 yds would be straight-line distance. I claim no level of accuracy on the course, though.
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Re: open water GPS distance accuracy [dcrainmaker] [ In reply to ]
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I wish I saw this back then. I bought a Spartan Trainer and it is so wildly off on the swim, I can't use it. The WHR is basically crap, too. So disappointed. Luckily I still have my Ambit 3 Sport. Now what to do with this useless POS watch. Too late to return to vendor.

Proud member of FISHTWITCH: doing a bit more than fish exercise now.
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Re: open water GPS distance accuracy [HalfSpeed] [ In reply to ]
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ebay
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Re: open water GPS distance accuracy [HalfSpeed] [ In reply to ]
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I always put my watch (910) in my swim cap when doing OWS. I'll also use a wrist-based watch (735), and the wrist one consistently under 'counts' the distance.

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Re: open water GPS distance accuracy [uw234] [ In reply to ]
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I have a Suunto Ambit 3 and have looked at my open water race results from the last 2 years - all of them have been -/+10% accurate to the actual race distance. For several of those races, the distance was adjusted afterwards due to swell / currents moving the buoys (these were all ocean swims and not triathlons - say a race was publicised to be 2.4km but actual distance was 2.2km).

Going by the -/+10% accuracy rule on a good watch - yes you might be zig zagging a bit. Do you have issues sighting? I used to have this problem until I got prescript goggles (seriously!) I'm not the fastest swimmer but I swim in a straight line.

I used to tuck another watch under my swim cap .. it worked ok and you could try his method to test the GPS accuracy, but this was too much fiddling around for me before a race..
Last edited by: snail: Mar 28, 18 12:17
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