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  • 310XT and open water swimming

    Since a lot of people ask about using the 310XT for swimming, I thought I'd post some results that I have had.

    I've worn the 310XT during two triathlons with open-water swims:

    http://connect.garmin.com/activity/11317098

    http://connect.garmin.com/activity/7832839

    Both of these swims were advertised to be 0.5 miles long. According to the 310XT data post-processed by Ascent, the 310 measured the swims to be 0.95 and 1.16 miles long.

    To measure the actual distance of the swims, I used Ascent to export the data as a kml files, and imported those files into GoogleEarth. Using the ruler tool, and estimating where the turns were based on the tracks, I measured the first swim to be 0.52 miles long, and the second swim to be 0.50 miles long, which are pretty close to the advertised distances. Note that the second activity had a permanent and well-defined small pier that we swam around, so this is probably a pretty accurate distance.

    I don't swim in a perfectly straight line, so I probably swam a bit more than the advertised and GoogleEarth-measured distances, but I would be very surprised if I swam more than an extra 0.1 mile.

    So, the 310XT measured the swim distance to be roughly twice the actual distance. Looking at the GPS track, that's not surprising. Before I did the second swim, I noticed that my 310XT estimated the GPS accuracy to be in the low-20 foot range. While I was waiting for the start of the swim, I put my hand in the water for a little bit, then brought it out, and the GPS accuracy was in the triple digits since the 310XT can't get a signal underwater. Once out of water, it seemed to quickly recover its good accuracy since it more or less really knew where it was from the last time it had a good signal. During the swim, this is repeated over and over, resulting in the tracks seen in the activities.

    No meaningful HR data were recorded in either swim.

  • #2
    Great information. Thanks! I guess the thing I take away from this is that the 310XT is not at all accurate for swimming if you're going to wear it on your wrist. It has to be in a position where it won't be constantly submerged. I suppose you'd have to go back to wearing it under your cap, as some have done with the 305.
    Phil M.
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    • #3
      310 Swim distance is better than the alternatives

      I get the same results. The swim distance reported is usually about double my actual distance. If you look at the track, it is back and forth a lot. But as was noted here, if you just look out the general outline of the swim such as with the measuring tool of Google Maps, you can come up with an very accurate distance. Try doing that with any other device (short of puting the 310 under your swim cap). Maybe someday Garmin can come up with an algorithm that does this automatically, trying to compute the actual corners and measure straight line distances. Garmins marketing is a little misleading at times but I don't think anywhere they claim that you can actually get a track during swimming so this is actually performing better than I would have expected.

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      • #4
        310XT -- Distance Reading While Swimming

        I'm having the same trouble. Swam 1500m in the pool and it said I swam nearly 3 miles. What setting gives you the closest accuracy? Have any of you tried "Distance - Nautical"?

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        • #5
          I fail to see why there is all this discussion, and disappointment, about the 310XT and it's lack of GPS (and HR) capabilities while swimming. It's never been sold as anything other than a timepiece in the water.

          Get over it! It's not Garmin's fault, it's physics apparently. The signal, as it stands won't transmit through water.

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          • #6
            As has been pointed out a number of times there are definite issues when swimming with the unit on your wrist, however, you CAN get accurate readings with the unit under a swim cap if the GPS track is important to you. This method only applies to open water swimming though, don't know any method that works well if you are swimming in a pool.







            Edited to add a few examples of swims from 1.2-2.4 miles. Units read respectively 2.58, 1.3, and 1.2 miles total. Say with some transition included that's pretty accurate and quite a good trace.
            Attached Files
            Last edited by KANSAIBEN; 09-11-2009, 10:44 AM.
            Anthony

            My GC Digest

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            • #7
              Originally posted by KKELLY9558 View Post
              I'm having the same trouble. Swam 1500m in the pool and it said I swam nearly 3 miles. What setting gives you the closest accuracy? Have any of you tried "Distance - Nautical"?
              Changing the units of display will do nothing to affect accuracy.

              AFAIK there is absolutely no setting on the device you can change to affect accuracy. The only thing you can do is to give it a better view of the sky, which means putting it on a different place on your body.

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              • #8
                Originally posted by PHILIPSHAMBROOK View Post
                I fail to see why there is all this discussion, and disappointment, about the 310XT and it's lack of GPS (and HR) capabilities while swimming. It's never been sold as anything other than a timepiece in the water.

                Get over it! It's not Garmin's fault, it's physics apparently. The signal, as it stands won't transmit through water.
                Nobody is really complaining in this thread. I started the thread because the accuracy of the 310xt while swimming is a common question, and I wanted to provide some real-world results.

                A couple of other people then asked (without complaining) a few other questions relating to accuracy. In their defense, not everyone understands that the GPS signal does not penetrate water, and not everyone realizes that the 310xt, worn on the wrist, most likely won't give an accurate distance. I do think that Garmin oversells this slightly, so this question is going to come up frequently, just like the "why doesn't my Forerunner give good elevation data" question.
                Last edited by DW152; 08-19-2009, 03:58 AM.

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                • #9
                  Originally posted by DW152 View Post
                  Nobody is really complaining in this thread. I started the thread because the accuracy of the 310xt while swimming is a common question, and I wanted to provide some real-world results.

                  A couple of other people then asked (without complaining) a few other questions relating to accuracy. In their defense, not everyone understands that the GPS signal does not penetrate water, and not everyone realizes that the 310xt, worn on the wrist, most likely won't give an accurate distance. I do think that Garmin oversells this slightly, so this question is going to come up frequently, just like the "why doesn't my Forerunner give good elevation data" question.
                  Agreed. And thank you for starting it. I for one have learned answers to questions that I could not get out of the salesman. A little late now, because I bought it and specifically for swimming. But nonetheless I am happy with performance on a run or a bike.

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                  • #10
                    I wonder is this as simple as the fact that your hand moves backward somewhat during the swim? With the 310xt on your wrist, it is moving forward then back slightly as you pull in the stroke, so it is not consistently moving and registering as a forward movement.

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                    • #11
                      Originally posted by RBIN.F.BROWN View Post
                      I wonder is this as simple as the fact that your hand moves backward somewhat during the swim? With the 310xt on your wrist, it is moving forward then back slightly as you pull in the stroke, so it is not consistently moving and registering as a forward movement.
                      I don't think this matters. The motion of your hand (~4 or 5 feet or so) is well within the uncertainty of your position, (typically > 20 feet) and indeed, it doesn't matter during the run when your hand is moving back and forth. If you look at the tracks that I posted, the jagged track during my swims was far more jagged than just me moving my arm around. Furthermore, the recorded motion is more random, and not a repeated pattern like you would expect if it was only caused by moving your hand back and forth.

                      The real issue is that every time you plunge your hand into the water, it loses the GPS signal, and it has to reacquire it. This was mentioned in the second-to-last paragraph of my original post. The latest firmware (2.9) is supposed to improve this, but I haven't had a chance to test it yet.

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                      • #12
                        I would say it's more than that, because the track moves all over the place. I think it's more a case of the unit losing the gps, and then reacquiring if but without a good lock, say only 2 or 3 sats tops before it's lost it again. Have you noticed with a gps device (say my phone) that when it's got a poor lock it shows your position as way away from where you are. I think of it as working that way. It's not getting enough time to acquire your exact position hence your track position jumps left, then 50mtrs right, then 100mtrs left etc etc.

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                        • #13
                          http://connect.garmin.com/activity/37710660
                          Swimming 900m in 50m pool. GPS measured 720m. Red line shows much more. I thing that is due to the new softw. version 2.90 where you can turn on in "other settings" menu "swimming". Which corrects the data somehow.

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                          • #14
                            Originally posted by DUSAN07 View Post
                            http://connect.garmin.com/activity/37710660
                            Swimming 900m in 50m pool. GPS measured 720m. Red line shows much more. I thing that is due to the new softw. version 2.90 where you can turn on in "other settings" menu "swimming". Which corrects the data somehow.
                            If it's an open air 50m pool then I think you need to practice your sighting

                            http://connect.garmin.com:80/activity/36538311

                            I put mine under my cap and got some pretty clean data. NB, I'm also using the QR kit so the unit sits quite happily on the top of my head without the strap flapping about which "might" make the initial seating of the unit tricky.

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                            • #15
                              Originally posted by DW152 View Post
                              I don't think this matters. The motion of your hand (~4 or 5 feet or so) is well within the uncertainty of your position, (typically > 20 feet) and indeed, it doesn't matter during the run when your hand is moving back and forth. If you look at the tracks that I posted, the jagged track during my swims was far more jagged than just me moving my arm around. Furthermore, the recorded motion is more random, and not a repeated pattern like you would expect if it was only caused by moving your hand back and forth.

                              The real issue is that every time you plunge your hand into the water, it loses the GPS signal, and it has to reacquire it. This was mentioned in the second-to-last paragraph of my original post. The latest firmware (2.9) is supposed to improve this, but I haven't had a chance to test it yet.
                              Correct. In my discussions with the engineering team, the primary challenge is the data dropouts - and the ancillary problem is the satellite acquisition phase. Because it loses the sats when it goes underwater, it has to re-aquire them. The reaquisition process inherently has to increase accuracy of a period of time (a few seconds). If the accuracy level is too low (such as +/- 200 feet), then in effect even if a data point is plotted - it's in the wrong location.

                              The 2.9 firmware update in openwater mode fixes this by looking at your data track as a whole during a post-processing analysis. Meaning, your distance is not available until after processing is done (once lap or stop is pressed). From there the software looks at the entire track and tries to determine which data points are invalid.

                              For an in depth look at how it works, and my discussions with the engineering team - check out this:

                              http://www.dcrainmaker.com/2010/06/n...openwater.html
                              Tri Blog: http://www.dcrainmaker.com

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