Sunday, June 12, 2011

Swimming with my Garmin 310xt, Part I

I've never swam with my 310xt.  It is a triathlon-oriented GPS watch, which basically means that it is water proof.  It doesn't mean, however, that the GPS signal works under water, or that the HR strap works under water.  When using the 310xt during a swim, it is basically a glorified stop watch.

There is one way to change things.  After doing some detective work on the internet, I found that the problem with the Garmin GPS signal is when the watch enters the water.  At that point in time, it loses the signal, and reconnects once exiting the water for the brief second that your arm is in the air during the recovery portion of a freestyle stroke.  The solution is to place the 310xt unit inside a swim cap, on the back of your head.  This places the 310xt in a position that is, theoretically, above the water during the freestyle stroke.

It's summer time, which means that the local high school pools are now open for public swimming.  My family was going to go there for an hour or so, so I decided to get in some laps while they played with the little kiddos.  I placed the 301xt in my swim cap, hopped in the pool and began my laps.  I wasn't too sure how this would turn out, but I figured I'd give it a try.  I only did 30 laps, but I figured that was a decent enough test distance.  So...drum roll is the resulting map:

It may look like it, but I promise, I was not drunk while doing this swim.
Basically, the GPS was having trouble locking onto my position during the swim.  And I didn't even do any flip turns! (<----mainly because I can't...)  All I did was swim back and forth over one of those little blue lines, 30 times.  My exact workout was this:

8 lap warm up followed by 1:00 rest
10 laps followed by 1:00 rest
10 laps followed by 1:00 rest
2 lap cool down

Here is what my 310xt recorded as my distances during each portion of my workout:

Splits 1, 3, 5, and 7 are the longer portions of my swim.  While splits 2, 4, and 6 were the rest intervals.
Maybe you noticed that the bottom line, "summary", shows a total distance of 0.85 miles.  That's because I went back and edited the total to show the actual distance that I swam.  If you add up the distances for each split, it only totals 0.23 miles...not even close to what I actually swam.  Splits 3 and 5 should have shown a distance of 0.28 miles, roughly.

For me, it was no surprise that the data turned out this way.  I was a little stunned at the distances being so far off, but I knew that having the 310xt go underwater periodically, and doing small back and forth laps, was going to cause some GPS issues.  I have read that the GPS does better during an open water swim, in which the GPS has more of a chance to "connect the dots" and make an estimation of your distance and path taken.  One of these days I'll head on out to the nearby lake (because we don't really have those all over the place in Arizona, there really is only one nearby lake that I can swim in).

GPS doesn't really matter when you're swimming laps anyways.  The only time it would be interesting would be during a race that is promoted as XX distance.  Hopefully I'll get it figured out enough to be able to take it into the swim during Ironman Arizona, and see how far I actually swim!

*I failed to include some key photos in this post.  To see those photos, head on over to "Swimming with my Garmin 310xt, Part II"!


  1. Hi! Nice post. Do you use 1 sec calc./rec. interval in your Garmin 310xt settings? It became available via 3.7 FM update. I m curious as it should make it more accurate... maybe...

  2. Thanks! I do currently use 1 sec calc./rec. in my Garmin 310xt settings. Every since firmware 3.7 came out, I switched. I think the issue was that I was swimming so incredibly fast that the GPS couldn't read well...or not. I haven't gone out and done another test since this, and I should. I'll make it a point to go out and try the different recording settings and post my results. Thanks again for the comment!

  3. Great idea Jason. I tried my 310 out in a lake for the first time this week, but the resulting data was worse than useless, it had me stopped for most of the time! I'll try this option next time.