4.19.2009

How to Use GPS to Measure Distance, Speed and "Straightness" of Open Water Swims

Open water swimming has many additional challenges that pool swimming does not. One of those challenges is not knowing how far your have gone. In pool swimming it is easy to count the number of laps and multiply it by 25 yards or 50 meters. In open water swimming often you do not know the distances that you are swimming unless it has already been marked out for you by the race director.

Here are some tips that I have come up with to help figure out how far your swims are.

The first thing I do is look at the body of water where I am going to be swimming on Google Earth or Google Maps. I zoom in to the exact area that I will be swimming and try to map out a swim, looking for landmarks near the shore that I can use to figure out where I am and where to turn around when I am in the water. I look at the scale in the bottom left hand corner of the screen and use it to estimate the distance of the swim. This method is not always accurate as water levels are always changing and may be higher or lower than the current photos on Google Earth, but it is a good place to start.

About a year ago my wife bought me a Garmin Forerunner 305 for Christmas. It is a great tool for running and cycling because you can measure your speed, distance, and you can upload the data to MotionBased and see your run/ride on an areal map. I was always curious of how, and if, it would work for open water swimming. After some searching I found this article and this video on YouTube.



I prefer the method outlined in the article because I do not like having something hanging around my neck while I swim. Before a swim I will turn on the Forerunner (after making sure that it is fully charged) and get it ready so that all I have to do it press "start". I generally wear two caps and put the Forerunner in a Ziplock bag between the two caps. I position the GPS device on the back of my head so that I get the best possible satellite signal. Once I am in the water, all I have to do is start and stop the data collection.

The fun part is getting home and uploading the data to MotionBased. You can see statistics on how far you swam, elevation change, speed, etc. You can also trace exactly where you swam on an aerial map and see how straight you are swimming. It is also fun to search for "Open Water Swimming" and see some of the swims that other people have done.

Here are some images of swims at Jordanelle Reservoir and the Deer Creek Open Water Marathon Swim using the methods described above.


How do you choose where you swim? How do you figure out how far you have gone?

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