How accurate is your GPS in Open Water Swimming?

Mark and Caroline test and discuss why GPS is often inaccurate in open water swimming.
Using a SUNNTO 9 and a Garmin Forerunner they swam the same lap 4 times.
Lap 1 - Two swimmers side by side
Lap 2 - Repeat of this
Lap 3 - Mark wears both watches
Lap 4 - Mark wears both watches on the back of his swim cap above water.
Inaccuracies vary between 12-70%, watch in full to find out more.
Pace and distance can both be wildly inaccurate, and even include elevation. Our conclusion is that wearing GPS in open water may not give you the information you think it does. We make suggestions about how you can structure an open water session on RPE and time instead.
Here are some reasons GPS may be inaccurate:
1. Water Interference: GPS signals do not travel well through water. Since swimmers spend a significant amount of time with their arms submerged, the GPS device on the wrist loses contact with satellites. This causes frequent interruptions in data transmission.
2. Movement: When attached to the swimmer’s wrist the GPS moves in a continuous circular motion during strokes. This results in erratic signal transmission as the GPS device only briefly clears the water on each stroke. The constant changes in position can cause the GPS to register fewer accurate data points.
3. Satellite Availability and Signal Reflection: In open water, especially in areas with natural obstacles like cliffs, dense forests, or buildings nearby, the GPS signals can bounce or reflect off surfaces, leading to inaccurate readings. Even in wide open spaces, if the satellite signal is temporarily blocked or weak, this can affect accuracy.
4. Motion and Speed Variations: Swimmers’ motion is irregular compared to running or cycling. The erratic pace of arm strokes combined with the GPS signal cutting in and out as the swimmer moves leads to errors calculating the actual distance and route.
5. Lack of Stable Reference Points: In open water, there are no stable landmarks to help correct GPS readings. This differs from land-based activities where GPS signals can often self-correct by referencing consistent signals from multiple satellite sources.
For more accurate tracking in open water swimming, some athletes use GPS devices that can be attached to a swim buoy or cap, keeping the device above the water for better signal reception. We tested our GPS units on the back of Mark's swim cap for the last lap to mimic this.

[@michaelhanson3150]

We have seen a similar thing in the docks where we swim, sometimes it misses huge chunks of laps out.

[@thebricksession]

In my experience it's important to teach this issue to novice triathletes at an early stage. It does have a very negative affect on some athletes who actually believe the numbers.

[@j-ph1150]

Man man man… what a waste of time

[@thebricksession]

@@j-ph1150 The GPS systems?