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The map ends in a straight line across a bay. The course was 1,500 meters, the watch says 1,340, and the track cuts a diagonal through open water that nobody swam. The swim felt honest. The file does not.
Open-water GPS is not broken; it is working with a poor view of the sky. The wrist spends much of each stroke underwater, and the watch fills the gaps with estimates. Once you understand that process, you can set up the watch, swim the course, and read the result without expecting pool-level precision.
Why the Track Looks Like a Zigzag
A GPS receiver needs line of sight to satellites roughly 20,000 kilometers away. Water absorbs the radio signal at the frequencies these watches use, so a fix is only possible when the wrist is above the surface. In freestyle, that is a small fraction of each stroke. The watch might get one good position every few seconds, or none for a minute in choppy water.
The antenna is also small and sits close to the skin, which is not an ideal place for reception. Add the motion of the arm and the result is a scatter of points that the software has to connect. The zigzag is not random noise; it is the watch doing its best with intermittent data. The science of outdoor GPS tracking explains the same constraints in hiking terms.
Get a Lock Before You Get Wet
Start the activity on dry land, in open sky, and wait for the watch to confirm a lock. Thirty to sixty seconds is normal. If you start the timer and jump in immediately, the watch may still be searching while you swim, and the first several minutes can be missing or misplaced.
A warm start, where the watch has recent satellite data, is faster than a cold start after a flight or a week in a bag. Some watches show a satellite page with signal bars; use it. Avoid starting under a boat canopy, beside a tall building, or in a dense tree line. Once the lock is stable, the watch can usually hold it between strokes, even if individual fixes are imperfect. The different satellite systems and what they add are covered in the comparison of GPS, GLONASS, and Galileo.
Stroke Type Changes the Track
Freestyle keeps one wrist underwater for most of the cycle and sends the other above the surface during recovery. Breaststroke brings both hands forward near the surface and gives the watch more frequent exposure. Backstroke is similar, with the watch arm often clear of the water. If you mix strokes, expect the track quality to change with each segment.
Which wrist you wear the watch on matters less than how high the arm rises. A tight strap helps the optical sensor, but it does not improve GPS reception. If you wear the watch under a wetsuit sleeve, the neoprene covers the antenna and can block the signal entirely. Wear it outside the sleeve, or use a strap extender so the watch sits clear.
Sighting Without Breaking the Line
Sighting is the swimmer's navigation system: lift the head, find a fixed point, and correct course. It also gives the watch a moment above water. Sight every six to ten strokes in open water, more often in current or waves. Pick a landmark that will not move, such as a building, a headland, or a moored boat, and line it up with the next buoy.
Do not rotate the whole body to look; lift just enough to see forward, then settle back into the stroke. A rushed sighting costs a stroke and disturbs the rhythm, while a calm one costs almost nothing. If the watch supports a compass or bearing screen, set the bearing before you start and check it at each sighting. The habits that make GPS tracking reliable on land, described in the guide to accurate outdoor tracking, apply even more strongly in water.
What the Watch Does With the Gaps
Between fixes, the watch draws a straight line. If you swam a gentle curve and the watch missed the middle, the line cuts the corner and the distance comes up short. If the points are noisy, the line wanders and the distance comes up long. Most open-water algorithms also apply smoothing and reject obvious outliers, such as a point on land or a jump of 200 meters in two seconds.
This is why a track can look plausible and still be wrong by five to ten percent. A 1,500 meter course may read 1,380 on a choppy day and 1,620 on a day with poor fixes. Neither number means the watch failed. It means the watch reconstructed a path from partial evidence, and the reconstruction has a margin of error.
Reading Distance, Pace, and Mode Choice
Treat open-water distance as an estimate with a tolerance, not a survey. Compare it with the known course length, then compare that ratio across swims. If your tracks consistently read eight percent short on the same course, you can adjust your expectations without adjusting the watch. Pace per 100 meters inherits the same error, so a fast-looking split may simply be a short track.
For training, use time and effort as the primary measures, and use the track for shape and navigation. A pool session and an open-water session are not directly comparable anyway: turns, walls, draft, and wetsuit buoyancy all change the cost of swimming. The comparison of pool and open-water data goes deeper into those differences.
Mode choice follows from the same logic. Pool mode counts laps from turns and accelerometer patterns, assuming a fixed length and a wall every 25 or 50 meters. In a lake there are no turns, so pool mode either records nothing useful or produces a lap count based on stroke interruptions. Open-water mode uses GPS and does not expect walls.
Using open-water mode in a pool is less harmful but still imperfect: the GPS signal is weak indoors, and distance may be estimated from stroke count. If you forget to switch modes, the file is not ruined, but do not read it as a precise measurement. Check the mode before every swim; it is the single most common cause of nonsensical swim files.
When the Track Still Fails
Some conditions defeat wrist GPS regardless of settings. Deep coves with high walls, dense tree lines along a river, heavy swell, and long periods of butterfly or underwater work all reduce fixes. A wetsuit sleeve over the watch, a low battery, or a watch that never acquired a lock before entry will also produce gaps.
If a track fails repeatedly at the same location, change the start point rather than the watch. Begin a few hundred meters away with a clear view of the sky, or swim a course with fewer turns. If you need a reliable route record for safety, carry the watch on a tow float or use a phone in a waterproof case; both keep the antenna higher and drier. The guide to GPS accuracy under cover explains why tree lines and buildings cause the same problem on land.
A Pre-Swim Routine That Produces Usable Files
Five habits cover most of the failure modes.
- Check the mode and battery before you leave the car.
- Start the activity in open sky and wait for a confirmed lock.
- Wear the watch outside the wetsuit sleeve and snug against the wrist.
- Sight every six to ten strokes and hold a steady line.
- Stop the activity on shore, not in the water, so the last fix is clean.
What matters most: a usable open-water track comes from good reception and honest reading, not from a better algorithm. Expect a few percent of error, compare the same course over time, and use the map for direction rather than proof.
Frequently Asked Questions
Why is my open-water distance shorter than the course? The watch connects scattered GPS fixes with straight lines, and those lines cut corners on curves and turns. A five to ten percent shortfall is common. Compare several swims on the same course before deciding the watch is consistently off, and check that you started the activity before entering the water.
Can GPS work underwater? Not usefully. Water blocks the signal at watch frequencies, so a receiver needs the wrist above the surface. Brief fixes during recovery strokes are enough to build a rough track, but long underwater segments will always be estimated. A tow float or a phone in a waterproof case keeps the antenna higher.
Should the watch go over or under a wetsuit sleeve? Over the sleeve. Neoprene covers the antenna and can block reception, especially when the cuff is tight. If the strap will not close over the cuff, use a longer strap or an extender, and keep the watch face on the outside of the wrist. Test it on a short swim before a long one.
Does breaststroke track better than freestyle? Usually. Breaststroke keeps the hands near the surface and gives the watch more frequent views of the sky. Freestyle still works, but expect wider spacing between fixes and a smoother, less detailed track. Stroke rate and chop matter as much as the stroke type.
How accurate is open-water pace per 100 meters? It is only as accurate as the distance estimate, which can be off by several percent. Use it for effort trends rather than exact pace. For precise splits, time a marked course or use a pool session with known lengths, and compare like with like.