GPS in open water swimming is inherently less accurate than GPS for running or cycling. Here’s why — and what you can do about it.
Why Open Water GPS Is Challenging
The GPS chip in your watch needs line-of-sight to satellites. When your arm is underwater, the signal is blocked or degraded. The chip compensates by estimating position between signal acquisitions — which introduces error.
For freestyle swimming, your arm is underwater roughly 70% of the time. The watch acquires satellite signal during the recovery phase (arm in the air) — about once per stroke cycle.
This means GPS position updates happen every 1–3 seconds during freestyle. Running watches update position every second continuously.
Factors That Affect Accuracy
Device hardware: Dual-frequency GPS (L1+L5) devices like Apple Watch Ultra 2 and Garmin Fenix 8 are more accurate than single-frequency devices. Multi-band GPS provides more robust signal acquisition in challenging conditions.
Stroke type: Freestyle provides the most GPS acquisitions because both arms exit the water regularly. Breaststroke and backstroke provide fewer arm-out windows.
Pool vs. open water mode: Always select “Open Water Swim” mode. Pool mode disables GPS.
Satellite acquisition: Wait for GPS lock before entering the water. Most modern devices need 20–60 seconds outdoors to acquire satellites. Entering the water before GPS locks significantly reduces accuracy.
Conditions: Dense cloud cover and tree/building shadows reduce GPS signal quality. Outdoor pools with surrounding structures can affect accuracy similarly.
Typical Accuracy by Device
For a 1km open water freestyle swim, expect:
- Apple Watch Ultra 2: ±2–3% distance error (~20–30m per km)
- Garmin Fenix 8 / Forerunner 965: ±2–3% distance error
- Apple Watch Series 10: ±3–5% distance error
- Older devices: ±5–8% or more
For most training purposes, ±3–5% is acceptable. If you need precise distance (e.g., for structured training targets), use the more accurate devices.
Improving Your Results
Practical tips:
- Activate the workout and wait for GPS lock before entering the water
- Swim freestyle for the majority of the session
- Check that your watch’s GPS sleeve isn’t covered by a tight wetsuit
- Review the GPS map after your swim — a clean, tight track line means good accuracy; a wobbly, erratic line means poor GPS acquisition
When GPS Accuracy Doesn’t Matter
For most recreational open water swimmers, GPS accuracy within 5% is perfectly sufficient for:
- Understanding your pace per 100m
- Logging training distance
- Reviewing your swim route
It only becomes meaningful when you’re doing specific distance targets (like training for a 5km race and wanting to know you swam 5km, not 4.75km) or when you’re analyzing your navigation efficiency.
For the latter — comparing your GPS distance to the straight-line distance of your route — even moderate GPS accuracy is sufficient to reveal whether your sighting is off.