How to Merge Garmin and Apple Watch Ride Data

Why Ride with Two Devices?

It's one of the most common setups in cycling: a Garmin Edge head unit on the handlebars for GPS, power, and cadence data, plus an Apple Watch on the wrist for heart rate and fitness metrics.

Each device excels at what it does best. The Garmin Edge has a superior GPS antenna and connects directly to ANT+ power meters and cadence sensors. The Apple Watch provides continuous optical heart rate monitoring that's more comfortable than a chest strap for many riders.

The problem? You finish your ride with two separate activity files — and neither one has the complete picture. Your Garmin file has great GPS and power but no heart rate. Your Apple Watch file has heart rate but lower-quality GPS and no power data.

What Each Device Records

Understanding what each device captures helps you decide which data source to use for each channel:

Garmin Edge (head unit):

• GPS coordinates — high accuracy with external antenna
• Power — from ANT+ or Bluetooth power meter
• Cadence — from ANT+ cadence sensor or power meter
• Speed — from GPS or speed sensor
• Temperature — ambient temperature sensor
• Elevation — barometric altimeter (more accurate than GPS altitude)

Apple Watch:

• GPS coordinates — decent accuracy, but less reliable than a head unit
• Heart rate — optical wrist sensor, continuous monitoring
• Calories — estimated from heart rate and motion
• VO2 max estimate — Apple's fitness tracking algorithm

The file formats also differ: Garmin exports .FIT files (binary, compact, rich metadata) while Apple Watch activities export as .GPX through Apple Health or third-party apps. TrailBlender handles both formats natively.

The Merge Challenge

Merging data from two different devices isn't as simple as copy-pasting one file into another. Here's what makes it tricky:

• Overlapping timestamps — Both devices recorded the same ride at the same time, but their clocks may be off by a few seconds.
• Different sampling rates — Your Garmin might record a GPS point every second, while the Apple Watch records every 5 seconds.
• Different coordinate precision — GPS accuracy varies between devices, so the same moment in time has two slightly different locations.
• Different data channels — You need to pick which device provides GPS, which provides heart rate, which provides power, etc.

Doing this manually in a text editor means aligning hundreds or thousands of data points by timestamp. TrailBlender automates the entire process.

Step-by-Step: Merging Your Files

1. Export both files — Download the .FIT file from your Garmin (via Garmin Connect or USB). Export the Apple Watch activity as .GPX from Apple Health or an app like HealthFit or RunGap.
2. Open TrailBlender and drag both files into the window.
3. TrailBlender auto-detects the overlap — It matches the recordings by timestamp and shows a preview of the combined track.
4. Select your primary GPS source — Choose the Garmin Edge. Its dedicated GPS antenna provides more accurate coordinates than the Apple Watch.
5. Select heart rate from the Apple Watch — The optical wrist sensor provides continuous HR data without needing a chest strap.
6. Keep power and cadence from the Garmin — These come from dedicated ANT+ sensors connected to the head unit.
7. Review the merged preview — Check the map for GPS accuracy and scroll through the data graphs to verify all channels look correct.
8. Export as GPX or FIT — Choose FIT if uploading to Garmin Connect, GPX for maximum compatibility with other platforms.

Choosing the Right Data Channels

The key to a good merge is selecting the right source for each data channel. Here's a quick reference:

• GPS / Location — Always use the head unit (Garmin Edge). It has a larger antenna and better satellite reception than any wrist-based device.
• Heart Rate — Use the Apple Watch for optical HR. If you also have a chest strap paired to the Garmin, compare both and pick whichever looks cleaner.
• Power — Always from the power meter source (connected to the Garmin). Apple Watch doesn't record cycling power.
• Cadence — From the cadence sensor or power meter (connected to the Garmin).
• Speed — From the Garmin's GPS or speed sensor. More reliable than wrist-based GPS speed.
• Elevation — From the Garmin's barometric altimeter. GPS-based altitude (Apple Watch) is significantly less accurate.

TrailBlender lets you pick the source for each channel individually, so you get the best data from each device in one file.