Format intake

FIT, GPX, and SRT formats for telemetry overlays

The file extension matters, but the recorded fields matter more. This guide explains what OVRLEY can usually read from each activity format and why two files with the same extension can produce different widgets.

OVRLEY turns activity data into video overlays: FIT, GPX, and SRT can all be useful, but they store data differently.

Use FIT when you want the richest sport and sensor data.
Use GPX when you mainly need a route, elevation, and GPS-derived movement.
Use SRT when your drone exports telemetry as subtitle data.

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Portrait cycling footage with VO2 max title, power, speed, and heart rate widgets.

FIT vs GPX vs SRT, without the mystery

Each format was built for a slightly different job and understanding the differences will help you choose the telemetry source to get the exact overlay you want.

FIT

Best for: Sport recordings with paired sensors and device metadata.
Usually contains: GPS, timestamps, elevation, speed, heart rate, power, cadence, temperature, laps.
Watch out for: Only shows sensors that were paired and recorded during the activity.
Recommended when: You want the richest activity file for cycling, running, hiking, or training footage.

GPX

Best for: Route-first activities and broad compatibility between apps.
Usually contains: Latitude, longitude, elevation, time, and sometimes app-specific sensor extensions.
Watch out for: Re-exported GPX files can remove sensor fields. The format is a wild west.
Recommended when: You mainly need map, elevation, distance, and GPS-derived movement widgets.

SRT

Best for: Drone action camera footage.
Usually contains: Timed text entries with camera GPS, speed, altitude, and camera-specific values.
Watch out for: Usually does not include external sensors; quite variable in what it contains.
Recommended when: Your GoPro, Insta360, DJI drone or other camera exported telemetry as subtitles.

These are typical scenarios, they can differ between devices.

Which overlay widgets can each format support?

There are two types of widgets - some directly display recoreded data. Others, such as speed or heading, can be calculated by OVRLEY from basic data such as GPS coordinates. In both cases, a widget can only show a metric if the file contains the source telemetry data.

Data support for FIT, GPX, and SRT telemetry overlay widgets.
Metrics / widgets	FIT	GPX	SRT
Course, Elevation, Altitude, Time	Direct	Direct	Direct
Gradient	Calculated	Calculated	Calculated
Speed, Pace, Vertical speed	Direct	Calculated	Calculated
Heart rate, Cadence, Power, Temperature, G-force, Air pressure, Ground contact time, Left/right balance, Stride length, Stroke rate, Torque, Gear position, Vertical oscillation, Core temperature	Direct	Possible	Unavailable
Vertical ratio	Possible	Possible	Unavailable
ISO, Aperture, Shutter speed, Focal length, EV, white balance	Unavailable	Possible	Direct

Course, Elevation, Altitude, Time

FIT Direct GPX Direct SRT Direct

Gradient

FIT Calculated GPX Calculated SRT Calculated

Speed, Pace, Vertical speed

FIT Direct GPX Calculated SRT Calculated

Heart rate, Cadence, Power, Temperature, G-force, Air pressure, Ground contact time, Left/right balance, Stride length, Stroke rate, Torque, Gear position, Vertical oscillation, Core temperature

FIT Direct GPX Possible SRT Unavailable

Vertical ratio

FIT Possible GPX Possible SRT Unavailable

ISO, Aperture, Shutter speed, Focal length, EV, white balance

FIT Unavailable GPX Possible SRT Direct

Empty widget typically means the source file did not include that field.

Decision order

Which format should you use?

When you have more than one export available, start with the original file if possible, or a one that preserves the most original data.

Build your overlay Original recordings usually beat re-exported copies.

Tunnel entrance footage with route, speed, power, elevation, and gradient widgets.

OPTION 01 FIT

Choose FIT first

sensor-rich activity

Use FIT for any sports activity if you have it available. It is most consistent.

OPTION 02 GPX

Choose GPX for route

track and elevation

Use GPX when the overlay mainly needs a map line, elevation profile, timestamps, or GPS-derived speed.

OPTION 03 SRT

Choose SRT for drones

subtitle telemetry

Use SRT when the camera exported it alongside the clip and when you want high update frequency.

Portrait mountain road footage with large speed, power, heart rate, balance, and elevation widgets.

Same format, different fields

Two files with the same extension can contain very different telemetry. The file format tells OVRLEY how to open the file, but the device and export settings determine what data is actually inside.

FIT is largely standardized, so metrics such as heart rate, cadence, power, and GPS data are stored consistently across supported devices.

GPX is less standardized. It primarily defines GPS coordinates, elevation, and timestamps, while additional metrics are up to vendor, stored under custom tags in extensions. As a result, two GPX files may contain very different telemetry.

The same applies to SRT files. There is no standard for telemetry stored in SRT subtitles, and even the same manufacturer may use multiple formats. For example, DJI devices use different SRT layouts depending on the product.

This is why two files that look similar in your file browser may support very different metrics in OVRLEY.

Embedded telemetry

What about telemetry inside MP4 files?

Some action cameras store GPS, motion, or camera metadata inside the video file instead of a separate activity file. OVRLEY can read telemetry from MP4 files of GoPro, DJI, Insta360, Sony, Cannon and other cameras, so you do not need an external separate telemetry file to create your overlays.

Telemetry data inside MP4 files is unfortunately not standardized which makes the process challenging. Different manufacturers, and even devices from the same manufacturer, may store telemetry differently.

GoPro primarily uses different versions of GPMF metadata. DJI is the most fragmented: many drones record telemetry as separate SRT files, while others embed telemetry directly in CAMM tracks, often encoded with protocol buffers (protobuf). Insta360 cameras use their own proprietary metadata embedded streams, with different generations of protobuf schemas and metadata layouts.

Because of this, one cannot simply "read telemetry from MP4", but it requires device-specific parsers accounting for several generations of metadata formats and layouts. See current supported files.

MP4 METADATA

MP4 is a container, not one telemetry format

A camera clip can hold separate streams inside the same file. Video and audio are standard tracks. Some vendors include metadata like GPS, gyro, and camera settings in a vendor-specific metadata track.

videoH.264 / H.265 frames

audiocamera audio

metadataGPS / gyro / camera

MP4 telemetry varies even between devices of the same vendor

Before you import, check the source

Good overlays start with good source data. You do not need to become a file-format expert, but it helps to know whether your export came from the original device and whether timestamps line up with the video. Compare export formats when you are ready to render.

Original export

Use the original activity export where possible instead of a re=export from service such as Strava or Komoot.

Clock sync

Ensure that the clock of your devices and camera are in sync to avoid drift. Account for timezones.

Sensors paired

Check that sensors were connected before recording; OVRLEY handles temporary disconnects.

Complete format

If you export from another app, make sure you export actual activity, and not GPS-plan.

OVRLEY processes files locally, so you can test imports without uploading your activity data.

Bring the cleanest activity file you have

OVRLEY is built for local telemetry overlay work: import the activity, choose the widgets the file can actually support, line it up with the footage, and export the result.

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Free, local, and built for action camera footage.