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How to Use gpx2srt to Create Timecoded Movement Subtitles

What gpx2srt does

gpx2srt converts GPX (GPS track) data into SRT subtitle files that display time-stamped location or movement information alongside video. This is useful for overlaying speed, distance, coordinates, or lap/time markers in video editors or players that support external subtitles.

When to use it

  • You recorded video and GPS data separately (action camera + GPS logger or smartphone).
  • You want a time-aligned textual overlay showing position, speed, distance, or timestamps.
  • You need an easy way to generate subtitle files for post-production or analysis.

Quick prerequisites

  • A GPX file with timestamps that align to your video timeline (UTC or consistent clock).
  • gpx2srt installed (command-line tool or script). If your GPX timestamps differ from video time, note the offset (see Step 4).
  • Basic familiarity with running commands in a terminal.

Step-by-step guide

  1. Inspect your GPX file
  • Open the GPX in a text editor or viewer to confirm it containspoints with entries.
  • Note the first timestamp and time format (ISO 8601 recommended).
  1. Install or obtain gpx2srt
  • Download or install the gpx2srt tool appropriate for your system (script repository or package). Ensure the executable/script is accessible from your terminal.
  1. Choose output fields and format
  • Decide what to show in the subtitles: coordinates, elapsed time, speed, cumulative distance, or formatted timestamps.
  • Plan concise text lines (SRT displays for a few seconds—keep text short).
  1. Align GPX timestamps to video time
  • If your camera’s clock differs, calculate an offset: video_start_time − gpx_starttime.
  • Many gpx2srt implementations accept a time-offset flag (e.g., –offset +00:00:05 to shift GPX forward 5s). Apply the offset so subtitle times match video frames.
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  1. Run gpx2srt with desired options
  • Example command patterns (adjust to your tool’s flags):
    • Basic conversion: 
      gpx2srt input.gpx -o output.srt
    • Include speed and distance (flags vary by implementation): 
      gpx2srt input.gpx -o output.srt –fields speed,distance –interval 1
    • Apply time offset: 
      gpx2srt input.gpx -o output.srt –offset +00:00:05
  • Use an interval or sampling rate to avoid excessive subtitle count (e.g., every 1–5 seconds).
  1. Review and edit the SRT
  • Open output.srt in a text editor or subtitle tool. Each entry has an index, start/end times, and text.
  • Trim or reformat lines for readability, shorten long coordinates, or convert decimals to degrees/minutes if preferred.
  1. Load subtitles into your video editor/player
  • Import the SRT alongside your video; adjust subtitle styling (font, size, position) in the editor.
  • Verify timing across different playback rates; adjust offset if needed.

Tips for better results

  • Use a consistent sampling interval (1–5s) to balance readability and precision.
  • Convert speed to the viewer’s preferred unit (km/h or mph) before exporting.
  • Round coordinates to a sensible precision (3–5 decimal places) to avoid clutter.
  • If showing distance, start cumulative distance at zero or show lap splits for races.
  • For live overlays, consider embedding the SRT or burning text into video during export.

Troubleshooting

  • No timestamps in GPX: the tool cannot align subtitles—use other sources to timestamp or sync manually.
  • Subtitle times off: re-calculate offset and re-run conversion.
  • Too many subtitles: increase interval or filter points by distance delta.

Example short SRT entry

1
00:00:05,000 –> 00:00:07,000
Speed: 12.3 km/h — 12.3456°N, 98.7654°W

Final note

gpx2srt turns raw GPS tracks into readable, time-aligned subtitles for video and analysis; with correct timestamp alignment and sensible sampling, it’s a fast way to add movement data to footage.

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