What your videos reveal: metadata, dates and GPS location

A video from your phone usually contains more than pictures and sound. Alongside them are the recording date, the codec settings, often the phone model and, if location was on for the camera, the GPS coordinates of where you pressed record. Anyone you send the original file to can read them. Video info shows what a file contains without uploading it, and this guide explains where those details live and how to keep them private.

What is inside an MP4 or MOV

MP4 and Apple's MOV share an ancestry: MP4 was based on the QuickTime file format. Both are built from nested boxes, also called atoms, each with a size and a four-letter type. Three matter here:

• ftyp names the file's brand, which tells software whether it is an MP4, a MOV or a 3GP.
• mdat holds the compressed video and audio, almost all of the file's size.
• moov is the index. It holds the movie header, with a creation time counted in seconds since 1 January 1904, and one track box per stream giving dimensions, a rotation matrix, the codec, the timescale and a table of every frame's timing. It also holds user data and metadata boxes, which is where cameras put the date text, the device and the location.

When moov sits before mdat the file is "fast start": a browser can begin playing before the whole file has downloaded.

Where the location is stored

Phones write location as a short text string in the ISO 6709 format, such as +51.5007-000.1246+012.345/: latitude, then longitude, then optionally altitude in metres, each with its sign. iPhones store it under a QuickTime metadata key named com.apple.quicktime.location.ISO6709; many Android phones use an older user-data box called ©xyz. Five decimal places of latitude resolve to about a metre, and phone GPS is typically accurate to a few metres outdoors, which is enough to identify a house, a school or a regular running route. A single video of your garden can reveal your home address.

Checking a video

Video info reads only the moov box, so even a multi-gigabyte file opens in a moment. It shows the duration, resolution, frame rate and frame count, the video and audio codecs, the overall bitrate, any rotation flag, the creation date, the iPhone model when the file names it, whether the file is set up for fast start, and the location with a clear warning if one is present. Full detail comes from MP4 and MOV files, which covers most phones and cameras; WebM and MKV files show only the basics. Creation dates come from the recording device's clock, so a camera with the wrong time records the wrong date.

What happens when you share

Whether the location travels depends on how the file is sent, not on the video. Platforms that re-encode uploads generally write a new file and do not carry over the original's location box, though what a company keeps on its own servers is a separate question. Anything that sends the original file keeps everything: email attachments, cloud storage links, file transfers, chat apps sending a file as a document, and uploads to marketplaces or forums that store files as they are. iPhones offer an Options switch in the share sheet to leave location out of photos and videos you send, but it is easy to forget. The same issues apply to photos, covered in What your photos reveal about you; the EXIF viewer checks images.

Removing location and other details

• Stop recording it. On an iPhone, turn off location access for the Camera app in Settings under Privacy and Security, then Location Services. On Android, look for a location or geotag option in the camera app's settings.
• Re-encode before sharing. The video compressor and the video trimmer record a brand-new WebM file in your browser, which carries none of the original's metadata. Run the result through video info to confirm.
• On a desktop, FFmpeg can copy the streams without re-encoding while dropping global metadata, using its map_metadata option with a value of -1. Check the output afterwards, since some tags live in places that option does not reach.

Removing location from the file does not remove clues in the picture itself: house numbers, street signs, school uniforms and distinctive views give away places just as precisely. Nothing you load into these tools is uploaded.