Manipulating stored geocoordinates in cellphone photos

exiftool to the Rescue

Although there is a go-exif2 library for Go which reads Exif tags, there is no easy-to-use library that writes new tags or refreshes the existing tags in a JPEG file. This is why Listing 1 resorts to the Unix utility exiftool. On Ubuntu, this jack-of-all-trades can be set up via the package manager (Listing 5, first line). The tool expects coordinates in a floating-point format and either creates or refreshes the corresponding Exif tags in a JPEG image file (second line).

Listing 5

Editing Exif Tags

01 $ sudo apt-get install exiftool
02 $ exiftool -gpslatitude=37.<xx> -gpslongitude=-122.<yy> file.jpg

The patch() function from line 42 of Listing 1 calls the tool from Go via the os/exec interface. It takes the name of a program including parameters. To collect the output of the command-line utility invoked, programmers provide a byte buffer, assigned to the Stdout and Stderr outputs. Run() starting in line 52 invokes the external process, grabs its output, and checks the exit code.

Normally, exiftool returns with an exit code of  . In this case, the error err in line 53 is equal to nil, and patch() returns to the caller (the main program) after the work is done. exiftool modifies the JPEG file given to it directly and stores a backup of the original in x_original, but geofuzz is not interested in this, and therefore simply ignores it, leaving it in the same directory.

To generate an executable geofuzz binary from the source code in Listing 1, Listing 6 shows the Go commands for initializing a Go module and compiling the code while automatically downloading any dependencies from GitHub. Then you just need to install geofuzz in a path listed in your PATH variable, and a quick call to geofuzz file.jpg quickly drops a privacy-friendly obfuscation layer over the geotags in the file. Curious online snoopers will be furious, because they'll be searching in the wrong part of town!

Listing 6

Generating the Binary

01 $ go mod init geofuzz
02 $ go build geofuzz.go

Infos

  1. Exif format: https://en.wikipedia.org/wiki/EXIF
  2. GeoImgr: https://tool.geoimgr.com
  3. "Programming Snapshot: Go program finds photos with nearby GPS coordinates," by Mike Schilli, Linux Magazine, issue 225, August 2019, p. 44, https://www.linux-magazine.com/Issues/2019/225/Programming-Snapshot-Go/(language)/eng-US
  4. Generating random locations nearby?: https://gis.stackexchange.com/questions/25877/generating-random-locations-nearby

The Author

Mike Schilli works as a software engineer in the San Francisco Bay area, California. Each month in his column, which has been running since 1997, he researches practical applications of various programming languages. If you email him at mailto:mschilli@perlmeister.com he will gladly answer any questions.

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