LocPic is a program for geotagging digital photos based on GPS track data. It is designed to work efficiently with large datasets, such as many years' worth of track logs and collections of hundreds of images taken by multiple cameras.
GPS track logs in GPX format are supported and can be loaded into a SQLite database for fast access. The locpic-track.pl script loads GPX files into this database so they can be used for tagging later.
In order to geotag a photo, a GPS fix close to the time it was taken needs to be found. The image timestamp is used, but an offset can be applied to correct for any small difference between the camera's clock and GPS time (many cameras only allow setting the clock to 1 minute resolution, which can introduce significant errors).
To calculate the camera's time offset, set the clock as close as possible to GPS time, then take a picture of the GPS screen showing the clock (or another clock synchronized to GPS/UTC). The locpic-align.pl script will read the camera's timestamp from this image, compare it to the GPS time entered on the command line, and calculate the difference between the two clocks.
When synchronizing camera and GPS clocks, time zones need to be considered. GPS tracks always recurd UTC time but camera clocks and GPS clock displays may or may not be in local time. locpic-align.pl takes an argument that sets the local time zone offset. It can also detect the time zone used by the camera (certain Canon cameras are currently supported) to automatically translate camera timestamps to UTC.
Once GPS tracks and a clock offset are available, the locpic-tag.pl script can be run to calculate location tags for one or more images and optionally write them to the image files.
- Store clock offsets history in a database for automatic lookup
- Interpolate/extrapolate camera clock drift
- Time zone map to guess the local time zone based on GPS location
- More options for controlling track matching, interpolation, and resolving multiple track matches
- Heuristics to guess clock offset based on track statistics (partially implemented)