I’ve played a bit with exiftool now and found that the commandline
exiftool -GPSimgdirection -if '$GPSImgDirection > 360' *.JPG
will find the images on the Virb which has broken values.
Now I just need to figure out how to subtract 4294966935 from GPSImgDirection in that commandline and write it to the images.
For your info the following is what Exiftool reports on one imagefile from the Virb:
Note that the problem is in the Virb image, not the JOSM plugin (which unlike i.e. the Mapillary website just happens to make some sense from the value).
Also note that my Virb is usually firmly mounted on my bicycle handlebar (when I’m not using it like an ordinary camera) and always pointing straight forward and that it doesn’t “just sometimes get the direction wrong”. It’s always when it’s pointing west.
I had tried to use decrement, but without any luck.
After reading http://u88.n24.queensu.ca/exiftool/forum/index.php/topic,4462.0.html though, I realize that increment is what i need to use:
exiftool -GPSimgdirection+=-4294966935 -if '$GPSImgDirection > 360' *.JPG
This commandline succesfully finds and fixes Virb images with broken GPSImgDirection values.
Maybe the Mapillary website could include this?