enhance exif subsecond and offset handling.

[tsteven4]

No description provided.

[tsteven4]

@jidanni Concerning your example https://commons.wikimedia.org/wiki/File:Eastern_part_of_Tanaga_Island_Alaska.jpg#Summary this allows a solution

gpsbabel -i kml -f FlightAware_UAL871_KSFO_RCTP_20221220.kml -x track,start=202212210325,stop=202212210330 -x interpolate,time=0.001 -o exif,offset=+08:00 -F Eastern_part_of_Tanaga_Island_Alaska.jpg

resulting in GPS tag info in the output file Eastern_part_of_Tanaga_Island_Alaska.jpg.jpg of

GPS Altitude : 9754 m Above Sea Level
GPS Date/Time : 2022:12:21 03:27:18.529Z
GPS Latitude : 51 deg 40' 2.47" N
GPS Longitude : 177 deg 49' 6.16" W

The track filter was used to reduce the number of points we interpolate over to save time and memory. The interpolation time was set to one millisecond. The new exif offset option was used to supplement the DateTimeOriginal tag as the image didn't record a OffsetTimeOriginal tag, and was using CST.

This horizontal position differs from the camera location listed in the wikipage by 146m.

[tsteven4]

BTW you could do this without this PR, but it is complicated due to the interpretation of DateTimeOriginal without OffsetTimeOriginal as being in the local time zone. In Colorado this requires a 15 hour shift = CST - MST = +08:00 - -07:00
./gpsbabel -i kml -f FlightAware_UAL871_KSFO_RCTP_20221220.kml -x track,start=202212210325,stop=202212210330 -x track,move=15h -x interpolate,time=0.001 -o exif -F Eastern_part_of_Tanaga_Island_Alaska.jpg
This gets the correct position, but the written GPSTimeStamp is wrong by the move amount.

[jidanni]

OK, I added a note to the wiki page. Thanks!