Does Geotagging account for the offset betweeen GPST & UTC?

[Bobbysmile]

Hi,

I've used exiftool for a while and never had any issues, recently I've been using the geotagging more to assign high precision GNSS data to aerial photos in post.

I've noticed on the quality reports there seems to be a bit of a shift against where the image gets geotagged, and where the photogrammetry software reconstructs the camera to, always in the along track direction (image attached). Red triangles are the computed positions, blue circles are the GPS tag locations, the differences will be due to different pause durations before shooting



I am using NMEA data to geotag the images which is output in GPS Time, currently GPS time is about 18s behind UTC which wouldn't be a bad guess for the position lag I'm seeing. I was hoping someone more familiar with the behind the scenes could jump in and clarify if this offset does get applied?

This is the command I am running, with the current offset in there to account for daylight savings in the UK

%exiftoolBin% -m -geotag %GeoTagFile% "-geotime<${createdate}+01:00" "%outputDirectory%\*.jpg" -overwrite_original

Would the command support seconds in the offset time to factor in this time slip (e.g. "-geotime<${createdate}+00:59:42") or am I better off finding a way to export in UTC

[Phil Harvey]

The GPS date/time is taken as whatever you use for the -geotime option.  You should be using the -geosync option if your times aren't synchronized to GPS.

This option also provides a linear time drift correction by specifying multiple synchronization points if your clock is running slow/fast.

- Phil

[Bobbysmile]

Fab thank you, looks like as my images are tagged with a local 'creation time' out of the drone (with no GPS time field as it's pulled from the mobile device which it's connected to) which is GMT (so GPS synchronized but with the GPST -> UTC correction applied) and my raw GNSS logger is GPST I'll need to remove that 18 seconds from the offset.

Will re-run the data and post here later if it fixes the issue in case others come across this thread in the future.