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I'm trying to use EXIF data from my photos, and I discovered the wonders of the tag "EXIF GPS GPSTimeStamp" (a timezone-independent timestamp).

However, I notice that it's often different from "EXIF DateTimeOriginal". Usually by a second or two, but I've seen up to 211 seconds difference. The GPS time is usually, but not always, behind (earlier) than DateTimeOriginal. The DateTimeOriginal seems to be the accurate one in the case of a discrepancy. I've checked this for a few hundred photos from 3 different phones.

Does anyone know when, exactly, "GPSTimeStamp" refers to on most phones?

Since GPS signals encode timestamps from literal atomic clocks in space, I assume if the phone receives any signal at all, it will be accurate to less than a second. So that can't be the source of the error.

My current theory is that the "GPSTimeStamp" tells you the time of the GPS fix described in the other GPS tags, not the time the photo was taken. That would explain why it's usually earlier than the DateTimeOriginal. Is that correct?

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Yes, it is the time at which the location was determined. That may be a second before the picture was taken, but it could be much more, if for example you are in a canyon and the GPS is unable to make a fix.

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    If the GPS is unable to make a fix at the time the photo is taken, won't it use the last good fix it was able to take prior to the time the photo was taken? – Michael C Jun 3 '16 at 4:30
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    @MichaelClark if the GPS update setting is at certain intervals, it might explain why the gpstimestamp is before the photo was taken? I seem to remember a GPS setting in my SLR where I could set it like this. Obviously you don't have this sort of control in phones, but maybe if the phone knows that it has very recently obtained a new location, its software will decide not to get a new one? – laurencemadill Jun 3 '16 at 9:59
  • @MichaelClark Yes, this is my understanding. – MirekE Jun 3 '16 at 14:21
  • @michael-clark: it solely depends on the software. Somebody should teach software to keep last fixed GPS time and if noone did it won't happen no matter how logical it seems to users. – Euri Pinhollow Jun 4 '16 at 9:02
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    @MichaelClark - The time stamp should be always before the exposure date/time. I wasn't clear in my answer and I changed the wording. – MirekE Jun 4 '16 at 23:34

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