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thebluepotato

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Posts: 5
 #1 
Hi!

I've recently purchased HoudahGeo for the purpose of (finally) organising the pictures of my 2105 USA trip. This involves syncing geotags and timezone information between 5 cameras :
  • 2 iPhones
  • 1 Sony with GPS
  • 1 Samsung Galaxy with GPS disabled
  • 1 Nikon without GPS functionality, timezone corrected on the camera only after a few days
Because the trip crossed over multiple timezones (of course), I'm trying to put each picture in the right timezone so they're displayed coherently in Photos. For now, the Nikon and Galaxy Photos are at very wrong times. So my plan is to first have the geotags and timezones correct for the iPhones and the Sony and then use some HoudahGeo magic to geotag the last two cameras.

My main concern right now is handling the iPhones and the Sony camera. Most have correct GPS info and have been imported with the right timezones (and have a correct GMT timestamp). Those without GPS info were imported in CEST (correct displayed time but expressed in the wrong timezone). Since there's no way to select only pictures with CEST I'm using HoudahGeo to write the timestamps on those.

My workflow is :
  • Select all pics from the iPhones and the Sony from the same day (to stay in one timezone) in Photos and Load them in HoudahGeo
  • Geotag those with missing info
  • And now I would like to write GPS, Altitude and Timestamp (I'm only interested in adding Time Zone offsets to the metadata) to the changed photos
  • Notify Photos that Timestamp, GPS and Altitude have changed
My question is: is it possible to "write changes and notify" only for the files which have effectively been tagged by HoudaGeo? I don't want any changes to be made to the pictures which were used as a reference to begin with. In other words, I want HoudahGeo to update only the pictures which were "gaps" without GPS info or whose info I've changed. Once all the pics have been tagged, how can I see which ones I've tagged in HoudahGeo and which ones were the references?

Would using the preservedAttributeKeys in the plist be of use here? Does it mean HoudahGeo never does anything related to those attributes or only that it doesn't touch them if they already exist?

And an even broader question: is my workflow even workable or should I remove all pics, tag them and import them again? This would only ba a last resort since those pictures don't exist on memory cards anymore, only buried in the Photo Library's Masters.

Looking forward to your assistance!

Joe
0
houdah

Moderator
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Posts: 2,966
 #2 
Hi!

HoudahGeo does not track which photos in a project have seen changes. If you don't want HoudahGeo to touch the iPhone photos, you will have to remove those from the project before exporting or select the other photos and have HoudahGeo only export to the selected ones.

One trick to find iPhone photos is to sort the list of images by name. iPhone photos have names that start with IMG_ followed by a number. With a little luck your other camera will have a different naming convention. Or use a different range of numbers.

You could also try to "flag" the iPhone photos after importing them into HoudahGeo. Add a dummy city or creator name. Later you can sort by city or creator and find the iPhone photos grouped together. The dummy name does not matter since you don't plan on exporting to the iPhone photos. 


The preservedAttributeKeys setting prevents HoudahGeo from writing certain tags during EXIF/XMP export. It does not check for 
existing values. This will not help with what you are trying to do.


Your workflow is workable with one caveat: Apple Photos may still show the "wrong" time at the end of the procedure. I.e. you may need to may an additional adjustment in Apple Photos. The timestamp will be correct in the library and allow for correct sorting of the photos. It may however appear wrong. HoudahGeo can only tell Apple Photos at what exact time a photo was taken. It cannot tell Photos how to display that time. E.g. HoudahGeo tells Photos a photo as taken at 1 PM GMT. Photos may decide to use the GMT time zone and show this as 1 PM. It may just as well decide to use the GMT+1 time zone and show 2 PM. 

When you import photos from your camera, Apple Photos assumes that your computer and camera are set to the same time zone. If you are at GTM+1 at that time, it will read a 1 PM time on your camera as 1 PM GMT+1.

Later when HoudahGeo tells Photos that the photo was actually taken at 1 PM GMT, Photos could decide to stick with showing a GMT+1 time zone and show the photo with a 2 PM time. It could just as well decide to use your current time zone. E.g. GMT+3 and show that time as 4 PM.

Of the top of my head I don't know which time zone Apple Photos uses in this situation. Give it a try. Try changing the time zone on your Mac before using HoudahGeo's notify feature. Does it make a difference?


BTW, I recommend you make a backup before writing timestamps to master images. If you get the times wrong, these can be tricky to correct/recover.

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thebluepotato

Registered:
Posts: 5
 #3 
Thank you for your very quick reply!

I did not think about using a dummy tag but it's a very good idea. I'm guessing this is the solution to my issue.

There are indeed additional steps in Photos: I have to select the modified pictures and set the correct timezone, but at least now the "physical" time is the same and they show up in the correct chronological order. It does indeed "stick" with the previously assumed timezone, the one when importing untagged pictures, i.e. CEST in my case.

It also seems the displayed time in macOS Photos is (always?) the local time when the picture was shot (or where the timezone is set). In iOS Photos, the time shown is the one in the current timezone, which is an absurd mess.

I continuously backup, but thanks for the reminder!

And a little bug report: I toyed a little with the preservedAttributeKeys and it works as advertised (not touching anything) but not for all tags. I had set it to never touch GPSTimeStamp and SubsecTimeOriginal. My discoveries:
  • GPSTimeStamp was left untouched but SubsecTimeOriginal was changed even though it was not supposed to (and I checked with exiftool that the tag name was correct, in lowercase)
  • SubsecTimeOriginal gets truncated to two digits (e.g. 435 -> 43, effectively losing 5 ms)
Cheers!

Joe
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houdah

Moderator
Registered:
Posts: 2,966
 #4 
Thank you for the update on how Photos handles times.

I think Photos is right to show times independently of the current time zone. I however think it is very wrong to assume camera and computer clock are set to the same time zone. When I travel, my Mac automatically changes time zones. My camera does not. Thus I get different results depending on where and when I import the photos: at the end of the day or next day after moving on.


preservedAttributeKeys is a hidden setting because it interferes with the inner workings of HoudahGeo. Things here can change. HoudahGeo writes the photo time using exiftool's composite SubSecDateTimeOriginal tag. This updates DateTimeOriginal and SubsecTimeOriginal. Thus there is no way to block updating SubsecTimeOriginal without also blocking DateTimeOriginal.

I will look into the possible precision with SubsecTimeOriginal.
 

__________________
Houdah Software s. à r. l.
https://www.houdah.com

HoudahGeo: One-stop photo geocoding
HoudahSpot: Advanced file search utility
Tembo: Easy and effective file search
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