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When the user tries to save a metadata import preset with blank fields checked, LR should issue a warning:
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Warning: You've checked the following fields but left them blank: User Comment, GPS, Altitude (and 5 more fields). This will clear those fields on photos imported with this preset. Is that what you want?
[] Don't show again Cancel Confirm
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Rationale:
A frequently asked question in LR forums: Why isn't LR importing GPS coordinates or (less frequently) some other metadata field?
Invariably, the user is applying a metadata import preset in which GPS or another field is blank but checked. LR does try to inform the user that this will clear the blank, checked fields:
But this is obviously not working for many users, perhaps because when they check a whole category, all the fields get that grey message, "Type to add, leave blank to clear", and at that point, they don't understand the consequences of the grey "leave blank to clear" messages and just ignore them. It's usually much later (days, weeks, or months) they notice the missing GPS fields and naturally don't connect the import preset with the missing GPS.