If the appropriate checkboxes are selected in the Post submit box when saving the Post, the plugin reads the EXIF data of the Post’s Featured Image and adds Post Tags and a custom meta value “location” containing the serialized GPS location data of the image.
If the Post has no Featured Image, or if the Featured Image’s EXIF data is incomplete, then nothing happens.
The EXIF data is read directly from the image file, not from a database entry.
If standard values “GPSLongitude” and “GPSLatitude” are not available in the Featured Image’s EXIF data, the post will not be updated with the custom meta field. (A pre-existing value will not be deleted, if this happens.)
When saving Post Tags, the standard taxonomy post_tag is set. If you need to modify this, for example in the case of a custom taxonomy, then you can use the filter hook update-post-with-exif-data/post_taxonomy to change it.
If you want to see the custom meta value containing serialized GPS location data in the WordPress backend, you’ll need to parse the data and add this field yourself. A good solution for this is the “Advanced Custom Fields” plugin by Elliot Condon, with field type “Google Map”.
Developers: see the code for a WordPress action hook update-post-with-exif-data/no-gps, which fires in the event that there is no valid GPS data available in the Featured Image.