Now that geotags are so common, it can be upsetting to see that some pictures don't have them when they should. Here I'll talk about why this may be, and how to find and fix the issue.
Here we'll learn how to add GPS locations to photos which didn't get geotagged. Even if you have the Camera app set to geotag your photos (Settings app -> Privacy -> Location Services), sometimes your location can't be determined at the time, so a GPS location is not added to the photo metadata. It can be very useful to add or edit the geotag, as shown below using the Photo Investigator iOS app.
When a photo is taken, different kinds of metadata are automatically added by the camera. Most of these kinds of metadata are well understood. The location of the photo is stored in the GPS metadata. Camera information is in the EXIF metadata. However, no one seems to know what is in the "Maker Apple" metadata inside iPhone and iPad photos.