I think I have found another Apple bug but since the exif spec is so vague, I wonder what more experienced folks think of my interpretation:
GPSTrack: the direction of travel. If I am on a bus, it should point down the road.
GPSDestBearing: the direction to the place I want to be. If I have a goal and the road is curved, it should point at the goal, not down the road. If I don't have a goal (not using a navigational guide), it should be null.
GPSImgDirection: the direction I am pointing the camera.
Does that seem like a correct interpretation?
On the images I took over the past five years with an iPhone 7, there is never a GPSTrack, and the other two are always pointing to the road/trail ahead.
Your interpretation is absolutely correct.
- Phil