On a hunch I checked the EXIF data attached to my first photos from my new iPhone. The intriguing news is that it has values such as:
GPSImageDirection : 2535/383
GPSImageDirectionRef: T
The GpsImageDirectionRef = "T" appears to be a constant.(Update: signifies True and not Magnetic north.)
Pointing the camera roughly N,E,S,W resulted in:
N : 2535/383 (and then, pointed 'up': 19845/113)
E : 27081/81
S : 20749/113
W: 28546/113
My guess is that this is a fraction representing degrees from N. The use of a denominator may indicate accuracy (?). When I first saw it, I thought it could encode tilt, but that doesn't seem to fit the data (at least to my eye). No other obvious EXIF source of tilt info.
More experimenting in order.
Ob-hack: Augmented reality. Place Godzilla / Cloverfield properly in all photos of Tokyo / New York.
[Update]{style="color: #ff0000;"}: It's just heading. Cue sad trombone.