Reverse projection photogrammetry allows one to measure and locate objects in an existing photo. The technique has application in technical fields such as engineering, forensics and law enforcement. In addition, the technique can be used in real-estate, photography, film making, or any time you want to locate the position of an existing photograph (for example, taking before-and-after photos).
The Reverse Projection application takes any photograph in your iPhones photo library as input, and creates an overlay which can be viewed on top of your iPhones camera view. The overlay consists of a line drawing (from a built-in edge detection algorithm run on the selected photo) and a semi-transparent version of the selected photo.
By moving around and adjusting the scale, rotation, and location of the overlay within the cameras view, you can find the position of the camera when the selected photo was taken. This is done based on aligning fixed objects in the selected photo with those same fixed objects at the scene. Once this point is found the user can take a picture to document the alignment, at which point a copy of the overlay is also saved.
From known measurements in the scene, or by placing objects of known dimension (such as rulers, surveyors rods, etc.) in the scene, unknown measurements or locations in the selected photograph can be determined*. Examples would be the length of skid marks from an old accident scene photo that are no longer at the scene, or the height of a person captured on a security camera.
A way to keep the iPhone steady and in a fixed position is very helpful, such as an iPhone tripod adapter (see my website for links). Also, in bright light a cloth large enough to shade the iPhone and your eyes is also helpful.
*This would be done using separate image viewing/editing software that allows you to overlay the photo taken with the iPhone over the original photo containing the unknown dimensions, and using the known measurements as a scale.