I have old photos of a town and landscape around it. I'd like to take these photos again, from the same locations. In some cases I am struggling with finding the original location of the camera.
I can identify objects on the photo, I know their position on the map in some cases even their dimensions, but I don't know anything about camera or lens. Are there any techniques (or even better any ready-made software) to calculate position of the camera from the photo?
The key is to find areas of the image with a lot of parallax, such as a foreground building and a background tree. Try to pick a point as close to one edge of frame as possible. Now walk left/right (green) to find the correct point of intersection from the old photograph.
Now that you've done that, you've established a straight line to move along (red).
Pick a different parallax intersection on the other edge of frame. Instead of walking left/right, walk along the red axis you established earlier. Once you've matched that parallax, without spoiling the first match, you've found the position of the camera.
Once you're in the same position, matching the lens is easy. You can just look through the camera and adjust until the framing matches, or measure the angle of view.
There is software that can calculate the position of the camera, but generally you need a 3D model of the scene as a basis.
With this tool...
...you can input any location and time (also in the past) and see the exact height and angle of the sun. As you do not know the exact location in this case, you could hopefully get an idea of the sun's height and angle from the actual photo. It's not perfect science, but hopefully it helps.
Disclaimer: I'm the creator of that tool. It's non-commercial and ad-free.
You don't require multiple photos. if you can see the angle between known landmarks you can use Resection, a triangulation technique used for surveying. As long as you can see 3 items whose location you know and you can measure the angle between them then you can calculate the exact location of the photo.
There is a nice paper I saw from the University of Liege. you can find it and some other algorithms for resection at http://www.telecom.ulg.ac.be/triangulation/