[OSM-dev] OpenStreetView: Digital Photos: Getting the field of view angle

[Nick Whitelegg]

Hello everyone,

Have been thinking a bit more about my own interests within the 
"OpenStreetView" project introduced by John McKerrell at SOTM. My 
particular interest is an in the field navigation tools for countryside 
users, particularly walkers/hikers/mountaineers etc, which I'm giving the 
codename "footnav" for the moment (seeing as John already has 
openstreetview and I'm more focusing on the countryside!)

One option is simply to have a computer generated 3D view from SRTM, OSM 
and 3D models of stiles, gates and so on. However I'm quite interested in 
using actual photos: the advent of "compass phones" such as the N97 and 
latest iPhone makes this a realistic possibility.

What I'd like to do is build up a database of photos taken along a path or 
footway, say every 10 metres or so. If a user wanted to navigate, the app 
could locate the nearest photo to them, based on location and direction of 
travel, and display it, with their position and OSM data overlaid on the 
photo. I was on top of a hill looking down at a valley the other day and 
was thinking "how cool would it be if I could see a photo of that valley 
and have all the footpaths overlaid on it"!

I've done a bit of OpenGL and believe something like this could be done if 
the position, direction, slope, aspect ratio and field-of-view angle of 
each photo were known. Most of these I have ideas on how to obtain, or at 
least where to go to look it up, *except* the field of view angle.

The thing I don't really know much about is: how can you get the 
field-of-view angle of a photo taken from a digital camera (either a 
standalone camera or within a phone)? I've seen a few explanations on the 
web on how you get it from the focal length (which appears to be quoted in 
e.g. the N95 specs) but they also require another parameter (the "size of 
the film"). Where might I get this from?

If anyone's an expert on photography theory I'd welcome any explanations 
on how to obtain the field-of-view angle of digital photos.

Thanks,
Nick