ortho-rectification theory

Hi,

I posted a while back on this, and thanks for the answers, I bought the
book, and feel a little bit more knowledge about forward and back
projection methods for rectifying imagery using a DEM.

I wish to estimate essentially the camera parameters using angles (the
look angle, and the angle around the equator, in a similar way to
spherical coordinates), and hence use trigonometry to estimate the
mapping function from image space to real space to do a fast but crude
orthorectification of a geotiff image using DEM.

Has anyone done this before, any advice etc., in particular can anyone
estimate the error involved in this method.

I can program so I am really looking for the theory, though sample apps
would be a great help :-).

Any help would be gratefully received.

Many thanks,

Norman Barker