Regarding whether photographs can be brought into GRASS and
registered: If the area you are working with is fairly flat,
i.register might give satisfactory results. Otherwise, i.ortho can be
used, along with a digital elevation model, to correct for terrain
distortions in the photo. i.ortho does require standard aerial
photography, however, including fiducial marks on the photo and a
data on the characteristics of the camera.
Jim Schmidt
US Forest Service
Sonora, CA. 95370
(209) 532-3671