The camera model summarizes the optic geometry from the optic center of the photo geometry (from the center of the object lens of the camera) to the image. Its parts are:
The focal length, and
The geometrical position of the fiducial points.
In case of the professional aerial photocameras, mainly of the older ones, the focal length is a constant at a certain camera. In the image plane, some pre-fixed points, the so-called fiducial points are placed. These are positioned near to the image corners and/or the halving points of the sides, their position is constant with respect to the image center (the principal point of the image). Their positions are expressed in a local coordinate system in the plane of the image, the origo is the principal point, the axes are parallel to the image sides. The positions are described in millimeters or centimeters (Fig. 54).
These meta-data are strictly needed for the exact geo-reference. For rectifying an archive aerial photo, the original camera type, and thus its camera model parameters are obligatory subjects of our investigation. For the ortho-rectification, the GIS softwares ask for these camera models. The necessary data of some ’standard’, widespread used cameras are often built-in. Also, we can define new camera models for our own instruments, knowing the necessary data.
Beyond the camera model, a further element of the internal orientation is the image coordinates of the frame points in our digital format image. In GIS software environment, it is practically given by moving the cursor to the frame points, and record them (e.g. by mouse click) in correct order.