The use of a Canadian (perspective) grid in deep-sea photography

The construction and characteristics of a Canadian (perspective) grid as applicable for oblique deep-sea photographs are described. The implementation of such a grid through an automated digitizing system allow fast accurate and precise calculations of sizes and densities of animals and other objects. Changes in the camera's inclination due to "nose up" or "nose down" motion over rough topography can cause erroneous calculations of size. The magnitude of this error increases quasi-exponentially with distance of objects from the lower edge of the image and is particularly large for camera systems with a small angle of inclination. The area in a photograph from which accurate counts can be taken, should be determined a posteriori for each type of object (e.g. species), based on a plot of density vs distance from the camera.