Page 190 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 6: Photogrammetric Techniques 169
35-mm format photographic film. Using a standard 35-mm black and white film, for example, PAN F, and calling the narrowest resolvable line on the negative d, then the smallest object resolvable on the seabed (D) is given by the formula:
(for the 15 (20) mm lens at height = H).
If d = 0.01 mm and H = 16000 mm, the smallest resolvable object is 8 mm,
and this will be the absolute limit of resolution of the system.
On a large underwater site where the visibility is poor or the water is shallow, an overall view of the site may not be possible. A simple solution to this problem is to take a series of overlapping photographs that can be joined together to form a photomosaic (Figure 6.2). However, it must be recognized that a photomosaic is always going to be a compromise. The slightest variation in vertical topography or camera tilt will inevitably lead to perspective distortions that will make it impossible to exactly match pho- tographs. Where there is a minuscule tilt, it may be possible to match one pair of photographs exactly, but there is little hope of matching successive pairs and attempting to do so is futile. Better results are gained by accept- ing a poor initial match in order to allow for more integrated subsequent matching. Provided the relief is not excessive, it is possible to produce a highly acceptable photomontage or mosaic of the site. This is a good simple
method for getting reasonably accurate site plans.
Figure 6.2 An example of a photomosaic showing the Pattaya wreck site in Thailand.
D=Êd¥Hˆ Ë 20 ¯