Page 189 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

168 Maritime Archaeology: A Technical Handbook, Second Edition
inadvertently been taken from an oblique viewpoint. Usually the under- water camera operator, with skill and practice, can take a reasonably true vertical photograph of a grid frame. However, unless special precautions are taken, it is unlikely to be exactly vertical, although it will normally be within 10°. To convert this to a true vertical, a perspective grid may be con- structed and the details transferred to a true plan (as described previously), or the print can be rectified in an enlarger to correct the perspective dis- tortion (see the following). Again, the conversion of photographs to a digital format opens up enormous possibilities in the area of modification and scaling of images. Photoshop is one of the most widely used computer pro- grams that is both sophisticated and flexible enough to do this.
Baker (1984) has used an interesting technique to obtain a vertical pho- tograph of a three-dimensional grid square. In this method, thin lines were attached between opposing midpoints of the upper and lower sides of the squares of the grid frames. Because the crosses on each square are sym- metrical, if the photographer lines up the cross-hairs then the camera posi- tion will be vertical to the grid frame (see Figure 6.1). However, with good manipulation of the camera this may well be an unnecessary complication. At the same time it is more relevant to level the grid frame to some mean- ingful plane on the site rather than be concerned that one is simply trying to obtain a good vertical photograph of a randomly orientated grid frame.
III. PHOTOMOSAICS
This section deals with noncomputer application of photomosaic work. In Section III.H issues relating to the application of computers to photo- mosaics will be discussed. However the fundamental principals of the pho- tomosaic apply in both computer- and noncomputer-based applications.
Visibility under water is scarcely comparable to that on land due to the effects of suspended matter (sediments and plankton) in the water and of scintillation caused by variations in the refractive index of the water. As a result, underwater photographs rarely have the clarity and sharpness found in photographs taken in air. If this is the norm, there are other factors which heighten the complexities of obtaining a clear underwater picture. If, for example, an overall vertical photograph of a 30-m long wreck site is taken using the Nikonos 15 mm lens (In reality, underwater this is a 20 mm lens), a camera-to-subject distance of more than 16 m is required. Apart from the problem this presents on a shallow water site, the resultant photograph would be, at best, a vague and indistinct outline of the site unless there was exceptional clarity. This unsatisfactory result would mainly be due to the visibility effects, but there is also a physical limit to the resolution of the