Page 204 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 6: Photogrammetric Techniques 183
ation and the complexity of the system will dictate the best solution. However, the digital solution is far more preferable than the printing, feather-edging, and gluing of large numbers of photographs.
IV. STEREOPHOTOGRAPHY
Much of the practical application of stereophotogrammetry has changed in the last ten years. Once photogrammetric laboratories contained large, if not huge, stereoplotters like the Wild B8 and A10. These behemoths weighed at least a ton and have now largely become redundant. Indeed to my knowledge, some have actually been used as boat moorings.
Through a series of progressions these analog instruments have largely been replaced with software programs that can be run on desktop and high- end computers. Where once mechanical movements provided the necessary adjustments, this is now done with software. As a result a machine that cost a huge amount of money now, relatively speaking, costs very little. Also, the software programs are easy to use and can also be used in the field. So, photogrammetry, which was once for an archaeologist an almost impossi- ble objective, unless one had dedicated personnel with expertise, can now be conducted by anyone with a good camera, a computer, and some training. The stereo principles still apply, but today software has removed most of the complexities. In some cases the discussion of earlier methods has relevance to the more current techniques.
In the past, using a pair of cameras on a stereobar with precisely defined optics, it was possible to apply the parallax equations under water to obtain three-dimensional coordinates. A number of different approaches were used to apply underwater stereophotogrammetry. In the 1960s and 1970s at Yassi Ada, Turkey, a series of photogrammetric surveys of shipwrecks was carried out using stereophotography. In some cases, single and, in other cases, pairs of SLR cameras in underwater housings were used to make three-dimensional plans of the sites (Karius et al., 1965; Rosencrantz, 1975). Later, Hohle (1971) used Nikonos cameras with a 28-mm lens in conjunction with a Stereotope to produce topographic plans of the Kyrenia shipwreck. These covered an area of 6 ¥ 12 m with a mean square error of ±10 mm in the vertical.
These early, simple underwater photogrammetric techniques generally consisted of a photo tower with a pair of cameras mounted on a sterobar. The cameras were optically aligned so that their optical axes were parallel. The stereobar was mounted on the tower so that it was symmetrical and level relative to the square grid at the base of the tower. One of the most common methods was to set the tower on the site in an unleveled orienta- tion (Figure 6.9b). It was anticipated that this system would be used when time was at a premium. A series of overlapping stereo pairs were then taken