Page 208 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 6: Photogrammetric Techniques 187
placed flat on the face of one bar (i.e., the camera bar) exactly in the center and a pin placed exactly in the center of the other target bar. An observer with the pin in front of the eye looks in the mirror on the other bar for the reflection of the pin. A second operator adjusts the angle of the camera bar with the mirror until the reflection of the pin in the mirror coincides with the pin in the other operator’s view. At this point, the camera bar with the mirror is at right-angles to the center of the other bar. The mirror and pin are interchanged, and the process is repeated to make the target bar parallel.
Now that the target bar and the camera bar are parallel, it is necessary to adjust the cameras so that their optical axes are parallel and at right angles to the camera bar. From this point onward, great care must be exer- cised to avoid disturbing the bars and thus upsetting this alignment. The observer now looks through one target at the corresponding camera mirror. The other person adjusts the camera until the reflection of the observer’s target appears central in the mirror. This will usually require a rotation in the horizontal and vertical planes. When the coincidence occurs, the camera is locked in position. The process is repeated for the second camera, and then the first camera is rechecked for alignment. It is worth casting epoxy resin blocks around the bases of the cameras so that they can be semiper- manently fixed in place.
V. PHOTOTRIANGULATION A. PHOTOMODELER
PhotoModeler, a computer program produced by Eos Systems, is one of the best examples of a phototriangulation program. The program uses a calibrated camera to measure the ray paths from the principle point of the camera, through the photographic image, to various points on the site and from this calculates the coordinates of points in the view. There are other similar programs, which the reader is encouraged to investigate, however, the basic system is largely the same and, as the author has had the greatest experience with this program, PhotoModeler will be used as the example for this section. With multiple views of the same points, providing the geom- etry of the camera and lens is known, it is possible to calculate angles and from the various camera locations the complete geometry can be resolved. The target points need to be well defined and some form of control is required to provide scale for these data. PhotoModeler has been used else- where for maritime archaeological work (Franke, 1999), although little has been published on its application to underwater archaeological sites.
The program was thoroughly investigated on the Institute for Nautical Archaeology (INA) Tektash expedition in Turkey from 1999–2001 (Bass,