Page 215 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

194 Maritime Archaeology: A Technical Handbook, Second Edition
64MB Smart Media card could hold 84 high-resolution pictures which in JPEG format were between 650 and 750 KB in size equating to 3.16 MB in TIFF format. The camera was capable of working in low light levels pro- vided it was held steady; it seemed to be more red-sensitive than normal color film; and it contained autofocus and autoexposure, so essentially it was point and shoot. What was being photographed could be seen in the LCD screen at the back of the camera, so it was relatively easy to frame the shots.
VI. STEREOPHOTOGRAMMETRY
The other photogrammetric program used at Tektash was the Georeal- ity VirtualMapper, a stereophotogrammetric program that can be used to plot three-dimensional features or terrain on sites. Like PhotoModeler, Vir- tualMapper is one of a variety of programs that essentially do the same thing, and this will be discussed here in detail. VirtualMapper requires two overlapping photographs to be taken (a stereo pair) that are then processed to produce an epipolar file that is then viewed by an operator or operators in stereo on a computer monitor using stereo glasses. The glasses are con- nected to the computer and the program rapidly alternates a left- and right-hand image on the monitor. The glasses have polarizing optics that sequentially turn the left- and right-hand optic of the glasses on and off. The program alternates this rapidly so that the left-hand eye sees the left- hand image (on the monitor) and the right-hand eye sees the right-hand image. This results in the operator being able to observe a stereo view. The program provides a cursor that can be moved around the site and appears, “floating,”, three-dimensionally in the view. By keyboard control, the cursor can be made to rise and fall in the view so that it is possible to “place” the cursor on any point or surface in the field of view. The cursor location is recorded in three dimensions with the program providing X, Y, and Z coor- dinates of its location. It can be used to obtain the coordinates of any point in the view or to track features using a polygon line tool.
At Tektash, there was a need for a contour plan of the site that would provide the background for the location of the artifacts in three- dimensional space. Both conventional tape trilateration and PhotoModeler were unsuitable for this work, the former being too time-consuming, and the latter requiring a large number of targets to provide spot heights. An experiment was made using PhotoModeler and white bathroom tiles as targets, however, even with about 50 tiles there was insufficient information to provide any sense of the complex nature of the site. The advantage of VirtualMapper and, by extension, any stereo system, is that it is not neces-