Page 188 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 6: Photogrammetric Techniques 167
size in the viewfinder. If necessary, the viewfinder can be opened up and a clear film inserted with the correct size grid square drawn on it. The grid can then be viewed through the viewfinder and the flying height adjusted to match the predrawn grid on the film.
In most cases it will be preferable to take a vertical photograph of the grid, because this will greatly simplify the processing of the information. Where the view is not exactly at right angles to the grid frame, the photo- graph will have to be rectified or a perspective grid drawn on the photo- graph. These techniques will be discussed later, but it should be remembered that the grid frame must lie in the plane of the object for the scale to be correct.
A. SITE SURVEYING USING GRID FRAMES
The use of a two- or three-dimensional grid frame to produce artificial control as an application for underwater work was a technique suggested and described by Williams (1969). This technique was initially for land archaeology but was later modified for underwater work (Williams, 1972). Two of the most common two-dimensional grid frame shapes are a cross and a square. The former has various defects including a lack of rigidity, but is easy to carry disassembled and assembled under water. The grid square, on the other hand, is rigid, but it is relatively difficult to carry and to assem- ble or disassemble.
The grid frame, when photographed on flat terrain, can be used to con- struct a perspective grid on the photograph. It is important to ensure that the plane of the grid lies in the plane of the terrain, otherwise the scale will be incorrect.
This use of the underwater perspective photograph and grid frame is limited. The technique was developed for the land situation where it is very difficult in normal circumstances to take high vertical photographs of a site without using a cherry picker, balloon, photo tower, aircraft, or other form of flying machine. Under water, the diver’s three-dimensional independence usually makes it feasible to obtain almost any viewpoint. The only physical limitations may be where the water is too shallow to allow an adequate camera-to-subject distance to include the whole view, or where the under- water visibility is restricted. This technique has largely been superseded by PhotoModeler and other computer-based programs. But where a more simple solution is required, it may well be advisable to make a photomo- saic. It is usually preferable in the underwater situation to take a vertical photograph rather than an oblique one. The perspective technique is there- fore mostly applicable in a situation where a vertical photograph has