There is a need, in archeaological site investigation, to survey in three dimensions Whilst there are a number of well established techniques for doing this under water they are, in general, either difficult, time consuming and limited to amateur diving depths or are prohibitively expensive. A possible alternative is the PhotoModeler system which involves only the acquisition of a number of photographs of an object from different directions with a single camera, (film or video, diver-held or ROV). These images are then analysed by the PhotoModeler PC program which generates a three dimensional model, exportable to CAD etc It is claimed by the producers of PhotoModeler that the package is widely used in on-land survey, and this paper describes the experiments (Ewins, 1997), conducted at the Underwater Science Laboratory, Institute of Marine Studies, University of Plymouth, to assess the viability of the system for underwater work It was concluded that the package is suitable for this type of work, though great care must be taken to keep errors acceptably low.
Much of archaological survey, on land and under water, involves the recording of positions (artefacts, points on structures, etc) in three dimensional space. This may be a comparatively simple task on land where use may be made of the theodolite and level. Of course, these devices cannot be used under water where the tape measure is usually the principal survey instrument For example, between 1971 and 1980 tape trilateration horizontal distances taken from at least two known points and plotted by ruler and compasses - was used extensively in the survey of Mary Rose, (Rule, 1989) In this method the thud (vertical) dimension is measured along a plumb line In the case of Mary Rose this approach was both difficult and slow and in 1980 the Direct Survey Method (DSM) was introduced In this method, (see e.g Anderson, 1969), three (at least) tape measurements are taken from three fixed datum points which are not in the same horizontal plane. A surveyed point is thereby fixed in three dimensional space The trigonometry involved becomes complex but is easily solved by a computer; Rule produced his very useful Web package for this purpose. This technique has been employed on a number of important sites including Mary Rose, Sea Venture and Amsterdam, (Adams and Rule, 1991) Although cheap in terms of equipment, DSM is expensive in terms of diving hours and is, of course, limited to diving depths.
An alternative approach may be by photogrammetry, "the technology of obtaining reliable information about physical objects and the environment, through measuring and interpreting images" (Slama, 1980). Surprisingly, the origins of photogrammetry are very closely linked to the very earliest experiments in photography, (a comprehensive history of the development of photogrammetry has been produced by Thompson and Gruner, 1980) Topographic mapping from ground based photographs was carried out extensively in the 19th century by the British, German and French mapping agencies.
