Page 50 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 3: Search and Survey 29
plotted on a chart. This neglect arises from the fact that at the time of loca- tion and during subsequent work on the site the position becomes familiar and can be relocated, using transits, on a day-to-day basis. Transits, however, are generally not associated with the use of a chart and, consequently, the position is often not properly recorded. Even after many thousands of hours have been spent on a site, the site can be completely lost in a few years, because the natural transits have disappeared or been forgotten. The tran- sience of various landscape features must be considered and care exercised in selecting transits. Geographical features must be chosen for their dura- bility. Thus, large public buildings, navigation aids such as lighthouses, etc., make better long-term transits than trees, walls, and flagstaffs, for example, which tend to disappear in time. A famous example of the loss of transits is the Mahdia site (Taylor, 1965), where the landmarks recorded in 1913 no longer existed in 1948. It required 6 days of searching to relocate the site.
If the surveyor chooses the junction of land with sea as a transit, there is another factor to be considered. Tides can often alter the position of the land–sea junction. The use of shoreline vegetation, such as growth of the foredunes, is best avoided, since over a period of time sand movement and patterns of growth can radically alter visual forms.
It is essential, even when transits have been recorded, that they should be identified on a topo- or orthographic map or an aerial photograph, and their intersection located to determine the position of the site. A good example of how transits can change in time is shown in Figure 3.4. The four illustrations show the appearance of a series of land transits over a number of years. Initially, the wreck site, which was found on the land during a mineral sands mining operation, was photographed against some buildings (a). The site was subsequently covered over. More than 20 years later the site could still be located by relocating the transits.
In order to determine an accurate geographical position of the site, the largest possible scale map of the area should be used for identifying the fea- tures used in the transits. Because of the photographic nature of an ortho- graphic map, it is usually easier to identify the transits on this type of map rather than a conventional topographical map which may not show the particular transit feature. However, orthographic maps are often difficult to obtain or are not available. In such cases an aerial photograph can be used and features on the photograph can be linked to the map. If it is not possi- ble to identify the transits on the map, there are two choices. One choice is to go out and survey the transit positions onto the map. Alternatively, and more simply, the photograph itself can be used to measure the angle, pro- vided the lens is accurately calibrated (see Section II.D). It is also possible