Page 233 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

212 Maritime Archaeology: A Technical Handbook, Second Edition
this was when the Western Australian shipwreck database was first put into a GIS. The database had been managed for a number of years as a simple, flat-file database. A great deal of effort had gone into the editing and quality control of these data. The moment the shipwreck positions were placed on a map of Western Australia, two sites showed up as completely wrong with one being in the middle of the Gibson Desert. It was obvious that there had been a transposition of the second and third digit in the longitude, but that was not something seen by looking at just the data.
The GIS for planning, urban development, management, etc., has much more sophisticated potential because of the wealth of data. For example, in the United States house numbers on streets have been geocoded so that it is possible to exactly locate the position of a street address. Demographic and geographic information is available about suburbs, counties, and states that can help to plan things as far apart as emergency response to flooding to planning the best location of a new McDonalds.
For the maritime archaeologist, there are three main areas where a GIS can be used: in survey, excavation, and site distribution. Although it is acknowledged that there are other things it is routinely used for now. In the following discussion, it will be assumed that the techniques of creating a GIS are understood and readers are referred to manuals and guides for the use of GIS in archaeology.
A. SURVEY
The most useful aspect of a GIS for survey is the ability to superimpose all sorts of two-dimensional images in a georeferenced form. Thus aerial photographs, charts, and topographical maps can be arranged in layers. Then historical maps and charts can be placed in the layer and georeferenced using the GIS programto complex georeferencing where the graphic image is stretched in a complex fashion in different directions so that selected points on the image align with geographical positions on the GIS. Most geo- referencing is simple scaling and rotating. In more complex situations there may be some skewing, and in the most complex situation the transforma- tion is nonlinear. In this case one could take a historical map and georef- erence it to a modern map. From this one would be able to determine the changes in coastline by comparing the historical map with the modern map.
Additionally, as indicated above, survey data can be included so, for example, the traces of a towed search can be traced on a chart showing what areas the survey has covered and what has been missed (see Figure 3.19 showing the towed search for the Portuguese ship Correio da Azia and Figure 7.5 showing the search area in Galle Harbour, Sri Lanka). This sort