To redraw these printed and scanned plans digitally, vector graphics applications (such as Adobe Illustrator) rather than standard GIS software and techniques were used because these made the digitisation process quicker and required only basic skills in using GIS. In particular, it was easier to digitise 'non-regular' graphic features, such as the curved lines of the fortifications, buildings and excavation trenches. Vector graphics applications use Bézier 'splines' to produce continuous, mathematically precise, curves and lines using 'end points' and automatically generated 'control points', whereas GIS uses a continuous string of XY coordinate pairs (called vertices) (see Allison et al. 2004, fig. 15). An arc, such as the fortifications of a Roman fort, is never perfectly smooth when drawn with GIS software. More importantly, while straight lines are simple to draw in GIS, closed shapes, particularly the type of irregular 'polygons', or multi-sided features, that represent areas common in archaeological plans, are not. A further advantage of maps drawn with vector graphic applications is that these high-resolution vector graphics files are of a higher quality for both print and web-based publications.
As part of the digital redrawing procedure the various features within the fort plans - the building or structural features, the building phases, the excavation areas - were each assigned their own 'layer', a system which is generic to all graphics programmes. The layer system also allows these individual features to be converted into 'shape' files, one of the formats used in GIS to create a comprehensive geo-referenced map where different aspects (e.g. building phases or reconstructions) can be viewed separately or together, as required. For Vetera I, each building was assigned a 'layer'. The most important 'layer', for plotting artefact distribution, however, comprised the 1629 trenches. Artefacts in the catalogues for Vetera I were provenanced to a trench or to a feature within a trench (e.g. a pit). Each of these trenches had to be drawn as a closed polygon, and then assigned a unique numerical identifier, or 'common key', within its boundary. For Vetera I this identifier consisted, in most cases, of the trench number and was assigned to the mid-point of the trench. Where there was more specific information a numeric identifier was assigned to that specific feature (e.g. Pit 6 within Trench 311). For the other sites, the 'layers' consisted variously of excavated areas, buildings, specific features, and construction phases, either of particular buildings or of the overall fort. This was because the artefacts in the published catalogues of these sites were provenanced to these features or aspects of the site rather than to specified excavation trenches. The next step in the procedure was to import the various 'layers' that made up the digitised site plan, into ArcGIS. This was done by using an Adobe Illustrator 'plug-in' supplied by Avenza (MaPublisher), which converted the layers in the graphics application into 'shape' files.
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Last updated: Mon Jun 30 2008