Internet Archaeol. 17. Allison. 6.3 Digitising the site plans

6.3 Digitising the site plans

The exercise of creating GIS data from printed maps or plans is a technically demanding task. It typically involves redrawing the original map detail, either on a digitising table (referred to as 'head-down' digitisation), or on a computer screen ('head-up' digitisation). In 'head-down' digitisation, the operator uses a hand-held digitiser device, or puck, to capture the locations of geographic features by converting their position on a map to XY Cartesian coordinates that are stored as computer files for processing and analysis by GIS software. In 'head-up' digitisation, the map is scanned and imported into the GIS software where the operator redraws the map features using the tools provided by the software. There are a number of considerations that need to be taken into account in order to produce useful results in 'head-down' digitisation mode: accuracy and precision of the digitising table; size of the digitising table; and operator competence. Similar considerations apply to 'head-up' digitisation. Even when all goes well, manually digitising maps and plans is time consuming and error prone.

6.3.1 Adobe Illustrator and MAPublisher

With these limitations in mind, we decided not to digitise the Roman military site plans using standard GIS software and techniques. Instead we used Adobe Illustrator and Avenza MAPublisher (a software 'plug-in' adding GIS functions to Adobe Illustrator) to redraw the plans and then convert them into GIS data formats for ArcGIS. We chose this methodology because the digitisation process was quicker and easier than using ArcGIS in 'head-up' digitisation mode.

It will not come as news to most archaeologists that Adobe Illustrator is widely used for archaeological illustration. We are not partisan users of Adobe Illustrator - other popular 'vector graphics' applications, such as Macromedia Freehand for example, are just as capable. Less well known is the role Adobe Illustrator can play in providing a low-cost, high-productivity, environment for 'head-up' digitisation.

The key technology that transformed Adobe Illustrator into a digitisation environment is Avenza MAPublisher [see http://www.avenza.com]. Essentially, MAPublisher is a 'plug-in', or software extension, for Adobe Illustrator and Macromedia Freehand. It enables the users to import, edit and export GIS data, as well as several other useful cartographic functions. MAPublisher is widely used in cartography, but when compared to standard GIS or CAD software, its GIS functions are limited. But, to repeat, we were not using MAPublisher to compete directly with ArcGIS, but rather as a useful tool for getting complex, difficult to digitise, plans into ArcGIS.

The speed advantages of Adobe Illustrator over ArcGIS are clearest when digitising 'non-regular' graphic features, such as coastlines, waterways, topographic contours, buildings and excavation trenches. The speed improvements stem from the fundamental differences between the vector drawing technologies used by Adobe Illustrator and GIS software. These differences are illustrated in Figure 15.

Figure 15: Comparison of Bézier Curve and GIS Arc (drawing C. W. Blackall)

Comparison of Adobe Illustrator and GIS drawing technologies

In Fig. 15 Adobe Illustrator uses Bézier 'splines' to produce continuous, mathematically precise, curves and lines using 'end points' and automatically generated 'control points'. Manipulating the Bézier control points changes the shape of the curve or line. By comparison, GIS software uses coordinate-based data structures to represent map features, meaning that each feature is represented as continuous strings of XY coordinate pairs (called vertices).

Bézier splines are clearly a more efficient method of drawing curves and lines than continuous strings of XY coordinates pairs. For example, the Bézier 'spline' is drawn using only two endpoints, whereas in Figure 15 above the same curve is drawn using 9 vertices, and a 'smooth' example using 17. But no matter how many vertices are created, an arc in GIS is never perfectly smooth. Importantly, the plans drawn with Adobe Illustrator retain their precision when viewed or printed at high magnifications, whereas the plans digitised with GIS software appear crude and 'blocky' in comparison.

Finally, an added advantage of using Adobe Illustrator for digitisation of plans is that it produces high-resolution files that can be used for print and web-based publications. GIS software also produces attractive output, but Adobe Illustrator is much better integrated with a wide range of industry-standard graphics, desktop publishing, and multimedia software, and has many more output options.

6.3.2 Digitising Vetera I

This section details the process of producing a digital plan of Vetera I from the published site plans (Lehner 1930, pl. II; Hanel 1995, pl. 169, plans. 1-12,) so that it was available for the spatial analysis of the artefacts and artefact assemblages from this site. As described, we needed to capture the various printed plans, to digitise them using Adobe Illustrator 10, import them into a GIS format with the aid of MAPublisher, and then create the final version of the digital plan using ArcGIS.

As noted in Section 6.2, no single and precise published plan exists for the entire site of Vetera I, with all of its buildings, structures, trenches and pits. Hanel's publication included a plan of the fortress at 1:5000 (Hanel 1995, pl. 169), and twelve plans of the trenches excavated, at various different scales (Hanel 1995, plans 1-12). For example, Hanel had published plans of the 1629 trenches at scales of 1:2000 (1995, plan 1), 1:400 (plans 2-3, 5-12), and 1:350 (plan 4). Because these plans were drawn at different scales, considerable effort was required to consolidate them to a common scale. In addition, few of these plans could be precisely registered alongside their neighbours because of differences in scale and alignment, but also because of errors and inconsistencies in the original drawings that became apparent after scanning.

6.3.3 Redrawing the plans in Adobe Illustrator

After the site plans had been scanned and the problems identified had been resolved (using Adobe Photoshop), a consolidated TIFF file was imported into Adobe Illustrator and assigned its own 'layer' (the capacity to place graphics into virtual layers is common in graphics software). Separate layers were also created to isolate certain areas of the fortress, or particular construction phases. This made redrawing the plans easier to manage and helped with the staged transfer of data from Adobe Illustrator to ArcGIS when the new digital plan was completed.

The overall tasks of redrawing the plan consisted of activating the relevant layer of the Adobe Illustrator file, and redrawing the details of buildings and other major features using Illustrator's Bézier drawing tools. As discussed, drawing with Bézier drawing tools is quite unlike the vertex-by-vertex digitisation process in GIS. Straight lines are simple to draw in GIS but closed shapes, particularly the type of irregular 'polygons' common in archaeological plans, are not. Polygons are multi-sided figures that represent areas on maps or plans in GIS. Every polygon contains one label point within its boundary (for example a unique identifier) and has attributes that describe the geographic features they represent. It is not uncommon for digitisation errors to occur when creating polygons, such as 'gaps' in boundaries that result in their being processed as lines. These gaps need to be 'joined' in order to create true polygons. In the Vetera I plan, the most important polygons were the excavation trenches. The major problem here was the number of excavation trenches that either overlapped or had missing boundaries, which meant that they had to be redrawn as self-contained, gap-free, polygons. Because of the errors and inconsistencies in the original plans, we made the decision to redraw each of the 1629 trenches using Adobe Illustrator. Although time-consuming, this process was the only way of effectively dealing with these problems.

After redrawing the Vetera I plans in Adobe Illustrator we had a digital plan comprising separate layers for each phase of fortification, each building and structural feature, including pottery kilns, and a layer for the 1629 excavation trenches. This overall procedure was followed for other sites we have used in this project.

6.3.4 Exporting the plan as GIS data

In the next step, each layer of the Adobe Illustrator file was converted into 'shape files' using the MAPublisher software plug-in. Shape files (with the file name extension .shp) are one of the file formats used by ArcGIS.

Before the Adobe Illustrator layers were exported as shape files, a unique identifier (a number) was assigned to each of the 1629 excavation trenches. A new list of trench numbers was developed by Fairbairn and Ellis because the trench numbering in the Vetera I plans and catalogue was inconsistent, but also because a common list of unique identifiers was required to link the digitised features in the Adobe Illustrator layers with the artefact information in the Access database. Thus, in database terms, the trench numbers provided the common database 'key' that enabled the various data sources to be 'merged', thus allowing successful import into ArcGIS.