1. Introduction

Archaeologists have been using vector-based digital tools for many years (Eiteljorg 1989), and today they are used routinely in many areas of archaeological work. This method of organising visual information using points, lines and polygons is a unique system that can inform the archaeological process. Unlike raster images, which are made up of an array of disparate pixels (Jones 1997), vector images exist within a defined set of coordinates (Eisenberg 2002, 2). The ability of a vector image to 'know where it is' is significant for a spatial discipline like archaeology. Vector graphics are resolution independent, and display the same clarity at any level of magnification (Laaker 2002, 7). Vector graphics can also be used to organise drawings into layers (Watt 2002, xvii), and the pan and zoom features make different forms of analysis possible as well (Eiteljorg 1989). Raster and vector graphics are not competing forms of visualisation; rather they are different formats for different purposes and are sometimes most powerful when used in combination.

Archaeologists use vector graphics in a variety of ways. While visual tools like raster-based Geographical Information Systems (GIS) and digital photography are very important to archaeology, there are many other forms of visualisation that are often best handled using the vector format. Some of the primary uses include the electronic illustration of artefacts, and computer-aided drawing (CAD) of excavation plans, sections and site maps. This is a significant list, bearing in mind that The Publication of Archaeological Projects: a user needs survey (PUNS), published by the Council for British Archaeology, places maps, plans and sections third in importance, only behind the introduction and conclusion in an archaeological report (Jones et al. 2001). The process archaeologists use to create these visualisations differs, as do the specific vector graphics programs appropriate for each application. Most have freeware, shareware, commercial choices and an industry standard, all adopted with varying degrees of utility for use in archaeology, and together they form a comprehensive group of effective tools from which to choose. While the majority of these tools were not created with archaeology in mind, archaeologists have always been resourceful and adaptable, and their use of vector technology is no exception.

Creating and interpreting excavation plans, sections, and site maps is an important part of visualisation in archaeology. Without this fundamental spatial information it would be almost impossible to make sense of any archaeological fieldwork. The transition of this information to a digital format began in the late 1980s with archaeological survey teams using vector-based CAD programs (Middleton 1998, 6; Eiteljorg 1989). CAD drawings can subsequently be incorporated into a variety of other formats that use vector-based information like GIS programs. Today, most GIS programs have the ability to create visualisations that are both raster and vector based, and to use them in combination (Wheatley and Gillings 2002, 16). In many cases CAD drawings are also the basis for three-dimensional Virtual Reality (VR) modelling, which also combines raster and vector technology to create a unique form of visualisation (Terras 1999).

CAD can be seen as a powerful tool in its own right for creating digital plan, section or site drawings, as well as an essential building block for much of the spatially derived digital visualisations currently used in archaeology. Designed for use with Microsoft's Windows operating system, AutoDesk's AutoCAD has been the leading vector design and drafting program since its release in 1982 (Anon. 2006a). AutoCAD's ability to plot images from information derived from a database in actual scale made it appealing to archaeologists early on (Eiteljorg 1988; Holloway and Lukesh 1994) and there are a variety of ways to convert drawings into digital format for manipulation. Like digital archaeological illustration, use of CAD speeds up repetitive tasks and creates very accurate drawings. Drawings can be expanded and updated after each field season to reflect new work or corrected information, unlike paper drawings that have to be redrafted each year. CAD can also be used to combine drawings created in a variety of scales into a document with a common scale (Beex 1995, 101) and data gathered using disparate technologies, like traditional survey and photogrammetry, into a single drawing (Batchelor 1995, 231).

Because the vector information in AutoCAD is tied to a database, data can be imported, exported and queried. In addition, CAD can organise information into thematic layers, which can be turned on or off to highlight different combinations of information. The layers in a single drawing can show structural details, the hypothetical location of structural details and how different types of artefacts or structures are distributed throughout a site. Layers can also show chronological change over time by dividing them into phases (Eiteljorg 1989). Because AutoCAD can create visualisations in three dimensions, the relationships of different archaeological elements can be explored in both horizontal and vertical space simultaneously. This can be very useful, especially in situations where it is difficult to get a complete view of how a site is structured (Main et al. 1995, 136). Creating a three-dimensional view is very difficult to achieve on paper, and for a user to change the view at will would be impossible. In some ways, the ability to view an AutoCAD drawing from different directions, in plan, elevation or isometric views is perhaps its most powerful attribute (Eiteljorg 1989).