It should be noted that, to date, the principal applications of GIS within archaeology have been restricted to 2-dimensional models, and at best 2.5 dimensional representations. The latter are a result of the inability of currently available analysis and display tools to adequately deal with truly 3-dimensional data. As a direct result, the issues we will be discussing here are concerned solely with the integration, management, analysis and archiving of representations of 2/2.5-dimensional space. Issues concerning 3-dimensional spatial representations will be discussed in detail in a forthcoming CAD Guide to Good Practice and no doubt in summary in future editions of the present guide.

3.2.1 The Vector model

In the vector model, the spatial locations of features are defined on the basis of co-ordinate pairs. These can be discrete, taking the form of points (POINT or NODE data); linked together to form discrete sections of line (ARC or LINE data); linked together to form closed boundaries encompassing an area (AREA or POLYGON data). Attribute data pertaining to the individual spatial features is maintained in an external database.

Figure 1: Vector and Raster Data - Figure created by Peter Halls using data from the Cottam Project directed by Julian Richards. Image copyright © Archaeology Data Service.

Cottam Project geophysics data displayed as a raster background and overlaid with an aerial photograph interpretation (blue line data), coins (red point data), and metal artefacts (black point data).

In dealing with vector data an important concept is that of topology. Topology, derived from geometrical mathematics, is concerned with order, contiguity and relative position rather than with actual linear dimensions. A good illustration of a topological map is that of the London Underground metro system. This well-known map is a precise representation of the stations (points or nodes) and the routes (arcs or lines) between them, yet provides only a very approximate indication of their relative locations and no indication of distances between them.

Topology is useful in GIS because many spatial modelling operations do not require co-ordinate locations, only topological information -- for example to find an optimal path between two points requires a list of the arcs or lines that connect to each other and the cost to traverse them in each direction. It is also possible to perform the same spatial modelling and interrogation processes without using stored topology, by processing the geometrical data directly, as in such GIS as ArcView and MapInfo, or by generating topology on the fly, as and when it is required. The latter is the approach taken by Intergraph, amongst other major GIS suppliers.

For a detailed discussion of the vector model see Aronoff 1989 and Burrough 1986.

3.2.2 Important information to record about vector files

The following information should always be recorded when assembling, compiling and utilising vector data:

• The data type: Point, Line or Area
• Type of topology which the file contains, such as line, network, closed area or arc-node
• Details of any automatic vector processing applied to the theme (such as snap-to-nearest-node)
• State of the topology in the file, particularly whether it is 'clean' (topologically consistent) or contains inconsistencies that may require further intervention or processing. This is particularly important where arc-node data is concerned
• Projection system
• Co-ordinate system

3.2.3 The Raster model

Here the spatial representation of an object and its related non-spatial attribute are merged into a unified data file. In practice the area under study is covered by a fine mesh, or matrix, of grid cells and the particular ground surface attribute value of interest occurring at the centre of each cell point is recorded as the value for that cell. It should be noted that whilst some raster models support the assignment of values to multiple attributes per discrete cell, others adhere strictly to a single attribute per cell structure.

Within this model spatial data is not continuous but is divided into discrete units. In terms of recording where individual cells are located in space, each is referenced according to its row and column position within the overall grid. To fix the relative spatial position of the overall grid, i.e. to geo-reference it, the four corners are assigned planar co-ordinates. An important concept concerns the size of the component grid cells and is referred to as grid-resolution. The finer the resolution the more detailed and potentially closer to ground truth a raster representation becomes.

Unlike the vector model there are no implicit topological relationships in the data, we are after all not recording individual spatial features but instead the behaviour of attributes in space. For a detailed discussion of the raster model see Aronoff 1989 and Burrough 1986.

3.2.4 Important information to record about raster files

The following information should always be recorded when assembling, compiling and utilising raster data:

• grid size (number of rows and columns)
• grid resolution
• georeferencing information, e.g. corner co-ordinates, source projection.