Section 4 - Structuring, organising, and maintaining information

4.2 Choice of vector, raster or combined forms of spatial database

The choice of vector, raster, or combined, forms for the spatial database may be determined by the GIS in use. For example, you cannot easily use the vector model within a raster GIS such as GRASS or IDRISI. Similarly, a vector GIS such as pc-Arc/Info cannot manipulate raster data.

Vector means of managing and manipulating the data are to be preferred for handling information relating to discrete points, delimited boundaries, alignment of linear features, etc. Thus a vector model would be used for storing, and manipulating, an excavation plan.

Raster means of managing and manipulating the data are to be preferred for handling continuous information such as altitude (see Digital Elevation Models, below), vegetation, etc., and are the digital form in which information from Geophysical Survey, Aerial Photography, and other forms of Remote Sensing and non-invasive survey, are delivered.

Where both data types are required to be used together a GIS capable of manipulating both is required - such as ArcView (with the Spatial Analyst option installed) or Intergraph MGE (with the Grid Analyst option installed).

When combining and integrating information from a variety of sources the following points should be kept in mind:

All spatial data must be recorded in the same co-ordinate system. Data which are recorded to some other system must be transformed/projected to the required co-ordinate system.
All spatial data should be to the same spatial resolution, or scale. It is not possible to get meaningful results from the combination of spatial data recorded to a scale of 1:250, as might be the case for an excavation site plan, with road alignments recorded to a scale of 1:250,000. In the former example 1mm represents 25cm, and in the latter example represents 250m. Spatial data recorded to scales of greater than around 1:10000 involve considerable generalisation of alignments to avoid features conflicting. This is especially true of paper maps drawn to such scales.
Non-spatial information to be combined, or integrated, must use the same field definitions, encoding regimes, etc. Where different schemes are used it will be necessary to convert or translate the data to the required scheme.

The National Geographic Data Framework (NGDF), recently established in the UK, is producing a series of guidelines for the definition and storage of spatial data such that it has maximum potential for future use. These guidelines include documentation standards. See http://www.ngdf.org.uk/ for more information.

From the point of view of specifically archaeological data, the recommendation must be to use one of the data formats defined in Section 6.