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GIS Guide to Good Practice
Section 4: Structuring, Organising, and Maintaining Information

4.1 Layers and Themes

The terms layer and theme are used almost interchangeably by many people - archaeologists and GIS practitioners included - yet are given very distinct meanings by some software suppliers and in some specific disciplines, for example in Computer Aided Design (CAD). For the purposes of this guide these terms will be used as follows. A theme is a collection of like objects, for example 'pottery', 'Iron Age sites', etc. A layer is a group of specific objects within a theme - for example, 'Stamford Ware' within the pottery theme or 'Hillforts' in the Iron Age site theme. In order to avoid confusion, it is important that the names given to such themes and layers are both descriptive and free from ambiguity.

Figure 2: Layers and Themes - Figure created by Peter Halls using data from the Cottam Project directed by Julian Richards. Image copyright © Archaeology Data Service. Two layers from the Cottam Project GIS (red layer with coin findspots and black layer with metal artefact findspots) are combined to represent the metal object theme. Two other themes are illustrated in this image, one for aerial photograph interpretations (line data) and another for geophysical survey data (raster image).
Vector and Raster illustration

The purpose of the theme/layer approach is to provide a framework for collecting together objects of similar nature - in terms of either representation and/or descriptive type. Thus, different Iron Age site types might be gathered together because they are related in terms of both the representational type -- a line or point object -- and because of their nature or purpose -- delineation of the landscape location selected for settlements in the Iron Age. In the same way, a database of finds of pottery might be defined in locational terms as a collection of points, each of which might relate to an individual object, or closely related group of objects.


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© Mark Gillings, Peter Halls, Gary Lock, Paul Miller, Greg Phillips, Nick Ryan, David Wheatley, and Alicia Wise 1998

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