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GIS Guide to Good Practice |
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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.
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. |
The right of Mark Gillings, Peter Halls, Gary Lock, Paul Miller, Greg Phillips, Nick Ryan, David Wheatley, and Alicia Wise to be identified as the Authors of this Work has been asserted by them in accordance with the Copyright, Designs and Patents Act 1988. All material supplied via the Arts and Humanities Data Service is protected by copyright, and duplication or sale of all or part of any of it is not permitted, except that material may be duplicated by you for your personal research use or educational purposes in electronic or print form. Permission for any other use must be obtained from the Arts and Humanities Data Service(info@ahds.ac.uk). Electronic or print copies may not be offered, whether for sale or otherwise, to any third party.
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