A place in history: a guide to using GIS in historical research

CHAPTER 2: THE WORLD AS VIEWED THROUGH A GIS

2.6 Bringing it all together with layers

Each separate theme of information is represented as a layer in the GIS. To build up a realistic representation of the study area a variety of layers are usually used. For example a DTM may be used to model the relief of an area, a raster surface to model its land-use, a point layer used to represent buildings of interest, a line layer to represent rivers, a network layer to represent the transport system, and polygon layers to represent field patterns and administrative boundaries. All of these contain the relevant attributes. Exactly when to use a separate layer is often a matter of choice. For example, if we have three types of buildings - churches, hospitals - and museums, these could be stored as three separate layers, or they could be stored on a single layer with attributes to say what type of building each point represents.

Most common GIS software packages can handle most of these types of data although the majority will concentrate on one. MapInfo and ArcView are both primarily vector systems that have limited raster and terrain modelling functionality. Spans and Idrisi on the other hand are primarily raster based.

A good example of a study area modelled through a GIS is provided by Pearson and Collier's work on the parish of Newport in Pembrokeshire (Pearson and Collier 1998). They were interested in land ownership and agricultural productivity in the mid-19th century. To investigate this they needed to combine environmental data such as soils, slope, htmlect and altitude, with historical statistics such as census data, and information from the tithe survey, a detailed and comprehensive survey of land ownership and land-use carried out in England and Wales during the early 19th century. Information from the tithe surveys was recorded as polygons. Each field was represented as a polygon with attribute information such as the owner, the occupant, the field name, the state of cultivation, the acreage and the tithe rent-charge. The environmental data started as altitudes for a regular grid of points 50m apart covering an area of 10km by 10km. This was used to create both a raster grid for which each cell had an altitude, and a terrain model that allowed slope and htmlects to be calculated. Bringing the data together in this way allowed a detailed reconstruction of the various factors affecting agricultural productivity in the mid-19th century, and provided a starting point for more sophisticated analyses.

A second example is provided by Siebert (2000). He was interested in reconstructing the history of Tokyo's development with a particular emphasis on physical features such as the shoreline and rivers, the administrative areas, the population distribution and the rail network. To do this he created a variety of layers for each type of feature: the rail network was represented by line layers; the shoreline and the rivers were represented either using lines (for small rivers) or polygons (for larger ones); and the administrative areas were clearly represented as polygons, as were the census data. As with Pearson and Collier's work, bringing the data together in this way allows a comprehensive picture of the study area to be built up.