Dave Martin's Population Surfaces
Martin (1989) introduces a method for the construction of such models using populaton-weighted zone centroid data from the UK census small area statistics. Basically, each centroid location is examined in turn and a local distance decay function is specified, dependent on the local density of other centroids (this is a proxy for zone size and therefore population density). The population associated with each zone centroid is then redistributed from each centroid into the cells of a regular geographical grid, creating a raster model with population estimates in each cell. Many cells remain unpopulated, thus reconstructing the settlement pattern. Much of the early work on this technique was conducted in collaboration with the late Ian Bracken, and an ESRC-funded research project Interpretive analysis of intercensal population change using surface models was completed in 1993. This resulted in a set of national surface models of the and retrieval software based on the 1981 and 1991 UK censuses, which are available online through the national datasets service at Manchester Computing (MIMAS). The Census Information Gateway provides lots of useful information about census data sources. Since the original work, the SURPOP V2.0 WWW interface to the information, surface models and retrieval software is now available, as part of a further ESRC-funded project with Nick Tate which includes 1991 models for Northern Ireland as part of the database.
Other researchers have used the models in a variety of ways, and Ian Turton, then at Leeds, assembled a gallery of models for individual British cities from these data. David Unwin (Birkbeck), Peter Fisher (Leicester) and others have developed the concept of the 'landscape analogy' between population surfaces and physical terrain. Julii Brainard at UEA developed an agro/forestry GIS application which uses the population models as a base population distribution. Unfortunately their original websites are now down, but the UEA team have published on this - use a search engine for references. Others currently working on applications of the population surface models include: Victor Mesev (Bristol), Kelvyn Jones (Portsmouth), Mike Farmer (SOAS), Jean-Charles Denain (Rouen) and Mike Batty (UCL).
Other developments include a version of the algorithm which constrains population totals within zones whose boundaries are known, evaluation of the accuracy of the MIDAS models, and various tools for the further analysis of modelled populations in this form. A 1996 paper in the International Journal of GIS compares these models with conventional zone-based representations.
An example of a population-constrained surface model of the city of Southampton, with ward boundaries for reference, is given below. The shading represents population density with the lighter values being the highest densities. The River Itchen (centre, north to south) and Test (across the south west corner) are clearly apparent, as are the common and municipal sports centre (two dark areas just west of centreline). Residential pattern is clearly revealed, and true population totals are preserved within each ED. The separate populated area visible at the top of the image is the southern part of Eastleigh and Chandlers Ford. The cell resolution of this model is 50m.
(Source: 1991 SAS and DBD data - ESRC/UFC-JISC purchase)
2001 Census data and boundaries are now available for output areas, and centroids are provided with the boundary data suitable for surface modelling using the SurfaceBuilder tool. SurfaceBuilder is a Visual Basic program which I have written to allow researchers to construct their own surface models of the type described here, although it currently only incorporates the basic modelling, and does not include tools for constraining within zone boundaries, for example. SurfaceBuilder will read centroid x,y,z data from a text file (csv or tab delimited, etc.), build a surface, display it on screen and export it to a range of ascii data formats.
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