The maps are a type of cartogram, where the areas, in this case local authority districts, have been simplified by forming them from hexagons (or parts of hexagons). The number of hexagons used for each local authority depends on the population of the local authority. For example, the largest local authority, by population, is Birmingham, so we have used 11 hexagons. Small local authorities, such as Rutland, become just half a hexagon. Hence the size of a local authority on the map is roughly proportional to its population.
Dorling and Thomas (2004) explain the rational behind making a cartogram:
This particular cartogram was developed by Bethan Thomas of the University of Sheffield, based on work by Danny Dorling and Helen Durham. It has been modified for this application by John Pritchard.
What we have mapped
The colours used on the map show local authority control for each year from 1974 to 2007. There aren't local elections every year, but the elections system is sufficiently complex that it makes sense to draw a map for every year.
The data has been compiled by Michael Thrasher and Colin Rallings of the LGC Elections Centre.
Geography, and changes in the map
The local government structure of Britain (especially England) is somewhat complicated, and has changed over the period that we are mapping. Some parts of England still have a two-tier local government system of County Councils and District Councils. For this series of maps, we have mapped the lower layer of local government - districts, metropolitan and London boroughs, and unitary authorities.
There have been changes to this level of geography in Scotland, Wales, and some parts of England. The cartogram therefore also changes; hexagons are broken up or merged. If you hover the mouse over the map, you should see the authority names. These names are for the period 1997-2007. Due to the changes, you may notice that, for example, the Isle of Wight is labelled as one local authority, but in the years up to 1995 there were two local authorities, and therefore often two different colours.
Dorling, D. and Thomas, B. (2004) People and Places: a 2001 Census Atlas of the UK. Policy Press, Bristol.