I’ve created a new visualisation, a dasymetric map of housing demographics which you can see here, which attempts to improve on the common thematic (a.k.a. choropleth) maps – a traditional example is shown below – where areas across the country are colour-coded according to some attribute. My visualisation clips the colour-coding to the building outlines in each area, leaving open ground, parks etc uncoloured.
The Traditional Approach
The shortcoming of choropleth maps is that each area is coloured uniformly. If the attribute being measured is a property of the houses in that area, such as much of the census data, then choropleth maps not only colour the houses in each area, but also the parks, rivers and mountains that might also be contained within the area, even though the data being displayed arguably only applies to the houses. This means that geodemographic classification results that predominate in rural areas tend to overwhelm a map at smaller scales – as can be seen in the map on the right – where the green represents a countryside geodemographic.
An alternative to choropleth maps is to use cartograms. These distort the area, elastically, to tessellating hexagonal groups or to circles (Dorling cartograms), to match typically population rather than geographic extent, so that the colours are represented more fairly, but cartograms are very difficult for most people to interpret and relate to familiar physical features. They can look very “alien”. One further alternative is dot distribution maps – these assign dots of colour, randomly within each area. This reduces the colour density correctly in sparsely populated areas, but distributes the dots evenly across empty parks and rows of houses, if both are in a single area, and imply single points of population.
Clipping the Choropleth Maps
My visualisation attempts be the best of both worlds, by retaining the familiar geographic shape of the UK and its towns and cities, but not swamping the map with colours in all areas, and indeed ensuring that unpopulated areas have no colour. This is possible because Ordnance Survey Open Data includes Vector Map District. The second release of this dataset improved the quality of building outlines considerably, allowing distinct rows of buildings on streets to be seen and even individual detached houses. Unfortunately building classifications are not included, so the process necessarily colours all buildings, rather than just the residential ones that formed part of the census data. This is why, for example, the Millennium Dome in Greenwich appears, even though no one (hopefully!) lives there.
The major shortcoming of doing this is that it falsely implies a higher level of precision within each Output Area, by often showing and colouring individual buildings, whereas the colour is representative as an average of the properties in the area concerned, rather than telling you something about that particular building itself. That is, the technique is showing no new or more detailed data than can be seen in the traditional choropleth maps, but tends to mislead the viewer otherwise. This is balanced by making the map seem more realistic, by not unformly covering everything in the area with a giant blob of a single colour.
The map can be considered to be a dasymetric map, albeit one where the spatial qualifier, population density, is one of two values – high (in a building) or zero (not in a building).
Booth’s Poverty Map
An inspiration for this kind of map is the Charles Booth Poverty Map of 1898-9, although my example is considerably less sophisticated. For this map, Booth (and his assistants) visited every house, to determine the demographic of the house, and then painstakingly coloured in the houses, along the streets. His map therefore did not suffer from the falsely implied accuracy – his map really was as accurate as it looks. The Museum of London, incidentally, has a “walk in” Booth poverty map, I featured it on Mapping London blog last year.
The photo above compares Booth’s map (from a photo of the map in the Museum exhibition, including a friend’s hand) with my map, for the Hackney area in London.
OAC, IMD and London
My main geodemographic map is showing the OAC (Output Area Classification), which was created by Dan Vickers in Sheffield in 2005, and is based on data from the 2001 census. The areas used are Output Areas, there are around 210,000 of them in the UK, each one with a population of roughly 250 people in 2001.
The OAC map is not particularly illuminating for London – the capital is considerably more ethnically diverse than most other parts of the country, but because the clustering process used to create OAC is run across the whole country uniformly, only one Supergroup appears to show such ethnically diverse areas – “7″ (Multicultural), rather than showing the variety within this group that extends across the capital. With this in mind I have created an alternative map, which colours the housing according to the IMD (Index of Multiple Deprivation) rankings. This covers England only, and the data is only available at larger spatial units, called LSOAs (Lower Super Output Areas) but is more up-to-date, being from 2010, and shows considerable more variety across London. Use the link at the bottom of the visualisation to switch between the two.
You can view the map here. It uses geolocation to attempt to zoom to your local area, if you allow it to – it will probably ask you to allow this when you visit the site.