Cartograms

Last week the BBC News Magazine featured an article on cartograms. The cartograms were produced by the University of Sheffield’s Geography Department and originally published on their Worldmapper website. I find the maps easy to understand and especially effective at representing global inequalities such as water use, but they also work well for population distributions and are most often used in elections to show the relative influence of each election district on the election outcome. Cartograms are produced by placing a regular grid over a conventional map of whatever it is you want to show- such as population- and then stretching or shrinking the grid cells based on the variable (in this case population) of interest.  A good analogy would be drawing a map of the UK on a balloon and stretching it to distort the size of southern England in relation to Wales if you want to show its high population density. Simple distortions in relative size have the limitation that the outlines on the regions of interest have to change in order to match up and remain contiguous. This makes the regions unrecognizable and the maps hard to interpret.  Gastner and Newman solved this problem (technical article)  through some fairly advanced physics, so now we can create cartograms that keep the outline of the regions we are distorting consistent and therefore much simpler to interpret. Tom Gross has created a Cartogram Toolbox for ArcGIS, whilst ScapeToad is very good free cartogram software. Using the former I have taken some of my surname data and produced the cartograms below. They provide a different and, in some cases, much more powerful method of highlighting the spatial distribution of names in the UK. You can compare the effectiveness of these maps with those produced by the National Trust Names website.

cheshire

This is a cartogram representing the relative frequency of the surname “Cheshire” in the UK. You can see it is an English name with very few, if any, occurrences in Scotland, Wales and Ireland. West Central England has become the most bloated suggesting the highest proportion of  “Cheshires” in this region. I have mapped a silhouette of a correctly proportioned (British National Grid) UK behind the cartogram representation.

lewis

The above cartogram shows the surname “Lewis”. Lewis is a common name throughout the UK but the cartogram shows its Welsh (particularly South Wales)  dominance as the country has greatly expanded beyond its normal proportions.

laity

This is my favorite cartogram. It shows how concentrated many Cornish names, in this case “Laity”, are in the UK and how they rarely exist beyond Bristol.

saltmarsh

The above cartogram is for the surname “Saltmarsh”. I have included this as it nicely represents the influence of landscape features on UK naming conventions. The sheltered South Eastern and Eastern coast of the UK is where you will find the majority of UK saltmarshes. The cartogram shows an expansion of districts on the South Eastern/ Eastern side of the UK and a reduction in district size along more exposed coasts (where there are fewer, if any, saltmarshes), therefore suggesting a close correspondence between the number of people with the name and their proximity to the landscape feature!