Spatial Estimation and Modelling Branch Open Meeting: CARTOGRAMS

Outline Maps in SEMB What are Cartograms? How do they differ from maps? What can they offer SEMB and ONS? Can we do them?

Maps in SEMB What does SEMB use maps for? For what audience? What methodologies do we employ? What are the chief qualities of these methodologies? Does this matter?

What does SEMB use maps for? Representing data variability across space - thematic maps Absolute or relative data by Electoral Wards, Local Authority Districts etc.

For what audience? Internal to division, for own reference; Internal to ONS; External publication, for dissemination world-wide.

What methodologies do we employ? Choropleth mapping on real-world boundaries No other methodologies used

Chief qualities of choropleth maps Advantages Simple, quick and therefore cheap All GIS software capable of producing them by default Disadvantages Urban areas, which are generally subdivided into a multitude of statistically interesting units, are often too small to be seen. Large rural areas with sparse population, dominate.

A blast from the SAEP past Proportion of HH with no access to a car (Tyne and Wear) By post code district, which are very roughly equivalent in terms of population Approximately four times as many postcode sectors are in the red ranges, yet green dominates the map

Why is this? A choropleth map aims to represent data on a graphical backdrop based on geographical reality. The relative size of component area is defined by their actual size. The shape of this backdrop will be affected by the chosen projection

How do cartograms differ? Cartograms are maps whose component regions are rescaled according to a given attribute (e.g. population). A region whose attribute data is relatively large will be represented by an area greater than its actual area, while a region whose data is relatively small will be represented by a smaller area. Cartograms give a sense of the proportion of the attribute statistic relative to the other regions.

How can this look? Source:

What is the benefit? The yellow and green colour ranges dominate the choropleth map, suggesting that they are relatively the most important. The cartogram (where symbol area is a function of population) gives a quite different overall pattern, revealing a “truth” which was quite hidden in the choropleth.

What are the weaknesses? Can be difficult to identify areas with confidence Can be difficult to produce Can be ugly

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