Spatial Estimation and Modelling Branch Open Meeting: CARTOGRAMS

2 of 35 What’s This All About? An opportunity to expand our minds! A look at how we currently visualize data Our our current methodologies good enough? Could we improve or extend them?

3 of 35 Outline of the Open Meeting Maps in Methodology Group (MG) Overview of Thematic Maps Overview of Cartograms How do Cartograms Differ from Thematic Maps? What can Cartograms Offer MG and ONS? Can we Draw Cartograms? Cartograms in ONS: Recommendations

Maps in Methodology Group (MG)

5 of 35 Maps in Methodology Group What does MG use maps for? For what audiences do we produce them? What methodologies do we currently employ? What are the chief qualities of these methodologies? Does it matter?

6 of 35 What Does MG Use Maps for? To represent data variability across space To illustrate patterns in absolute or relative data about the population, as revealed by differences between areas (Electoral Wards, Local Authority Districts etc.)

7 of 35 For What Sort of Audience? For our own reference within the division; For other divisions, but internal to ONS; For external publication.

8 of 35 What Methodologies Do We Currently Employ? Choropleth mapping on real-world boundaries Err… that’s about it

9 of 35 Does It Matter? From the MG Business Plan 2001/02. The group will...be a leader as well as a guardian of the ONS/National Statistics way of doing things, to ensure quality and integrity in our products and service. Emphasis in the original. So, we have a duty to use the best methodologies. and to encourage their use elsewhere in ONS. This applies equally to maps.

Thematic Maps

11 of 35 Thematic Maps: show the spatial distribution of a particular geographical phenomenon" or theme (Dent 1996); display the "structural characteristics of some particular geographical distribution" (Robinson 1975); depict "distance and directional relationships, patterns of location, spatial attributes of magnitude change" (Dent 1996); consist of a base map, where relative size and location of each component area is geographically ‘normal’, and a thematic overlay.

12 of 35 Qualitative or Quantitative Qualitative: "spatial distribution or location of kind" (Dent 1996) Quantitative: “a symbolised generalisation of the information contained in a table" (Jenks, 1976)

13 of 35 Some Methodological Challenges for Thematic Maps Choices relating to symbology Choices relating to colours Portrayal of: Urban areas Thinly inhabited areas Areas where several variables are mixed together Areas where a scattering of one variable exists among others

14 of 35 A Selection of Thematic Map Methodologies symbols (+ proportional) pie charts (+ proportional) dot maps grid-based maps area-class maps Surface/contour mapping 3-D visualisations And the humble choropleth

15 of 35 Can GIS Help Us?

16 of 35 Further Examples

17 of 35 Choropleth Mapping Each component area is shaded or coloured according to the value of the map’s subject variable(s). Component areas may be EDs, Wards, etc, or combinations thereof. Boundaries may be displayed or suppressed.

18 of 35 Example Choropleth Maps

19 of 35 Qualities of Choropleth Maps Advantages Relative simplicity – but beware! All GIS software capable of producing them Disadvantages An arbitrary zone system is imposed on the data Bear no relationship to the built environment Urban areas, often subdivided into a multitude of statistically interesting units, have no visual impact Large rural areas with sparse population, dominate Often very misleading and hard to interpret

20 of 35 A Blast From the SAEP Past “Proportion of HH with no Access to a Car”. By Postcode District, which are very roughly equivalent in terms of population. In Tyne and Wear, approximately four times as many postcode sectors are in the red ranges as green, yet green dominates the map. This makes the map very hard to interpret.

21 of 35 Prop.n of HH w. No Access to Car

22 of 35 Why? A choropleth map aims to represent data on a graphical backdrop based on geographical reality. The relative size of “component areas” – e.g. wards - is fixed by their actual size, rather than the value of their data. (The shape of this backdrop will be affected by the chosen projection).

Cartograms

24 of 35 How Do Cartograms Differ From Thematic Maps? Cartograms are diagrams whose component units are scaled according to a given variable (e.g. population). AREA  VALUE. The area of a unit is directly proportional to the value of the variable it represents. Cartograms give a sense of the value of the variable in one unit relative to its value in other units.

25 of 35 Two Types of Cartogram Areal Transformation Where the component areas into which the data is divided (e.g. wards, LADs) are themselves transformed: in size in shape Distributional Transformation Where a unit of data is represented by a given area of ink, placed systematically on a normal geographical backdrop

26 of 35 Areal Transformation Cartogram 1 Source: Areas to symbols

27 of 35 Areal Transformation Cartogram 2 Areas to geometries

28 of 35 Areal Transformation Cartogram 3 Areas to distortions

29 of 35 How Might We Gain? The cartogram (where symbol area is a function of population) reveals pattern within data completely obscured by choropleth thematic maps. Abandoning the geographical underlay, and spatial re- arrangement, prevents area symbol overlap in urban areas, and permits a clearer, yet smaller product to be created.

30 of 35 At What Cost? Cartograms: Can make it difficult to identify areas with confidence Can be difficult to produce May not be aesthetically pleasing (...but then what’s intrinsically pleasing about real boundaries other than familiarity?) Users may need to be persuaded of their value

31 of 35 Distributional Transformation Cartogram Very similar in concept to the “dot” type of Thematic Map. However: Data units coalesce or expand as required into solid areas. No overlap of data. All data in the area of the map can be shown. Urban population given equal weight to rural population. Nevertheless, data no longer displayed in true location, so does not show the real pattern of settlement. Opaque methodology undermines trustworthiness. Best example: Teleki.

32 of 35 The Teleki Cartogram

33 of 35 What Could Cartograms Offer MG and ONS? Opportunities: to “Lead and Guide” (MG Business Plan); to broaden the range of ONS data visualization tools; to aid data interpretation; to enhance the impact of MG and corporate publications. for close and fruitful collaboration with ONS Geography; to draw on academic experts (Dorling).

34 of 35 Can We Draw Cartograms? Yes, but not yet sure how Probably using Adobe Illustrator / MAPublisher plug-in Not a feature of standard GIS tools WWW. Research and collaboration with ONS-G (and academics?)

35 of 35 Cartograms in ONS: Recommendations Areal Transformation Cartograms. Some templates for these types of cartogram could usefully be researched jointly by MG and ONS-G. Distributional Transformation Cartograms. Too subject to bias: should not be used. Wider use of approved types should be encouraged in ONS, with methodological guidance/vetting from a corporate centre of expertise.