spatial estimation and modelling branch open meeting: cartograms

Spatial Estimation and Modelling Branch

Open Meeting: CARTOGRAMS

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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?

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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)

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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?

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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.)

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For What Sort of Audience?

For our own reference within the division;

For other divisions, but internal to ONS;

For external publication.

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What Methodologies Do We Currently Employ?

Choropleth mapping on real-world boundaries

Err… that’s about it

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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

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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.

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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)

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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

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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

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Can GIS Help Us?

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Further Examples

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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.

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Example Choropleth Maps

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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

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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.

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Prop.n of HH w. No Access to Car

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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

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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.

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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

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Areal Transformation Cartogram 1

Source: http://www.geog.qmw.ac.uk/hgis/conference/cartogram.htm

Areas to symbols

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Areal Transformation Cartogram 2

Areas togeometries

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Areal Transformation Cartogram 3

Areas to distortions

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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.

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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

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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.

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The Teleki Cartogram

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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).

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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?)

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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.