spatial estimation and modelling branch open meeting: cartograms
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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.