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GIS and census design

Professor David Martin

School of Geography, University

of Southampton

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GIS and census design: what’s the connection?

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What’s the connection?

• Census as driver for

neighbourhood geographies

• GIS for census geography design

• Neighbourhood definitions

prominent in public policy

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Overview

• UK population census basics

• 2001 census geography

• Neighbourhood statistics

• Towards 2011

– Address listing

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UK population census: basics

• 10-yearly data collection

• Questionnaire delivered

to every household

• Legal obligation: 96%

coverage

• Multiple uses and users

• Single most important

social data source for

small areas

Source: ons.gov.uk

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Census topics

• Age, sex, marital status,

household structure

• Country of birth/ethnicity

• Health/disability

• Economic status

• Housing, amenities

• Travel to work and cars

• Qualifications

Source: ons.gov.uk

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100+ years of census mapping

Charles Booth: poverty

in Pimlico 1898-9

Neighbourhood

Statistics: households lacking amenities 2001

Source: neighbourhood.statistics.gov.uk

Source: booth.lse.ac.uk

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Need for information

• Policy Action Team 18:

Neighbourhood policy requires

accessible baseline data

• No definitive neighbourhoods

• Definitional anarchy

– Neighbourhood,

– community, ward,

– catchment…

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2001 census geography

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Previous census geographies

• Enumeration-based EDs

• Wide variation in population,

some sub-threshold: no data

• Significant manual redesign at

each census

• No match to postcode geography

• Not ‘neighbourhood’ building

blocks

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GIS for enumeration design

Source: ONS

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Case for redesign

• Demands from census users:

– Postcode and statutory geographies

– Uniformity of population sizes (all above

threshold)

– Control over shape (observe settlement

pattern and topographic features)

– Internal homogeneity of population

– Compatibility with previous census

geographies!

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Overview of design methods

• Synthetic unit postcode building block

polygons

– Nesting within wards (Dec 2002) and

parishes, incorporating topographic

features

• Automated zoning procedure

– Iteratively recombination of building

blocks seeking best trade-off of design

constraints

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Address-based Thiessen polygons

• Thiessen polygons

around individual

ADDRESS-POINTS

intersected with ED,

ward, parish

boundaries and road

centrelines

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Unit postcode building blocks

• Address polygon

boundaries dissolved

to form unit postcode

polygon building

blocks

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AZP (Openshaw, 1977)

• Initial random aggregation of building

blocks into output areas within ward

or parish polygon

• Compute design statistics

• Evaluate effect of possible swaps of

building blocks between output areas

• Keep swaps that improve the overall

solution

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Automated output area design

Group addresses into

postcode building blocks

Group addresses into

postcode building blocks

Automated GIS-based

output area design

procedure

Automated GIS-based

output area design

procedure

Design Constraints

(Contiguity, Thresholds,

Shape, Size, Homogeneity)

Design Constraints

(Contiguity, Thresholds,

Shape, Size, Homogeneity)

2001 Output

Areas

2001 Output

Areas

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OA design (1)

Initial random

aggregation of

postcodes into

potential output

areas

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OA design (2)

Choose one

postcode at

random as

candidate for

swapping into a

different output

area

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OA design (3)

Make the swap

and evaluate the

impact on the

overall solution

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OA design (4)

If swap does not

result in an

improvement, go

back to the

previous

configuration

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OA design (5)

Choose another

postcode at

random as

candidate for

swapping into

another output

area

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OA design (6)

If the swap results

in an overall

improvement,

keep it as part of

the solution and

examine a new

potential swap…

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Contiguities, thresholds, urban/rural

• Output areas assembled from contiguous postcodes (NB treatment of stacked postcodes)

• Output areas above 100 person and 40 household thresholds (NB treatment of sub-threshold parishes)

• Initial postcode classification to urban/rural based on DETR boundaries

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Size and shape

• Output areas should be as

uniformly sized as possible; target

125 households

minimize Σ(OApop-target)2

• Output areas should be as compact as possible

minimize distance to OA centroid…

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Distance to OA centroid

PC mean

centroid

OA centroid

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Intra area correlations

• Maximize intra-area correlations

(IAC): ratio of area level to individual

level variance

• Higher correlations ~ greater internal

homogeneity

• Tenure (4) and dwelling type (7)

categories used

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Combination of constraints

• All constraint statistics recomputed at

each iteration

• Must always meet contiguity and

threshold requirements; urban/rural if

possible above threshold

• Population, shape and homogeneity

constraints combined with equal

weighting

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Digital census geography

• 2001 Output

Areas (England

and Wales)

• Mean 297

persons; 123

households

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UK geography?

• Northern Ireland: same algorithm

as England and Wales but

digitized postcode boundaries

• Scotland: own algorithm to

maximise continuity with 1991

output areas

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Neighbourhood statistics

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Neighbourhood statistics

• NeSS as primary delivery

mechansim – originally 1998 wards

• Addition of census, and

administrative data streams

• Post-2001 geography built on

postcode > census output area >

Super OA

• Cannot continually reconstruct

small areas: data confidentiality

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NeSS geography

• Aggregated source records from

multiple suppliers

• Single hierarchy based on census

OAs (and hence primarily on

postcodes, with lookup tables)

• Estimation of attribute values to

standard geography if necessary

• Intended as durable core geography

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• OAs as building blocks of NeSS

geography

• “Lower” (pop mean 1500; n=35000)

– Automated zone design, nesting within

census wards: now created

• “Middle” (pop mean 7500; n=7000)

– Automated design, not constrained to

wards, some local consultation

Super Output Areas

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Key

Lower Layer Zones

Greenwich lower layer SOAs

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New SOA data streams

• Indices of Deprivation 2004,

2007

• Fires attended

• National curriculum assessments

• Benefits claimants

• Land use

• Air quality

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Neighbourhoods?

• More flexible building blocks and richer data than previously

• Irregular boundaries

• Little regard to landscape features

• Little regard to social geographies

• Never intended as definitive ‘neighbourhoods’

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Towards 2011

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Towards 2011

• Essentially conventional 2011

census

• Mail-out and mail-back

• Critical nature of address lists

• Internet data collection

• 4-page per person questionnaire

• Big focus on population count

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Complex geography requirements

• Stability and change

• Data supply

• Landscape features

• Social homogeneity

• Reflect contemporary housing

development

• ‘Real’ neighbourhoods!

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Address listing

• No definitive address list in

England and Wales

• USA, Scandinavia, South

Africa… seen as essential

• Address Point/Postcode Address

File/Mastermap address layer

• National Land and Property

Gazetteer

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Address strategy

• Use combination of OS

Mastermap AL 2 and NLPG

• Pre-listing and checking by

enumerators

• Can the information be shared?

• Counting the Population review –

possible action on addressing?

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Where do we go from here?

• Proposed principle of stability

• Research on residential change

– based on address and

postcode geographies

• Evaluation of population change

requiring essential OA splits and

mergers <5%

• Need maintenance methodology

• Need disclosure policy details

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UK geography?

• Northern Ireland: total change -

enforced

• Scotland: minimal change –

consulting now

• Convergence to whole-UK

approach extremely unlikely

• Counting the Population –

definitive address list???

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Research…

• Methods for improving OA

boundaries

• Data ownership issues more

important barriers than technical

ones

• How best to reflect housing

change?

• Small area geography beyond

census?