Creating consistency in British Creating consistency in British Census SpaceCensus Space

Nigel Walford and Kelly HaylesNigel Walford and Kelly HaylesCentre for Earth and Environmental Science Research, School Centre for Earth and Environmental Science Research, School

of Earth Sciences and Geographyof Earth Sciences and Geography

GISRUK06, 5-7 April 2006GISRUK06, 5-7 April 2006

Presentation OutlinePresentation Outline

Research AimResearch Aim Issues and previous researchIssues and previous research Data SourcesData Sources Methods Methods

Areal InterpolationAreal Interpolation Dasymetric MappingDasymetric Mapping

DiscussionDiscussion ConclusionConclusion

Research AimResearch Aim

To develop and implement a methodology for To develop and implement a methodology for producing consistent small geographical units with producing consistent small geographical units with associated estimates of key census counts spanning associated estimates of key census counts spanning at least the 1981-2001 British Population Censuses.at least the 1981-2001 British Population Censuses.

ObjectivesObjectives Assess alternative GIS-based methodologies for Assess alternative GIS-based methodologies for

deriving this information from existing data sourcesderiving this information from existing data sources Examine alternative ways of visualising demographic Examine alternative ways of visualising demographic

and socio-economic changes between censusesand socio-economic changes between censuses..

IssuesIssues

Previous workPrevious work Change filesChange files Census tracts (England and Wales)Census tracts (England and Wales)

1971-19811971-1981 Reformatting SAS (Scotland)Reformatting SAS (Scotland) LCT – Linking Censuses Through time LCT – Linking Censuses Through time

large spatial unitslarge spatial units 1971-19911971-1991

Issues cont’dIssues cont’d

CATT – Consistent Areas Through Time CATT – Consistent Areas Through Time (Daniel Exeter)(Daniel Exeter) LUTsLUTs Scotland onlyScotland only

Issues cont’dIssues cont’d

Census comparabilityCensus comparability Definitions changeDefinitions change Questions changeQuestions change Population base changesPopulation base changes

GeographiesGeographies No digital boundaries existed of 1971-81 Tracts or No digital boundaries existed of 1971-81 Tracts or

1981 EDs prior to Walford (2005) research1981 EDs prior to Walford (2005) research Changes occurring each census year for enumeration Changes occurring each census year for enumeration

purposespurposes

Data SourcesData Sources

1981-2001 1981-2001 Census Census CountsCounts

CDU, MIMASCDU, MIMAS 142 counts for 1981 (~130,000 EDs)142 counts for 1981 (~130,000 EDs)304 counts for 1991 (~148,000 ED/OAs)304 counts for 1991 (~148,000 ED/OAs)454 counts for 2001 (~218,000 OAs)454 counts for 2001 (~218,000 OAs)Extracted counts and joined to DBFs due Extracted counts and joined to DBFs due to Casweb download limitsto Casweb download limits

1991-2001 1991-2001 DBFsDBFs

Edina ® Edina ® UKBORDERSUKBORDERS

Downloaded as whole countries for Downloaded as whole countries for England, Wales and Scotland England, Wales and Scotland

Either as EDs or OAsEither as EDs or OAsSplit into GOR and joined to count tablesSplit into GOR and joined to count tables

2001 National 2001 National OutlineOutline

Edina ® Edina ® UKBORDERSUKBORDERS

England, Wales and Scotland DBFs England, Wales and Scotland DBFs extracted and intersected with Strategi® extracted and intersected with Strategi®

depict populateddepict populated non-populated areasnon-populated areas

Data Sources cont’dData Sources cont’d

1981 ED 1981 ED DBFsDBFs

Walford (2005)Walford (2005) England, Wales and Scotland EDs were England, Wales and Scotland EDs were joined to counts from CDUjoined to counts from CDU

1971-81 1971-81 Census Tract Census Tract DBFsDBFs

Walford (2005)Walford (2005) Available for England and Wales, Available for England and Wales, identical over 2 years or amalgamations of identical over 2 years or amalgamations of two or more EDs from one or both yearstwo or more EDs from one or both years

1971-81 1971-81 Census Tract Census Tract DefinitionsDefinitions

Data ArchiveData Archive Definitions comprised 490 counts linking Definitions comprised 490 counts linking 1971-81 for England and Wales only1971-81 for England and Wales only

Strategi®Strategi® Edina ® Edina ® Digimap ®Digimap ®

Vector data derived from 1:250,000 Vector data derived from 1:250,000 topographic base. topographic base. Urban, water, forest features extracted Urban, water, forest features extracted using feature codesusing feature codes ‘ ‘Other’ by polygon overlayOther’ by polygon overlay

Data Sources cont’dData Sources cont’d

1971-81 Census Tracts1971-81 Census Tracts

DBFsDBFs

1971-81 Census Tract Counts1971-81 Census Tract Counts

Data Sources cont’dData Sources cont’d

Strategi®Strategi®

England National OutlineEngland National OutlinePolygonised Polygonised Strategi®Strategi®

MethodsMethods

Areal InterpolationAreal Interpolation Given data on one source zone, determines Given data on one source zone, determines

values based on another target systemvalues based on another target system Splits one boundary based on another and Splits one boundary based on another and

proportions the counts based on areaproportions the counts based on area Allows aggregation back to another Allows aggregation back to another

geographical level.geographical level.

Disadvantages:Disadvantages: Assumes uniform distribution over ED/OAAssumes uniform distribution over ED/OA Does not account for areas of zero population.Does not account for areas of zero population.

Areal Interpolation cont’dAreal Interpolation cont’d

1981 EDs1981 EDs

East AngliaEast Anglia

2001 OAs2001 OAs

Areal Interpolation cont’dAreal Interpolation cont’d

7126 (0.39%)7126 (0.39%) 1821052 1821052

1981 EDs which now have been 1981 EDs which now have been proportioned to the 2001 OA boundariesproportioned to the 2001 OA boundaries

EDED 18219131821913

ED_AIED_AI 18210521821052

LOSSLOSS 861 (0.047%)861 (0.047%)

Dasymetric MappingDasymetric Mapping Using a-priori based weights to proportion counts by land Using a-priori based weights to proportion counts by land

useuse Water and forest = 0Water and forest = 0 Urban = 0.9Urban = 0.9 Other = 0.1Other = 0.1

Weight populated EDs where both ‘urban’ and ‘other’ exist Weight populated EDs where both ‘urban’ and ‘other’ exist Weight all forest and water land use (x 0)Weight all forest and water land use (x 0) Aggregate based on ‘FEATLAB’ – land use and ED (value Aggregate based on ‘FEATLAB’ – land use and ED (value

not sum)not sum) Uses areal interpolation once weighting has been Uses areal interpolation once weighting has been

undertaken – disaggregate and aggregate on new 2001 undertaken – disaggregate and aggregate on new 2001 lableslables

More accurate analysis as accounts for areas with no More accurate analysis as accounts for areas with no population.population.

Dasymetric Mapping cont’dDasymetric Mapping cont’d

Intersection of Intersection of Land use with Land use with 1981 ED 1981 ED DBFsDBFs

Dasymetric Mapping cont’dDasymetric Mapping cont’d

3. Weight selected query for all fields by the ‘rate_luse’ field3. Weight selected query for all fields by the ‘rate_luse’ field

4. Select non populated land uses and weight (x 0)4. Select non populated land uses and weight (x 0)

2. Join the frequencies to the MapInfo Table and perform a 2. Join the frequencies to the MapInfo Table and perform a SQL Query to select EDs where both ‘urban’ and ‘other’ are SQL Query to select EDs where both ‘urban’ and ‘other’ are > 0> 0

1. Create frequencies in SPSS of all land use types1. Create frequencies in SPSS of all land use types

Discussion cont’dDiscussion cont’dTotal: 100Total: 100

Other Other (100)(100)

Urban Urban (100)(100)

Total: 100Total: 100

Other Other (10)(10)

Urban Urban (90)(90)

Total: 100Total: 100Total: 100Total: 100

Other Other (10)(10)

Urban Urban (90)(90)

Water (0)Water (0)

Forest (0)Forest (0)

Urban Urban (100)(100)

Water (0)Water (0)

Total: 100Total: 100

Dasymetric Mapping cont’dDasymetric Mapping cont’d

Aggregate data (sum) where 2001 OAs contain Aggregate data (sum) where 2001 OAs contain the 1981 EDs.the 1981 EDs.

EDED 18219131821913

ED_DMED_DM 18215331821533

Loss Loss 380 (0.02%)380 (0.02%) 5273 (0.29%)5273 (0.29%) 18215331821533

Loss in total population of 380 using DM, whilst 861 missing in AI.Loss in total population of 380 using DM, whilst 861 missing in AI. Less people in external boundary in DM than AILess people in external boundary in DM than AI

Loss of population in outer boundaryLoss of population in outer boundary

Discrepancies in internal boundariesDiscrepancies in internal boundaries

DiscussionDiscussion

1981 and 20011981 and 2001

1991 and 2001 (clipped 1991 and 2001 (clipped boundaries)boundaries)

Discussion cont’dDiscussion cont’d

EDED 18219131821913

ED_AIED_AI 18210521821052

LossLoss 861 (0.047%)861 (0.047%)

EDED 18219131821913

ED_DMED_DM 18215331821533

LossLoss 380 (0.021%)380 (0.021%)

Areal Interpolation - 1981Areal Interpolation - 1981 Dasymetric Mapping - 1981Dasymetric Mapping - 1981

Discussion cont’dDiscussion cont’d

Complexity of processingComplexity of processing Computationally intensiveComputationally intensive Large number of filesLarge number of files Aggregation, then disaggregation and Aggregation, then disaggregation and

aggregation againaggregation again

ConclusionConclusion

Issues with both techniques and Issues with both techniques and boundaries not aligningboundaries not aligning

Issues with assigning weights for land Issues with assigning weights for land uses – what is best weight to useuses – what is best weight to use

Land uses – assumption that they are the Land uses – assumption that they are the same for both years, not always the casesame for both years, not always the case

Dasymetric mapping complex process, Dasymetric mapping complex process, areal interpolation more simplistic to areal interpolation more simplistic to processprocess

ConclusionConclusion

Is Dasymetric Mapping a valid technique?Is Dasymetric Mapping a valid technique? Is Dasymetric Mapping using land use Is Dasymetric Mapping using land use

weighting more superior than Areal weighting more superior than Areal Interpolation?Interpolation?

ReferencesReferences Walford, N (2005) Creating historical and contemporary small-area Walford, N (2005) Creating historical and contemporary small-area

geography in Britain: The creation of digital boundary data for 1971 geography in Britain: The creation of digital boundary data for 1971 and 1981 census units, and 1981 census units, International Journal of GISInternational Journal of GIS, Vol.19, No.7 , Vol.19, No.7 August 2005, 749-767.August 2005, 749-767.

Copyright StatementsCopyright Statements

Source: 2001 Census Output Area Boundaries. Crown copyright 2003. Crown copyright material is reproduced with the permission of the Controller of HMSO.

This work is based on data provided with the support of the ESRC and JISC and uses boundary material which is copyright of the Crown and the ED-LINE Consortium.

© Crown Copyright/database right 20(yy). An Ordnance Survey/EDINA supplied service.