grid models of population temporal comparison by fixing the geography
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Grid models of population: temporal comparison by fixing the geography . David Martin, School of Geography University of Southampton. Overview. Problems of comparison over time Merits of using gridded data Data sources Grid modelling principles Grid modelling resources

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grid models of population temporal comparison by fixing the geography

Grid models of population: temporal comparison by fixing the geography

David Martin,

School of Geography

University of Southampton

overview
Overview
  • Problems of comparison over time
  • Merits of using gridded data
  • Data sources
  • Grid modelling principles
  • Grid modelling resources
  • Example: Southampton 1981-2003
  • Conclusions
problems of comparison over time
Problems of comparison over time
  • Population changes location and composition
  • Boundaries and definitions are recast between data collection events; measurement success varies
  • Cannot disentangle true change from representation and measurement
  • If geographical units remained the same, one part of the problem would be addressed
southampton 1991 eds
Southampton 1991 EDs
  • N = 417
  • mean address count = 228
  • Population 197,000
southampton 2001 oas test data
Southampton 2001 OAs (test data)
  • N = 762
  • mean address count = 125
  • 6.2% overall population increase
intersection of eds and oas
Intersection of EDs and OAs
  • N = 1771
  • mean address count = 54
  • Where was the increase?
  • (and ED to ED was no better)
merits of using gridded data
Merits of using gridded data
  • Grid cell data comparable through time
  • Settlement geography is retained
  • No boundary data required for geographical comparison
  • Readily combined with gridded environmental models
data sources
Data sources
  • Great Britain: 1971 census gridded data published but not available
  • Northern Ireland: 1971-2001 grid square data published
  • No general GB grid counts 1981-
  • Spatial modelling required
    • 1971-2001 population-weighted centroid locations published
    • Postcode locations can be treated as centroids
grid modelling principles 1
Grid modelling principles (1)
  • Population-weighted centroid(s) as summary points of local distribution
  • Locally adaptive kernel estimation based on inter-centroid distances
  • Redistribution of centroid counts into grid
  • Could use dasymetric and other methods
  • This is NOT interpolation
grid modelling principles 2
Grid modelling principles (2)

Weighting

wij

a

Distance

dij

k

Centroid

grid modelling principles 3
Grid modelling principles (3)

Centroids and boundaries

Gridded population model

ni surfaces 1991
NI surfaces 1991
  • Construct surfaces from NI 1991 ED centroids
  • Experiment with dispersion, postcode centroids, directional interpolators
  • Selective comparison with grid square data
  • Martin, Tate and Langford (2000)
slide14

Standard model

Postcode centroids

Empirical kernel width

Directional kernels

grid modelling resources
Grid modelling resources
  • NI grid square dataare true count data for download
  • SURPOP provides 1981 and 1991 ready-made surfaces
  • SurfaceBuilder allows construction of own surfaces from centroid data
  • (URLs on final slide…)
surpop
Surpop
  • Requires census data registration
  • 200m cells
  • 1981 or 1991
  • Select variable
  • Extract window
  • Export to GIS
surfacebuilder
SurfaceBuilder
  • Download and install VB program
  • Download X,Y,Z centroid data file
  • Specify surface parameters
  • Run and preview model
  • Export to GIS
example southampton 1981 2001
Example: Southampton 1981-2001
  • 25 x 25km region centred on City of Southampton
  • Population change 1981-2003 from census and postcode data
  • 1980s: large-scale greenfield development at urban fringe
  • 1990s: combined with brownfield redevelopment within urban area
slide25

1981-91-01 change, 0.2km

1981

1991-81

1991

2001-91

2001

slide26

1981-91-01 change, 1km

t2-t1

(t2-t1)/(t2+t1)

1981

1991

2001

contemporary change postcodes
Contemporary change: postcodes
  • All Fields Postcode Directory providing quarterly postcode record
  • Downloadable from UKBORDERS
    • Very big files!
  • Postcodes as centroids with household counts can be modelled onto grid
  • Census counts could be reallocated
slide28

2001-02-03 change, 0.2km

(AFPD households)

2001

2002-01

2002

2003-02

2003

conclusions
Conclusions
  • Grid-based modelling offers specific advantages over small areas
    • Longer-scale neighbourhood change
    • Monitoring contemporary change
    • Practical modelling advantages
  • Problem of noise due to centroid relocation (requires smoothing)
  • Challenge of definitional drift and measurement error remains
key urls
Key URLs
  • Northern Ireland grid square data http://census2.mc.man.ac.uk/cdu/2001/ni/GRID/index.htm
  • Surpop http://census.ac.uk/cdu/software/surpop/
  • SurfaceBuilder http://www.geog.soton.ac.uk/users/martindj/davehome/software.htm
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