Grid models of population: temporal comparison by fixing the geography David Martin, School of...
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Transcript of Grid models of population: temporal comparison by fixing the geography David Martin, School of...
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 Example: Southampton 1981-2003 Conclusions
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
N = 417 mean address
count = 228 Population
197,000
Southampton 2001 OAs (test data)
N = 762 mean address
count = 125 6.2% overall
population increase
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
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
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)
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)
22
22
ij
ijij
dk
dkw
Weighting
Distance
wij
dij k
Centroid
Grid modelling principles (3)
Centroids and boundaries
Gridded population model
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)
Standard model
Postcode centroids
Empirical kernel width
Directional kernels
Grid modelling resources
NI grid square data are 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
Requires census data registration 200m cells 1981 or 1991 Select variable Extract window Export to GIS
SurfaceBuilder
Download and install VB program Download X,Y,Z centroid data file Specify surface parameters Run and preview model Export to GIS
SurfaceBuilder sequence
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
1981 population count 0.2m
1991 population count 0.2m
2001 population count 0.2m
1991-1981 population diff 0.2m
2001-1991 population diff 0.2m
2001
1981
1991
1981-91-01 change, 0.2km
1991-81
2001-91
2001
1981
1991
t2-t1 (t2-t1)/(t2+t1)
1981-91-01 change, 1km
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
2003
2001
2002
2001-02-03 change, 0.2km
(AFPD households)
2002-01
2003-02
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
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