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