Defining areas using postcodes

John Langley

27th April 2007at The Riverside Centre Derby

Define the area by…

• Moving from the area “on the ground” to a useful abstraction

• Using large scale OS maps

• Drawing a boundary line defining the area

• Fitting postcode centroids to the area

• Example: Manton Community Alliance (Neighbourhood Management Area)

Digitise the boundary (or just draw round it)

Which spatial dataset?

Codepoint?

Which spatial dataset? NHS Postcode Directory

Codepoint vs NHS PC

Which spatial dataset? NHS PC Dir has Current postcodes

Which spatial dataset? NHS PC Dir is also historical (coloured symbols)

Method

• Each quarter, use GIS to compare the latest NHS Postcode Dir with your boundary (“point in polygon”)

• Use the resulting file of postcodes recognising that this contains a historical dimension

• This allows linking to datasets where ‘old’ postcodes are included

Problems

• NHS Postcode Directory only accurate to 100m (good enough?)

• The ‘historical’ postcodes may be lost if the Royal Mail reuses the terminated postcode (2 year period of grace?)

• ‘Postcode Drift’ – we do not yet know the extent of this – needs work?– what error does this introduce?– in which areas?– over which time period(s)?– how can we allow for it?