GIS and Socioeconomic studies

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GIS and Socio-economic studies

• Martin Frost

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Fundamental Ingredients

• Points (dimension 0)
• Lines (dimension 1)
• Areas (dimension 2)
• Surfaces (dimension 3)
• Can be reduced to sets of points (nodes) and
  lines joining those points (links) in an ordered
  topological structure

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Fundamental Ingredients

• In most human applications of GIS the
  fundamental elements are integrated using a
  vector approach
• this builds combinations of nodes and links into
  shapes, or as simple point events
• this allows the accurate measurement of distances
• it provides the possibility of compact storage of
  information
• but relies on having an underlying sense of
  position (determines the precision of the
  anal.)

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The issue of position

• Almost all means (except globes and spheres) of
  representing the earth as a map rely on
  projections that translate spherical
  coordinates onto a flat sheet of paper.
• All projections imply some warping of the
  surface they represent the choice of projection
  depends on the purpose to which they are put
• Some maintain true measurement of distances, some
  maintain true comparisons of area, or true
  measurement of direction but all are
  intrinsically compromises

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The issue of position

• More details on the range of possible projections
  can be found at
• http//www.colorado.edu/geography/gcraft/notes/map
  proj/mapproj_f.html
• Or enter map projection in Google!
• The British National Grid is produced on a
  Transverse Mercator projection with a nominal
  basing point to the west and south of the Scilly
  Isles

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The issue of position

• By far the most common grid references used in
  socio-economic applications of GIS are six
  figure grid references based on the Ordnance
  Survey National Grid
• Birkbeck College in Malet Street has a reference
  of 529801, 181961
• this is approx 530kms east of the base and 182kms
  north

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The issue of position

• Some cautions
• beware of false accuracy the six figure
  reference implies accuracy to the nearest meter
  often not matched by the real quality of the
  information!
• some commercial maps (eg Bartholomews street
  maps) look as though they follow an OS standard
  base but are, in fact, slightly different to save
  licence payments

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The issue of position

• In practice we rarely run up against projections
  directly. This is because
• Many sources of information come with grid
  coordinates already attached
• Most modern GIS software can register images or
  scanned maps onto any grid using four points with
  known co-ordinates (sometimes known as tick
  points)
• But care is always needed always read the small
  print on how files have been generated and on
  what bases

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Linking data to areas

• A key feature of almost all GIS systems is that
  they contain within them a relational database
• Relational databases allow us to join two sets of
  information that have a set of codes in common
• One of the most commonly used set of codes are
  the labels applied to administrative areas
• e.g. a Census ward code might look like 09UCFA
  (thats actually Ampthill Ward in Mid
  Bedfordshire)
• This label needs to be found within the data file
  and the file containing the boundaries of the
  areas

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Linking data to areas

• Butthe codes must match exactly for a join to
  be successful
• This means that
• We have to work with clear unambiguous codes that
  apply to all administrative areas (in Britain
  that is the responsibility of the Office for
  National Statistics) that everyone uses (see
  www.statistics.gov.uk/geography)
• Joins based on names are rarely successful or
  reliable

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Linking points to areas

• In addition to a relational join we can have a
  spatial join in which points can be allocated
  to areas (sometimes known as the point-in
  polygon problem)
• This is achieved by taking the coordinate points
  that identify a point-based occurrence and
  identifying what area it lays within
• The coordinates of the points must be the same as
  the coordinates on the boundaries
• In Britain this often means the National Grid

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But, where does all this stuff come from?

• Area boundaries
• It is possible to take any map and translate its
  boundaries into streams of nodes/links using
  either a digitising table or by digitising
  on-screen and using the GIS software to join
  things up
• This is time-consuming, tedious and rather error
  prone!
• We usually try to buy, borrow or obtain files
  where another agency has already undertaken the
  digitising. Often these files are produced by
  Ordnance Survey as a by-product of their mapping

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But, where does all this stuff come from?

• Ordnance Survey Products
• Boundary Line administrative areas (eg wards,
  local authority boundaries)
• Strategi features such as roads, paths,
  railways, towns etc
• Landline- topographic maps
• Raster base maps pictures of urban and rural
  areas at various scales
• The problem is usually cost!

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But, where does all this stuff come from?

• Output Area Boundaries
• The one breakthrough!
• As part of the Census Access Project for 2001
  Census of Population Output Area boundaries are
  available almost free in a GIS format (ARC
  shapefile)
• Be warned in all there are approx 175,000
  areas, but subsets can be ordered
• Can be used for larger areas by dissolving
  boundaries

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But, where does all this stuff come from?

• Desirable data features
• Needs to be machine-readable if at all possible
  (there is nothing worse than entering your own
  data! particularly if there are many values)
• Needs clear accurate coding of areas that will
  match the coding on appropriate boundary files
  OR if it is a point-located feature needs a
  direct way of allocating a grid co-ordinate
• Usually best to obtain data in either a
  spreadsheet or database format

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But, where does all this stuff come from?

• Possible data sources
• Neighbourhood Statistics Web site
  http//neighbourhood.statistics.gov.uk/
• contains a growing range of socio-economic data
  in appropriate formats for GIS analysis
• Other government surveys (eg Labour Force Survey)
  sometimes difficult to access (Data Archive,
  Univ of Essex)
• Historic Censuses (eg 1981 and 1991) often
  accessed through academic links (eg CASWEB based
  at Manchester Univ)

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But, where does all this stuff come from?

• Possible data sources
• Point-based data often depends on an address as
  the key to geo-referencing (postcodes!)
  sometimes useful for location of
  facilities/services
• Data can be available from administrative
  processes of local authorities and Health
  Authorities often problems with confidentiality
  at household level
• There are commercial lists of services (eg CACI
  Retail) that are geo-referenced, but often
  expensive