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Introduction. Today we will do two things: Consider some of the potential pitfalls associated with choropleth maps Discuss some aspects of geovisualization. Choropleth Mapping. Decisions made in constructing a choropleth map include: 1. Choice of areas: number and location.

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  • Today we will do two things:
    • Consider some of the potential pitfalls associated with choropleth maps
    • Discuss some aspects of geovisualization
choropleth mapping
Choropleth Mapping
  • Decisions made in constructing a choropleth map include:
  • 1. Choice of areas: number and location.
    • May be predetermined
    • Small numbers problems
    • Heterogeneity
  • 2. Choice of indicator.
    • The need to normalise (i.e. rates, densities, means)
    • The need to standardise
    • Probability mapping
choropleth mapping 2
Choropleth Mapping(2)
  • 3. Number of classes
    • Too few classes
    • Too many classes
  • 4. Choice of class intervals
    • Spatial v frequency distribution (e.g. skewed data)
    • Divisions of range (and outliers)
    • Percentiles (by number of areas, by area, by population)
    • Geometric progressions (of cutoffs, of class widths)
    • Natural breaks
    • User-specified
choropleth mapping 3
Choropleth Mapping(3)
  • 5 Choice of tones
    • Gradation
    • Clearly distinguishable, yet evenly ‘spaced’
    • Monochromatic v dichromatic
  • Even making sensible decisions, a choropleth map may still give a misleading impression
  • Small high density areas may be difficult to see; whereas large sparsely populated areas may dominate the map
  • Solution may be to redraw areas to reflect their population (isodemographic maps, cartograms)
  • Five properties of areas may be inferred from a traditional undistorted map:
    • Size
    • Shape
    • Distance
    • Direction
    • Contiguity
  • These properties help us recognise which area is which.
  • If size is distorted, then so must at least one other property
  • Objective is to minimise distortions of other properties to permit areas to be recognised
cartograms 2
  • Graph paper was used in the pre-computer era, but this produced very rectangular shapes (e.g. Scotland)
  • Apart from being ugly, this example is particularly difficult to interpret:
    • Area shapes are distorted
    • Relative location is distorted
    • Contiguity not preserved.
cartograms 3
  • By abandoning shape and contiguity, it may be possible to retain relative location.
  • Alternatively, it may be possible to retain shape by drawing an ‘exploded’ map (e.g. by shrinking all but the most densely populated areas).
  • This would also preserve relative location, but could leave a lot of empty space. Better results might be achieved by sacrificing distance and/or direction.
  • Preserving shape works best if the areas are readily identified from their shape (e.g. countries of the EU).
cartograms 4
  • Various algorithms have been devised in the computer age.
  • One of the most successful was devised by Kocmoud and House.
  • This partially preserves shape, and relative location, whilst completely preserving contiguity.
  • Contiguity constrain causes shape of Califormia to become distorted.
3d displays
3D Displays
  • Traditional maps use contour lines to show the third dimension. However, even using colour to create a hypsometric map, some people find them difficult to read.
  • Alternative approaches include:
    • Hillshading (using a computer to do the number crunching)
    • Block diagram
    • Stereoscopic images
  • Traditional maps are static.
  • Using GIS we can create a series of maps (‘scenes’) to make a movie.
  • Could be used to show changes over time (e.g. weather systems, landuse changes).
  • May also be used for fly-through animations, etc.
user interaction
User Interaction
  • Could link histogram with moveable divides with the class intervals on a choropleth map to allow user to interactively explore the distribution.
non visual displays
Non-Visual Displays
  • Sound may be used to create maps for blind people.
  • Haptic maps (based on a sense of touch) can also be created.