Introduction

1. Introduction • Today we will do two things: • Consider some of the potential pitfalls associated with choropleth maps • Discuss some aspects of geovisualization

2. 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

3. 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

4. Choropleth Mapping(3) • 5 Choice of tones • Gradation • Clearly distinguishable, yet evenly ‘spaced’ • Monochromatic v dichromatic

5. Geovisualization • 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)

8. 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

9. 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.

12. 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).

14. 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.

17. 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

18. Animation • 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.

19. User Interaction • Could link histogram with moveable divides with the class intervals on a choropleth map to allow user to interactively explore the distribution.

20. 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.