Introduction

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Introduction - PowerPoint PPT Presentation

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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Presentation Transcript
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.
• 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)
• 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)
• 5 Choice of tones
• Clearly distinguishable, yet evenly ‘spaced’
• Monochromatic v dichromatic
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)
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
• 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
Animation