Thematic maps
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THEMATIC MAPS PowerPoint PPT Presentation

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THEMATIC MAPS. Mercator projection. Why did my plane from Paris go so far out of the way to get to Chicago?. Mollweide Projection. Maybe it didn’t go too far out of the way…. Azimuthal projection (north pole). That explains it! Remember that every projection is a distortion.

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Mercator projection

Why did my plane from Paris go so far out of the way to get to Chicago?

Mollweide Projection

Maybe it didn’t go too far out of the way…

Azimuthal projection (north pole)

That explains it!

Remember that every projection is a distortion.

How is this projection distorted?


  • Every map employs some kind of projection to transform spherical surface to flat surface

  • Choice of projection reflects priorities of cartographer in regard to preserving one of the following (or compromising between all)

    • Area

    • Distance

    • Direction

    • Shape


  • All maps involve decision-making process

  • There is no “natural” way to draw a map

  • Maps are not reflections but rather selections of reality

  • Maps can be analyzed just like literature to determine what the cartographer believed and thought about the world (J.B. Harley, John Pickles, etc.)

Medieval “T&O” map

What common words in the English language reflect this tradition of mapping?

General idea

  • Thematic maps represent one or two variables (population, ethnicity, income, language, religion, etc.) in map form

  • The “language” of thematic maps is quite varied, and involves the use of color, shape, pattern, light and dark, etc.

  • Thematic maps serve two main purposes:

    • Analysis of data

    • Presentation of data

Isoline map of population

Map with pie chart callouts

Choropleth map

Cartogram and choropleth

A cartogram is a boundary map in which the areas are distorted systematically: every partition’s area shows its value for a particular variable

Choropleth maps

Choropleth maps

  • Tremendously common and useful

  • Use some existing system of boundaries (countries, states, counties, voting districts, etc.)

  • Group data into 2 or more levels or classes using slicing values

  • Show spatial variation of one or two variables at a time by using color, shades of grey and/or patterns

Percent of Population White (not Hispanic)

Percent of Population White (not Hispanic)

How can such different looking maps show the same variable?

  • Cartographic reasons

    • Different slicing values

    • Different levels of spatial aggregation

  • Geographical reasons

    • Uneven distribution of minorities at the state scale as well as at the national scale

    • Concentration of minorities in cities, particularly in northern states

Different ways of “slicing” data

  • The Data {42, 50, 55, 57, 61, 77, 79, 97}

  • Equal interval

    • Three classes based on range 40 to 100

    • {42, 50, 55, 57} {61, 77, 79} {97}

  • Quantile

    • Quartiles (lowest 1/4 of observations, next 1/4, …)

    • {42, 50} {55, 57} {61, 77} {79, 97}

Different ways of “slicing” data

  • Natural breaks

    • {428505552574616772791897}

    • {42} {50, 55, 57, 61} {77, 79} {97}

  • Standard deviations

    • Mean = 64.75, Std. Dev. = 15.977

    • {42} {50, 55, 57, 61} {77, 79} {97}

Natural Breaks

Equal Interval

Quintiles (quantiles based on division into 5 classes)

Which map would be preferred by each of the following users?

  • The ACLU

  • The KKK

  • A geographer studying the relationship between ethnicity and poverty

  • A spokesman for the Georgia branch of a charitable assistance association targeting minorities

  • Not very useful this way

  • Generally it helps to re-order the data

quartiles: what is 52 ÷ 4 ?

What if you wanted to use quintiles?

How informative would “natural breaks” be in this case?

Problems with choropleth maps

  • Make it easy to slant data to suit the cartographer’s purpose (by adjusting the slicing values)

  • Create the illusion of rapid breaks whereas data varies continuously and gradually in the real world

  • Allow small areas (like major cities) to overwhelm the data of large regions (like states)

Process Maps

How can you map a process?

Map with pie chart callouts

Expansion of the EU

Religious diffusion

Charles Joseph Minard’s map of the disastrous Napoleonic campaign in Russia (1812)

Mei-Po Kwan

Mei-Po Kwan

Paul Adams


  • Why use an isoline map instead of a choropleth map?

  • Why use a choropleth map instead of an isoline map?

  • What kind of map is unbiased?

  • What kind of map reflects the interests, intent, and biases of the mapmaker?

  • What are some problems with choropleth maps?

  • What are some processes that people have managed to map?

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