Geographic Data Science - Lecture IV

Mapping Data

Dani Arribas-Bel

Today

  • Visualisation
  • Geo-Visualisation
  • Mapping data
    • MAUP
    • Choropleths

Visualization

“Data graphics visually display measured quantities by means of the combined use of points, lines, a coordinate system, numbers, symbols, words, shading, and color.”

The Visual Display of Quantitative Information. Edward R. Tufte.

ML data ML pic

[Source]

Visualization

  • By encoding information visually, they allow to present large amounts of numbers in a meaninful way.
  • If well made, visualizations provide leads into the processes underlying the graphic.

The Visual Display of Quantitative Information. Edward R. Tufte.

Geovisualization

Tufte (1983)

“The most extensive data maps […] place millions of bits of information on a single page before our eyes. No other method for the display of statistical information is so powerful”

MacEachren (1994)

“Geographic visualization can be defined as the use of concrete visual representations –whether on paper or through computer displays or other media–to make spatial contexts and problems visible, so as to engage the most powerful human information processing abilities, those associated with vision.”

GeoVisualization

  • Not to replace the human in the loop, but to augment her/him.
  • Augmentation through engaging the pattern recognition capabilities that our brain inherently has.
  • Combines cartography, infovis and statistics

A map for everyone

Maps can fulfill several needs, looking very different depending on the end-goal

MacEachren & Kraak (1997) identify three main dimensions:

  • Knowledge of what is being plotted
  • Target audience
  • Degree of interactivity

MacEachren & Kraak (1997) map cube

[Source]

Making good data maps

  • “Containers”
  • Choropleths

Data “containers”

Modifiable Areal Unit Problem (Openshaw, 1984)

MAUP

Scale and delineation mismatch between:

  • Underlying process (e.g. individuals, firms, shops)
  • Unit of measurement (e.g. neighborhoods, regions, etc.)

In some cases, it can seriously mislead analysis on aggregated data (e.g. Flint, MI!!!)

Always keep MAUP in mind when exploring aggregated data!!!

Choropleths

Choropleths

Thematic map in which values of a variable are encoded using a color gradient of some sort
  • Counterpart of the histogram
  • Values are classified into specific colors: value –> bin
  • Information loss as a trade off for simplicity

Classification choices

  • N. of bins
  • How to bin?
  • Colors

How many bins?

  • Trade-off: detail Vs cognitive load
  • Exact number depends on purpose of the map
  • Usually not more than 12

How to bin?

Unique values

  • Categorical data
  • No gradient (reflect it with the color scheme!!!)
  • Examples: Religion, country of origin…

Unique values

Equal interval

  • Take the value span of the data to represent and split it equally
  • Splitting happens based on the numerical value
  • Gives more weight to outliers if the distribution is skewed

Quantiles

  • Regardless of numerical values, split the distribution keeping the same amount of values in each bin
  • Splitting based on the rank of the value
  • If distribution is skewed, it can put very different values in the same bin

Other

  • Fisher-Jenks
  • Natural breaks
  • Outlier maps: box maps, std. maps…

Color schemes

Align with your purpose

  • Categories, non-ordered Qualitative
  • Graduated, sequential Sequential
  • Graduated, divergent Divergent

TIP: check ColorBrewer for guidance

Tips

  • Think of the purpose of the map
  • Explore by trying different classification alternatives
  • Combine (Geo)visualisation with other statistical devices

Creative Commons License
Geographic Data Science’19 by Dani Arribas-Bel is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.