How to Lie with MapsUnderstanding Distortions in Maps and Geographic Data

by Treg Christopherfor NR 25

Purpose of a Map:Communication of (Spatial) Information

Spatial Information Flow• Collection -> GIS -> Cartographic Presentation (Maps)

• Information is processed, converted, analyzed, manipulated, categorized, selected, abstracted, generalized, and symbolized throughout these steps.

• In other words……

…All maps lie!

…because maps are representations (or simplifications, or models) of reality (the earth), it is impossible not to “lie”

• “No map is an objective, neutral artifact as many subjective decisions are made about what to include, how the map will look, what the map is seeking to communicate.”

• “They are artifacts imbued with the values and judgments of the individuals who construct them and they are undeniably a reflection of the culture in which those individuals live.”

• Maps are “subject to distortions arising from ignorance, greed, ideological blindness, or malice.”

1. Distortions from Data Collection

• Sampling design- pattern and density

• Recording unrealistically high instrument precision (exceeding human precision and accuracy)

• Positional accuracy- compass, GPS unit

• User error

2. Cartographic Distortion

• Graphic design errors– Common map elements:

• A map’s purpose and consequential distortions– Artistic– Commercialism: Enticing customers, copyright traps– Propaganda and Politics– Informational: General audience or specialists?

• Common and obvious types of distortions:– Distance– Orientation– Boundaries– Color– Shape– Area

Distance and/or scale bars, Title, Data frame, Inset map, Legend, Date, North Arrow

Art

Distance, Orientation

Political Boundaries

War and Politics in Bosnia

Color: “The red menace”

3. Distortions in GIS

A. Human error

B. Projections and Datums

C. Conversion between data types (ascii, raster, vector)

D. Conversion between feature types (e.g. poly to point)

E. Generalization and simplification of data

F. Classification/Categorization

B. Projections: Distortions from making the Earth flat

• As the areal extent of a map increases the amount of distortion increases

• List the 4 types of distortion and the name of the map type that is designed to minimize this distortion:

Conformal – shapeEqual-area – area Equidistant – distance between 2 placesAzimuthal – direction (angle)

Global Projections: Mercator

• Rhumb Lines (Bearing) are straight. The original purpose was to aid in navigation at sea.

• Minimizes distortion around the equator

• Northern latitudes become larger than normal

• Politically, this means that Europe becomes over emphasized

Converting between Projections

Global Projections: Peters

• Minimize distortion of area

• Increases distortion of shape

Global Projections: Sinusoidal

• Minimize distortion of area

• Increases distortion of shape (but not as much as Peters)

Datums

Take Home: When you send a file with GPS coordinates make sure you tell the person what projection and datum you were using when collecting that data…Don’t assume they know what datum you are using.

C. Conversion bw/ data types

Raster Data TypeVector Data Type

D. Conversion bw/ feature typesHighways and Railroads Towns

Conversion Examples 2

Avg Annual Precipitation (inches) Avg Daily Temp, Mon, Oct24, 2005

List other examples from weather/nature….

E. Simplification

Aggregation

F. Classification and Categorization

• Chloropleth (Color Aggregation):– Aggregates all data values within a specific range to a

single number…displayed with a single color– Method of aggregation affects spatial pattern and

interpretation– Colors affect interpretation

• Cartograms:– Similar to Chloropleth but weighted by something else

(e.g. population)

Chloropleth vs. Cartogram

• Electoral Map for Election 2000

• Above: Chloropleth– Area of state is true

but what does area have to do with a population-based voting system?

• Below: Cartogram– weighted by

population

Statistical Methods of Classification

Why distort at all?

– Emphasize certain objects– Reduce visual clutter

• Most people can distinguish between only seven classes.

– Help interpretation of data– Reduce printing time– Reduce storage space requirements (GIS)– Reduce processing time (GIS)

What is scale?

• Cartographic– expressed as representative

fraction– (e.g. 1:24,000)– A larger denominator means a

smaller scale– (e.g. “small scale” means “map of

earth”

• Ecological– Spatial scale referring to extent

(and resolution)– (e.g. “large scale” means “large

extent” i.e the whole earth– Just the opposite of cartography!

Distance) (GroundDistance) (MapScale

Carto:Small Ecol:Large

Carto:Large Ecol:Small

Example of distortion from scale• Feature Conversion:Polygon to line• Selection• Simplification

Distortions from increasing scale (ecological scale)

• 1. Polygon to line• 2. Simplification

National Map Accuracy Standards

Map Scale Error (for 90% of points) on paper map

Error (for 90% of points) on the ground

1:12,000 1/30 inch 33 feet

1:24,000 1/50 inch 40 feet

1:100,000 1/50 inch 167 feet

http://rmmcweb.cr.usgs.gov/nmpstds/nmas.html

As the scale of the map broadens, more area needs to be represented on a given size of map paper. This creates a greater need to simplify, generalize, abstract…..distort

“Solutions” to Distortions

• Follow metadata (FGDC) and map accuracy standards (USGS)

• Know your target audience• Put important information on map• Develop online, interactive maps that let users

create their own printable maps• Calibrate data• Avoid false precision in labeling and attributes• Provide confidence intervals or probabilities instead

of just a single, mean value

• No map can represent all of the objects and features that exist in reality; a map that tried to do so, like the map in Lewis Carroll’s Sylvie and Bruno, would be useless:

• “We very soon got six yards to the mile. Then we tried a hundred yards to the mile. And then came the grandest idea of all! We actually made a map of the country, on the scale of a mile to the mile!”

• “Have you used it much?” I enquired.• “It has never been spread out, yet,” said Mein

Herr: “the farmers objected: they said it would cover the whole country, and shut out the sunlight!

• So now we use the country itself, as its own map, and I assure you it does nearly as well.”

Sources

• http://www.collectionscanada.ca/education/008-3060-e.html

• http://www.colorado.edu/geography/gcraft/notes/notes.html

• http://jcmc.indiana.edu/vol5/issue4/dodge_kitchin.htm

• How to Lie With Maps, by Mark Monmonier. Chicago: University of Chicago Press, 1996. 207 p.

Review Questions

• What are the 3 types of geometric projections?• What are 4 types of distortion that can occur?• A conformal projection minimizes which distortion? • List some examples of converting between points,

polygons and lines• List 3 types of symbol generalizations• Describe a chloropleth vs. a cartogram• Describe cartographic scale vs ecological scale• As the scale of the map becomes finer, figures

become more or less distorted?