Area Symbols: the core of thematic mapping

Objectives of lecture

1. Map of the Day
2. Area Symbols - particularly choropleth maps
   
   

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Area symbol maps share a characteristic situation :

a spatially exhaustive collection zones specify location, shape and size.
an attribute has been assigned to the whole zone

To represent this situation,

the zone (polygon) acts as the symbol;
an area symbol must try to show the value of an attribute
through either color or texture (each subdivided as required)

They may LOOK the same, but the underlying measurement framework varies ­ collection zones or coverages

• collection zones control space, measure attribute (through a PRIOR nominal variable, the identifier of the object: e.g. census tracts, counties)
• coverages control attribute, measure location (land use, soils, climate...)

Coverage maps

Nominal attribute used to generate the boundaries. Display differences between categories...

Choropleth maps

• First (in the "base map" stage), use a NOMINAL variable to create an exhaustive coverage of areas.
• Then assign each area an attribute value using some rule
• Choropleth use the area as the symbol
• Usually through CLASSIFICATION (from ratio/interval to Ordinal) [next lecture]
• Graphic elements should show the continuous orgins of the classes by using value scale / intensity scael / hues in some logical sequence... perhaps texture (orientation, spacing, etc.) depending on constraints.

Nominal data are possible, but not the core method (similar to coverages).

Classless Choropleth maps:

scale darkness ("value") to the attribute
(could use linear or other scaling of the density function)
Quite hard to read off a specific value...

Most choropleth maps convert continuous attributes to ordinal classes.
Reasons: Identifiability of the shading patterns (What is the value of this zone?)
Simplification of the data. (General spatial pattern, not exact value)
Classification methods are the subject of next lecture

Bivariate methods

There are always attempts to cram two variables or more onto a choropleth presentation. Some are possible, but others confuse readers.
Any such attempt must use color hue and color value in some combined manner or some other pair of graphic elements...

Dasymetric maps

Dent (like most cartographers) presents John K. Wright's map of Population Density on Cape Cod as if it were simply a kind of area symbol. It represents a complex TRANSFORMATION of the original population data with ANOTHER hidden layer (first uninhabited land, then land use).