Geography 353 Cartography and Visualization

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Geog 353 Lecture Outline: Map Symbolization
Update: 10/21/13

We will review chapter 9 (Map Symbolization) from the Making Maps 2nd book. Additional information and examples can be gleaned from the material below.






Map Symbolization and Choropleth Maps

Once decisions have been made regarding the classification of the data you have to design an appropriate graphic symbolization of your data

The visual variables are primary means of guiding cartographic symbolization



Abstract and Realistic Point and Line Symbols

Abstract point/line symbols: the symbol doesn't look like what it represents




Realistic point/line symbols: the symbol looks like what it represents




Most maps have a mixture of Abstract and Realistic Symbols: National Park Service Symbols




Introduction to Symbolizing Data in Areas

Symbolizing statistical thematic map data in areas is more complicated than data at points or along lines


Phenomena Space:


Two dimensions in "Phenomena Space"


Discrete versus Continuous variation

A continuum: phenomenon may fall at either end, or somewhere in between



Abrupt versus Smooth variation

Also a continuum: phenomena may fall at either end, or somewhere in between



Data examples in phenomena space:



The next step is to match this to an appropriate symbolization method...




Symbolization space: same dimensions as phenomena space graphic



Representing discrete phenomena with smooth variation: us a dot map:



Representing discrete phenomena with abrupt variation: use a graduated symbol map:



Representing continuous phenomena with smooth variation: use a isarithmic map (surface map):



Representing continuous phenomena with abrupt variation: use a choropleth map:



MacEachren fig. 3.25: AIDS data mapped with four different area symbolization techniques


What to do?

It is ok to map the data with a choropleth map but it reflects the way the data was collected rather than the phenomena

But in reality AIDS as a phenomena is not so continuous (it isn't really everywhere) and it is not so highly contagious (like influenza or measles) as to be easy to catch with casual contact.



Back to symbolization space: most appropriate is the middle technique: the "chorodot"



AIDS example is useful in that it reveals

Problem: you may not be able to easily choose the best area mapping method






The Choropleth Map: Symbolizing Data in Areas

1. The Choropleth Map: Characteristics and History

When to use a choropleth map: symbolization space:


Choropleth map defined: "A method of cartographic representation which employs distinctive shading applied to areas other than those bounded by isolines. These are usually statistical or administrative areas." Dent.



The choropleth map is now the most common way to map areal aggregated data



In most cases, the data is classified



Important question to ask in choropleth mapping: Is the data appropriate?

Mistake: using magnitude data for choropleth maps

In most cases, the enumeration units on a map are unequal in area


Different view of data when looking at density versus totals:


Choropleth maps are optimal for:



2. Designing Choropleth Maps

Appropriate Legend Design


Range grading

Better to assign noncontinuous than continuous values in the legend



Appropriate Symbolization for Choropleth Maps

Several visual variables which can be used to differentiate the variations in data on a choropleth map, but value is the one that you should use for typical single variable choropleth maps



A few more choropleth map design guidelines

With printed maps most people tend to see dark as more, light as less


This may be different on computer screens, but the dark = more is a convention that you should violate with caution


Also try and avoid white and black at the extremes of your range of values


Remember that most people can identify about 6 to 8 different values


Pay attention that the map projection is appropriate




E-mail: jbkrygier@owu.edu

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