Choropleth Maps, Color & PlacementGeography 59

May 24, 2007

Refer to: Krygier & Wood, pages 202-225 Krygier & Wood, pages 255-271

Monmonier, pages 163 – 173 (+ plates)

Notes & Review

● Homework 4, Part II due May 31st

● Extra credit next week (Projections)

● Graded Lab 3 – overall very good

** Review type placement and labels in K & W, (page 245)

Choropleth Mapping

● A map that uses different colors to show how some phenomenon varies across space.

● Often, choropleth maps are based on raw data that has been standardized in some way

● Data is categorized into classes (legend)

Choropleth Maps

Data source? Title?

Data Classification – common methods

1.Equal Intervals, Equal Steps

2.Quantiles

3.Natural Breaks

Other methods include algorithm-based analysis,

mean-sd, clustering

Lab #4 is a sequential set of data (years)

How does the data look?

Equal Intervals

If you have raw numerical data (Lab #4 does not work this way), compute your intervals:

range of data = high – low# of classes = # classes

example:58.5 – 0.7 = 11.56 5

Quantile Division

Each division has an equal number of observations

Natural Breaks

Visual inspection helps adjust classes – use a histogram or dispersion graph

Colors are important too

The Electromagnetic Spectrum

Visible Light

Color: Human Perception

The Dimensions of Human Color Perception

HUE: The electromagnetic energy wavelength (color)

VALUE: lightness or darkness of hue

SATURATION: color intensity or brilliance (“chroma”). Most subtle.

See K & W page 259 for examples

HUE is the most common, but you may manipulate color using the other properties too.

Guide to Use

● HUE: Discern qualitatively different features– Roads from rivers

● VALUE: Vary to show quantitative differences– Large roads from small roads

● SATURATION: – Binary data (present or not present)

From K & W, page 259

Color: Considerations

● Light source– Colors appear different under different

lighting– If you know the expected lighting conditions,

use guidelines as in K & W (page 258)– Intuitive!

● Less light? Use more intense colors● Bright light? Use less intense colors

COLOR SYSTEMS

Color Systems: RGB

● Primary colors: red, green, blue ● Colors on a computer monitor

● More common on a scale 0-255, but can also be expressed in terms of percentage (1-100) of primary color in hue

Color Systems: Process Color

● Process color (CYMK) is used for full-color printing

● One through the printing press per color (Cyan, Magenta, Yellow, K is black) -> 4 passes

● Values for C,M,Y specified in percentage

● Inkjet printers use this system

Visible Light

Color Systems : HSB

Hue is measured in angular degrees around the cone starting and ending at red = 0

Yellow is 60 degrees, Green is 120 degrees

Saturation is measured in percent from the center of the cone to surface (0 to 100)

Brightness is measured in percent from black to white (0 to 100)

Color Systems: Greyscale

It is essentially the HSB system holding saturation as zero (or the same as the brightness axis alone)

The human eye can disntinguish 10 shades of grey!

• Black = 0% brightness, 100% grey.

• White = 100% brightness, 0% grey.

• Grey is most often specified from white = 0, thus 10% grey = 90% brightness.

Color: Practical Use?!

Basic ideas:● Representative colors● Data type (e.g. binary)● Sequential data (gradients)● Relate trend to color if possible (numbers)

Colors: Practical Use

Always think about: – Color blind users?– Elderly users?– Culturally unique viewers/Sensitive

colors!

Tools that can help

● ColorBrewer is a program designed specifically for color selection in choropleth maps! (Penn State)

● K & W and Monmonier both have useful information

Homework #4, Part II

● Due May 31st (next Thursday)

● Do not forget to turn in Part I & Part II.

● Note Extra Credit available on Part II.

Tips for Lab 4, Part II

● See the back side of the handout for tips

● There are two data sets floating around, US_Bees, and afrbeep020.tar.gz, the latter requires clean up. AK, HI, PR, and VI should NOT be in the final map

● Make sure you can start ArcToolbox

Lab 3 – Review & Tips

● Very good overall● Review rules of type placement● Review rules of point label placement● Example

Map Examples – Project Ideas

● Example 1: Scientific - Reference● Example 2: Scientific - Thematic● Example 3: Social Thematic - Choropleth● Example 4: General Reference

Scientific – Thematic Map

General Reference

Reference Map - Location

Projects

Please turn in a one-paragraph description of your project idea by next Thursday, May 31st. You may email it with subject heading: Project Idea