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Color Key to Presentation of Understandable Scientific
Data
February 14, 2003
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Use of color to indicate population density
and change in population from Census 2000. More
images.
Credit: Cynthia Brewer
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Denver – The scientific establishment is drowning
in data, but whether it is census data or the vast amounts
of satellite and computer-generated information created
every day, visual representation and the use of color
can help scientists understand and extract important
patterns from this deluge, according to a Penn State
cartographer.
“The smarter we are at mapping data, the more
likely it is that we will see relationships,” says
Dr.
Cynthia Brewer, associate professor of geography.
“We can look at complicated data using color intelligently
and see these relationships and generate hypotheses,”
she told attendees today (Feb 14.) at the annual meeting
of the American Association for the Advancement of Science
in Denver.
Presenting more than one variable is sometimes difficult,
but careful color and texture choices can make representations
much clearer. One option is to use a color scheme for
the actual data and a hatched texture to indicate that
the data is uncertain.
“Sometimes, it is important to overlay something
about uncertainty to prod the viewers into thinking
about what the data means,” says Brewer. “The
color says here is the data, but the hatching says this
data is suspect.”
No more than three types of data can be shown using
color. Confusion sets in at the fourth variable, but
three variables can be successfully shown on a map.
In topographic data for example, color hues could show
the direction of slopes, while saturation of the colors
could indicate the steepness of slope. Lightness represents
the third variable – shape -- and is the illumination
on the relief produced by an arbitrary sun above the
map. With these three variables, the map takes on shape
and conveys information. With only the first two, it
is difficult to read.
Brewer warns that there are mistakes that can be made
when choosing colors for maps. If ordered data is being
represented, choosing a group of random color hues obliterates
any patterns. Also, if vivid colors are used for values
that are unimportant, the color pops off the page and
causes the viewer to give more weight to that variable
than it warrants.
“For diverging spectral color schemes, make sure
the central value, often bright yellow, is actually
matched with an important data range,” Brewer says.
“If mapping areas of drought versus flooding, use
the central yellow as the normal precipitation level
moving to orange brown in one direction and blues in
the other.”
Another error is designing a map without considering
those who are color blind. Most color blind people are
red-green color blind and schemes that contain versions
of red, orange, yellow and green with the same lightness
can be impossible to read. Often, simply removing the
green from a spectral scheme will improve readability
for the color blind. But avoiding unreadable schemes
and finding ones that are pleasant and best show the
data is not as difficult as it seems.
Brewer and Mark Harrower, assistant professor of geography,
University of Wisconsin, Madison, developed an online
web tool that can provide pre-designed color palettes.
ColorBrewer (www.ColorBrewer.org)
allows you to identify the number of classes you will
have on the map, the type of data being represented
– sequential, diverging or qualitative – and
then a color scheme. Schemes are marked to indicate
color blindness friendly choices, usefulness for projection,
on a computer screen, photocopyability or printability.
The program also indicates the color specifications
for commercial printing, computer screens or web page.
“For those who need to do full four-color printing,
a paper soon to be published in the January issue of
Cartography and GIS will provide color swatches and
color-separation ink codes for a large variety of color
schemes,” said Brewer.
With the advent of color monitors for computers, scientists
in every discipline embraced the use of color to clarify
and illuminate data, but back then, monitors could only
display the eight basic colors. Consequently, many scientists
adopted a simple rainbow color scheme for their work.
“Now, computers can show millions of colors and
researchers are still using just eight,” says Brewer.
“There is a whole world of subtlety and shade available
now.”
**aem**
EDITORS: Dr. Brewer is at 814-865-5072 or at cbrewer@psu.edu
by e-mail.
Contacts:
A'ndrea Elyse Messer (814) 865-9481 aem1@psu.edu
Vicki Fong (814) 865-9481 vfong@psu.edu
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