CHOROPLETH
MAPPING
Cartographic Design for GIS (Geog. 340)
Prof. Hugh Howard
American River College
THE CHOROPLETH
DEFINED
CHOROPLETH DEFINED
•
•
The Place – Value map
A map in which shaded areal symbols
represent the magnitude of an attribute
Represents
quantitative
data
CHOROPLETH DEFINED
•
Data are represented by Enumeration
Units
– Areal units for which data are collected
and reported
– Typically political or administrative areas
such as Census tracts, counties, states
CHOROPLETH DEFINED
•
Perhaps the most widely used form of
thematic map
– Relatively easy to create
– Relatively easy to interpret
– Works well, as long as its weaknesses are
recognized and efforts are made to
minimize them
WEAKNESSES of the
CHOROPLETH
WEAKNESS ONE
•
Gives the impression that a feature or
phenomenon is uniformly distributed
within each enumeration unit
Raw Population
Persons not
uniformly
distributed
WEAKNESS ONE
•
Sometimes, the phenomenon being
mapped is uniformly distributed
Choropleth is most
appropriate with this
type of phenomenon
WEAKNESS ONE
•
Choropleth is commonly used to
represent non-uniform distributions
– An appropriate use of the choropleth, but
can be misleading
Persons not uniformly
distributed
WEAKNESS TWO
•
Exact values cannot be determined
within a given enumeration unit
– The value at a particular point falls within
the range of the enumeration unit’s class
WEAKNESS THREE
•
Larger enumeration units tend to attract
more attention
– Can cause misinterpretation of data
– Enumeration units of uniform size and
shape are most effective
Health Care Employees
DATA
STANDARDIZATION
and the CHOROPLETH
DATA STANDARDIZATION
•
The Choropleth can represent raw
data, but is most appropriate for use
with standardized data
– Raw data can be standardized to account
for the sizes of enumeration units
DATA STANDARDIZATION
•
Various approaches to standardization
can be taken
– Density, proportion, relationships,
statistical summaries
MAP PROJECTIONS
and the CHOROPLETH
MAP PROJECTIONS
•
Equivalent (Equal Area) projections are
most appropriate
– Distortion of relative sizes of areas can
mislead the map user
Equivalent
Conformal
CLASSED versus
UNCLASSED
CHOROPLETH MAPS
CLASSED vs. UNCLASSED
•
Classed choropleth maps represent
data in groups, or classes
– A series of 48 data values can be grouped
into 5 classes, represented by 5 symbols
CLASSED vs. UNCLASSED
•
Unclassed choropleth maps represent
every data value on the map using a
unique areal symbol
– 48 data values = 48 unique symbols
CLASSED vs. UNCLASSED
•
Classed Choropleth
– Depict data with less accuracy (values are
generalized into a few classes)
– Are easy to interpret (limited number of
symbols are easily distinguished)
– Good for map
presentation
– More common than
unclassed
CLASSED vs. UNCLASSED
•
Unclassed Choropleth
– More accurately depict data (every unique
value is represented)
– Are more difficult to interpret (we can only
differentiate a small number of symbols)
– Good for data
exploration
– Less common than
classed
CHOROPLETH
COLOR SCHEMES
COLOR SCHEMES
•
Color will be covered in greater detail
in the color lecture
– Basic information is provided here
•
Color schemes are initially determined
by the characteristics of the data
– Unipolar Data
– Bipolar Data
– Balanced Data
COLOR SCHEMES
•
Unipolar Data
– Have no dividing point or complementary
parts
– Sequential color schemes are appropriate
COLOR SCHEMES
•
Unipolar Data (cont.)
– Monochromatic or Part-Spectral
sequential schemes work best
Monochromatic Sequential
Part-Spectral Sequential
COLOR SCHEMES
•
Bipolar Data
– Have a natural dividing point (zero, mean,
etc.) with values falling above or below
– Diverging color schemes are appropriate
COLOR SCHEMES
•
Balanced Data
– Consists of two complementary
phenomena
– Diverging color schemes are appropriate
COLOR SCHEMES
•
Sequential schemes (most common)
– Recommended for unipolar data
– Should increase from light to dark
– Darker colors should be associated with
larger numbers
– Colors should be distinguishable from one
another
Monochromatic
Part-Spectral
COLOR SCHEMES
•
White can be reserved for “no data”
– Polygons for which data aren’t available
– Include a note below the legend
•
White can also be used to represent 0
– The absence of color is appropriate for
representing an absence of value
COLOR SCHEMES
•
Caution should be exercised when
using black as a fill color
– Boundaries between enumeration units
can disappear (if boundaries are black)
COLOR SCHEMES
•
Caution should be exercised when
using black as a fill color (cont.)
– Use a dark gray instead of black to
represent the highest data class
Darkest fill is
80% Black
COLOR SCHEMES
•
Caution should be exercised when
using black as a fill color (cont.)
– Make all enumeration unit boundaries
dark gray instead of black
Boundaries are
80% Black
COLOR SCHEMES
•
Caution should be exercised when
using black as a fill color (cont.)
– Make boundaries between adjacent, black
enumeration units dark gray
Boundaries
between black
polygons are
80% Black
(midway between two
highest classes)
CHOROPLETH
LEGEND DESIGN
LEGEND DESIGN
•
Choropleth legends can be oriented
vertically or horizontally
– Vertical legends are most common
LEGEND DESIGN
•
Representative symbols are placed
– To the left, and defined on the right (for
vertical legends)
– Above definitions (for horizontal legends)
We read from left-to-right
and top-to-bottom
We first see what needs
to be defined, and then
we see its definition
LEGEND DESIGN
•
Higher values are shown either
– At the top: more = darker = higher
– At the bottom: more logical ordering
•
Higher values on a horizontal legend
must be at the right
LEGEND DESIGN
•
Representative symbols are connected
– Enumeration units on the mapped area
are usually connected
– Connected boxes help to emphasize that
a gradation of values is being represented
LEGEND DESIGN
•
Representative symbols are identical to
those on the mapped area
– Ensure that line width and colors are the
same
LEGEND DESIGN
•
Representative symbols are
– Horizontally centered
– Vertically centered with definitions
LEGEND DESIGN
•
•
Ranges of numbers are normally
separated by a hyphen, or the word “to”
For clarity, spaces should be included
to the right and left of each separator
LEGEND DESIGN
•
A legend heading is often included to
further explain the map’s theme
– The unit of measure and enumeration unit
are often included
LEGEND DESIGN
•
The legend heading looks best when
– Larger than legend definitions, but smaller
than subtitle
– Multiple lines are horizontally centered
– Horizontally centered above legend
MAP ELEMENTS of the
CHOROPLETH
CHORO. MAP ELEMENTS
Frame/Neat Line
• Encompasses all other map elements
• Subtle and thin
Mapped Area
• Geographic region being represented
• Contains dominant thematic symbols
• Large as can be, while leaving room…
• Visually centered within frame line
Inset
• Not necessary in this case
• South America is assumed recognizable
Title and Subtitle
• Convey the map’s theme
• Top center
• No “Map of”
• Horizontally centered (center justified)
CHORO. MAP ELEMENTS
Legend
• Visually centered within available space
• Legend heading further explains title
• Heading horizontally centered
• Heading horizontally centered w/ legend
• No “Legend”
Data Source
• Where thematic data were obtained
• “Source:”
• Publication names italicized
• Smallest type on map
• Horizontally centered (center justified)
• Below legend, horizontally centered
CHORO. MAP ELEMENTS
Bar Scale
• Allows you to take measurements
• Visually centered within available space
• Round, easy to use maximum value
• Appropriate unit of measure
• Simple and subtle
Orientation
• Not necessary in this case
• Simple and subtle north arrow if used
Frame/Neat Line
Mapped Area
Inset
Title/Subtitle
Legend
Data Source
Bar Scale
Orientation
CHOROPLETH
MAPPING
Cartographic Design for GIS (Geog. 340)
Prof. Hugh Howard
American River College