CHOROPLETH MAPPING

Cartographic Design for GIS (Geog. 340)Prof. Hugh HowardAmerican 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 effectiveHealth 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 PROJECTIONSand 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

CHOROPLETHCOLOR 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 is80% Black

• Caution should be exercised when using black as a fill color (cont.)

– Make all enumeration unit boundaries dark gray instead of black

COLOR SCHEMES

Boundaries are80% 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 are80% Black

(midway between two highest classes)

CHOROPLETHLEGEND 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 ELEMENTSFrame/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 ELEMENTSLegend• 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 ELEMENTSBar 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 LineMapped Area

InsetTitle/Subtitle

LegendData SourceBar Scale Orientation

CHOROPLETH MAPPING

Cartographic Design for GIS (Geog. 340)Prof. Hugh HowardAmerican River College