choropleth mapping
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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