Symbolization

1
Symbolization
Lecture 10
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Data Symbols

• Most thematic maps are representations of data
• Data, in order to be visualized, must be
  converted to symbols
• Symbols, representing maps themes, co-exist with
  base-information on the maps
• Since there are many types of symbols,
  cartographers must be expert in recognizing data
  type and matching those types of information with
  appropriate symbolization

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Data Types

• Qualitative
• Names of things
• Quantitative
• Amounts of things
• Enumeration units (districts)
• Statistical areas from which data is collected
  and recorded

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Data Framework

• Qualitative
• Quantitative

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Locational Nature of Data

• Point
• Area
• Line

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Measurement Level of Data

• Qualitative
• Quantitative
• Ranked

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Qualitative Data
Data that distinguishes kinds or types of
phenomena with no indications of quantities,
intensities, or relative status
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Ranked Data
Information about the relative status of various
phenomenamost likely made ranked by design or
intent having had absolute values to determine
ranked status.
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Quantitative Data
Data that is neither qualitative or ranked and is
expressed numerically.
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3 X 3 Scheme for Matching Data to Symbol Type
Geographic Nature of Data
Measurement Level of Data
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3 X 3 Scheme for Matching Data to Symbol Type
Geographic Nature of Data
Measurement Level of Data
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Quantitative Data at Points

• Continuous
• Isarithmic map
• Isolines
• Smooth statistical surface
• Discrete
• Spot symbols
• The simple bar graph example
• Circle and squares
• Spheres and cubes
• Accumulated symbols
• Spot symbols for grouped data

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Quantitative Data at Areas

• Dot mapsa area symbol in which spots symbols
  (dots) represent a certain quantity of a
  phenomena are placed within an area
• Cartogramswhere size of area changes according
  to quantity
• Choroplethtraditional treatment for ratios or
  densities from Greek words choros (place) and
  plethos (magnitude)

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Quantitative Data at Points

• Isolinesa area symbol in which spots symbols
  (dots) represent a certain quantity of a
  phenomena are placed within an area
• Flow magnitudeswhere size of area changes
  according to quantity

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Dot Maps
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Definition

• The dot is a symbol used to map quantitative
  data at areas
• Dot maps, while using a spot symbol (dots) are
  not spot symbols maps like proportional circles
  or squares
• Dots represent numbers of phenomena and each dot
  is placed at a selected point
• Dots maps do more than simply show an absolute
  quantity at a selected area. They also show the
  distribution of a phenomena

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Theory Behind Dot Mapping

• The dot map uses a single graphic element that
  never changes. What does change is the number of
  elements in various areas across the map
• Each dot represents the same number of
  occurrences or items
• for example if the population of Pennsylvania
  is 20 million, and each dot equaled 10,000
  residents, 2,000 dots would be placed on the
  map
• the issue would then be, where would they be
  placed?

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Advantages of Dot Mapping

• Easy to understand
• Effective way of showing spatial distribution
• Original data may be gleaned or recovered from
  map by simply counting the dots
• More than one data set can be used (different
  dots symbols)
• Maps using dots are easy to make

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Quote by Michael R.C. Coulson
It is not the dot size, or the value, or even
the placement, that give the real power of the
dot mapassuming some reasonable decisions have
been made. Rather, the power of the dot map is in
the overall pattern of the distribution that is
reveled.
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Visualizing Distribution
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Enumeration Units

• The dot map represents geographic quantities that
  have been collected at some statistical
  areathese areas are called enumeration units.
• (1) a statistical area or enumeration unit is
  something like a state or a county or a
  metropolitan area
• (2) the smaller the area, the greater the
  accuracy of the map (see Fig. 8.4, Dent, pp.
  165)
• (3) since quantities are assigned or associated
  with each unit, dots representing the total sum
  of these quantities are placed strategically
  within areas of the unit that are likely to
  feature the phenomena being mapped (see next
  section dot placement)

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Dot Placement
Enumeration units, while being neat containers
for data collectors such as the census, are not
uniform in terms of the spatial distribution of
individual phenomena
for example there may be hog farming across a
state, but that does not mean that every area of
the state has hog farms (see Fig. 2.20, Mattson,
pp. 34, and Fig 8.3, Dent, pp. 165) therefore,
in order to give an accurate representative of
the actual distributions, dots need to be placed
in areas that are likely to have actual
occurrences of a given phenomena
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Dot Size and Value
Map size and density affect perception of the dot
map Too small dots give a false impression of
precision Large dots appear crude and deny the
reader the ability to interpret actual
values Fig. 8.5, Dent, pp. 165 Review general
guidelines on pp. 166, Dent and Fig. 8.7, Dent,
pp. 166 Dot coalescence The desired condition
where a dot size and value is chosen so that
dot just begin to overlap each other in the most
dense enumeration unit (see Fig 8.6, Dent, pp.
166)
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• Area Symbols and Choropleth Maps

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Area Symbols
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Overview of Area Symbols

• the map is made-up of a series of spatial
  collection zones which specify location, shape
  and size
• attributes assigned to the whole collection zone
• the zone (polygon) acts as the symbol
• an area symbol must try to show the value of an
  attribute through either color or texture (each
  subdivided as required)

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Choropleth Maps

• A symbolization technique for displaying
  quantitative data at areas
• A method of cartographic representation which
  employs distinctive color or shading applied to
  areas other than those bounded by isolines.
• The most popular type of map
• Often used to display statistical information
  like the census that is attached to enumeration
  units
• Easily accomplished through the use of computers

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Visualizing Choropleth

• The choropleth map can be thought of as a
  three-dimensional histogram or stepped
  statistical surface
• If looked at as such, each prism is raised
  according to the value it represents
• In a choropleth map, each prism is colored
  according to the value it represents

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Choropleth Construction
To make a choropleth map, the cartographer
collects data associated with an enumeration unit
and then devises scheme that attaches color/value
symbols to corresponding areas.
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What Readers Observe

• the value of a specific area
• overall pattern on the map
• pattern on the map as compared with the pattern
  on other map(s).

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Limits of Choropleth Maps

• Appropriate data is characterized by its
  spatially discrete nature that can be confined by
  predefined borders
• Continuous phenomena such as elevation or
  temperatures must not be mapped this way
• Absolute number data such as total population
  should not be mapped using color shades or
  patterns

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Appropriate Data

• Choropleth maps are reserved for proportional
  values, such as
• population per square mile
• percent of population that is Latino
• percent of population with DVD players

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Assumptions

• the spatial unit used for shaded mapping is the
  smallest detail that shaded map can represent
• due to this fact an assumption must be made that
  the phenomena within this unit is uniformly
  distributed

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Considerations (1)

• Consider variables mapped or geographical
  phenomena
• before map is produced the mapmaker should have
  some background knowledge about the variable or
  must use a common sense to make the right
  decision. It does not make sense to map number of
  subway stops per sq. mile for Philadelphia, as
  they are located only in specific places and
  along certain routes

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Considerations (2)

• Consider map scale
• cartographer must consider necessity and
  available spacein other words, is the map scale
  large enough to display enumeration units whose
  colors can be identified

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Considerations (3)

• Consider map number of aerial units vs.
  distribution detail
• as the number of units mapped increases more
  detailed distribution can be achieved
• if you have more units the difference in value
  between one of them and the next higher/lower in
  value becomes smaller

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Types of Choropleth Maps

• classed choropleth map where data are grouped
  into classes to simplify readers task of map
  interpretation, each map unit falls into one of
  the predetermined classes
• classless choropleth map where each map unit is
  in fact a graphical class by itself, each value
  is represented by a different shade or pattern
• dasymetric maps are form of choropleth map where
  zones of similar values are used separated by
  escarpments of rapid value change
• bipolar choropleth maps have zero in the middle
  two scales (two hues)