Thematic Maps

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Thematic Maps

• Choropleth, Proportional/Graduated Symbol,
  Digital Image, Isoline/Isopleth and Dot
  Distribution Maps

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• Choropleth Maps
• In these maps, each area (spatial unit) is
  filled with a color or pattern.
• These are the maps we see most often.

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• Choropleth Map
• Maps that show the density information about an
  area per unit area are choropleth maps. Also,
  a map that would show who owns what areas of land
  would be a choropleth map.

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Choropleth Map Colorado Counties
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2000 Colorado Population Distribution Map
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• Advantages of Choropleth Maps
• It provides an easy way to visualize how a
  measurement varies across an area.
• When defining regions is important to a
  discussion (as in an election map divided by
  electoral regions), choropleths are preferred.
• Choropleth maps are also appropriate for
  indicating differences in land use, like the
  amount of recreational land or type of forest
  cover.

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• Disadvantages of Choropleth Maps
• They give a false impression of abrupt change at
  the boundaries of shaded units.
• It can be difficult to distinguish between
  different shades in the maps.
• They have terrible problems with induction (e.g.
  even though not everyone voted Republican in a
  state, when they color it red on a map on TV it
  LOOKS like everyone did)
• boundaries of unit areas are arbitrary sometimes
  (e.g. the South)

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• Digital Images
• These maps are actually digital images
• A grid of cells (pixels) are assigned a color
  that corresponds to a specific value or range of
  values

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Digital Image Map This kind of map is being used
to map environmental occurrences seen from remote
sensing systems like satellites
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Population Density Map 2000 World Population
Distribution
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Digital Image Map African Population Density
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Digital Image Map Asian Population Density
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Digital Image Map Population Density North
America
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• Proportional Symbol/Graduated Symbol Map
• In this type of a map, a symbol is selected,
  usually a circle because its easier to work with.

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• Proportional Symbol Map
• The symbol will vary in its size from place to
  place on the map. This is because the symbol
  represents different quantities in different
  places.

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• Advantages of proportional symbol maps
• They show data occurring at points
• They show absolute values (precise numbers)
• The circles can be subdivided to show percentages
  of the whole 
• They are easy to make

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• Disadvantages of Graduated Symbol Maps
• Densities/ratios/rates are hard to show with this
  map
• Its hard to tell the relationship between the
  different circle sizes

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Isoline/Isopleth Maps These maps are used to show
quantities that vary smoothly over the earths
surface. These maps can take two forms 1.
Lines of equal value (iso means equal) or
2. Ranges of similar value are filled with
similar colors or patterns.
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• Advantages of Isoline Maps
• Good for showing gradual change over space
• They avoid the sudden changes which boundary
  lines produce on choropleth maps. Temperature,
  for example, is something that should be mapped
  using isopleth maps, since temperature exists at
  every point (is continuous), yet does not change
  abruptly at any point (like population density
  may do as you cross into another census zone).

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• Disadvantages of isopleth maps
• They are not suited for showing discontinuous or
  'patchy' distributions
• A large amount of data is required for accurate
  drawing.

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• Dot Distribution Maps
• Dot distribution maps use dots to show the
  quantity of something in a given area or areas.

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Population distribution in US
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• Advantages of Dot Distribution Maps
• easy to understand
• show the varying density of a phenomena
• dot placement placement of dots takes into
  account the distribution of other phenomena a
  dot map of crop acreage would not place dots in
  urban areas, a dot map of population would
  cluster dots near urban areas, etc. Therefore,
  patterns are meaningful.
• Allows for mapping with another distribution of
  some other phenomena e.g. use multi-colored
  dots, or use dot maps over choropleth map.
  Therefore, they are good for complex maps

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• Disadvantages of Dot Distribution Maps
• There is a human tendency to underestimate
  density and its easy to do with this map
• If dots get too dense, it is difficult to read
• Dot placement computer placement of dots is
  often random thus, patterns are often not
  meaningful if the map is misleading.

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• Dot Distribution Maps
• If not created properly, these maps, and other
  maps, can be confusing.
• In the map you will see, dots were placed
  randomly to identify alligator populations in the
  countries of North America
• What does the map you will see imply?

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Dot Distribution Map Alligator Population
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Cartograms These are maps that show the amount
of something by increasing or decreasing the size
of the area in which the something is found.
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• Advantages of Cartograms
• Good at showing the relationship between spatial
  units
• Disadvantages
• Difficult to read