Transcript Thematic Maps - GonzalesatBerthoud

Thematic Maps
Choropleth,
Proportional/Graduated
Symbol, Digital Image,
Isoline/Isopleth and Dot
Distribution Maps
Choropleth Maps
• In these maps, each area
(spatial unit) is filled with a
color or pattern.
• These are the maps we
see most often.
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.
Choropleth Map: Colorado
Counties
2000 Colorado Population Distribution
Map
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.
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”)
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
Digital Image Map
This kind of map is being
used to map environmental
occurrences seen from
remote sensing systems like
satellites
Population Density Map: 2000 World
Population Distribution
Digital Image Map: African
Population Density
Digital Image Map: Asian
Population Density
Digital Image Map: Population Density
North America
Proportional
Symbol/Graduated Symbol
Map
• In this type of a map, a
symbol is selected, usually
a circle because it’s easier
to work with.
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.
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
Disadvantages of Graduated Symbol
Maps:
•Densities/ratios/rates are hard to show
with this map
•It’s hard to tell the relationship
between the different circle sizes
Isoline/Isopleth Maps
These maps are used to show quantities that
vary smoothly over the earth’s 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.
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).
Disadvantages of isopleth maps:
They are not suited for showing
discontinuous or 'patchy' distributions
•
• A large amount of data is required for
accurate drawing.
Dot Distribution Maps
• Dot distribution maps use
dots to show the quantity of
something in a given area or
areas.
Population distribution in US
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
Disadvantages of Dot Distribution Maps
• There is a human tendency to underestimate
density and it’s 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.
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?
Dot Distribution Map: Alligator
Population
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.
Advantages of Cartograms
• Good at showing the relationship between spatial
units
Disadvantages
• Difficult to read