thematic maps and visualization of georeferenced … maps and visualization of georeferenced data...
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Thematic Maps and
Visualization of
Georeferenced Data
What is a Map
Visualization of geographic and georeferenced
information is normally made through maps
A map is drawn/printed representation of the Earth
Cartography is the study and the making of maps
A person who makes maps is called a cartographer
Most maps are flat representations of a region
A globe is also a map, but it’s shaped like a sphere
What Are Maps Used For?
Locating places
Measuring distances
Planning trips
Visualization of
georeferenced data
Many types of maps…
Political Maps
Physical Maps
Contour Maps
Road Maps
Street Maps
Transit Maps
Thematic Maps
Resource Maps
Inventory Maps
…
They can be classified into three main types:
1) General reference maps
2) Mobility maps
3) Thematic Maps
General Reference Maps
Show political or physical features
Boundaries or borders for regions
Geographical features
Bodies of water
Cities and towns
Political and Physical Maps
Political maps show boundaries of countries and provinces
Physical (aka relief) maps show the location of physical features found on the Earth’s surface such as mountains, rivers, lakes, and valleys
Mobility Maps
Mobility maps help people find their way
from one place to another (wayfinding)
Examples include transit maps, road
maps, street maps, …
Thematic Maps
A thematic map shows georeferenced data,
numeric or character, by colors or symbols.
Specific data displayed in this manner is
referred to as a theme
Different types of thematic maps, useful for
showing different types of thematic variables
Commonly used to show natural resources,
vegetation areas, population, types of soil,
climate, temperature, …
How Does One Read a Map?
Need to understand the following:
Map Legends
Scale
Geographical grids
Map indexes
Latitude and Longitude (map projections)
Map Legends and Symbols
Map legends list and
explain the symbols and
colors found on a map
Sometimes the map
symbols look like the
features they represent
(tree represents a forest)
Map Scale
The scale on a map shows the relationship
between the distances on a map and the
distance on the earth’s surface
Scale is shown with a bar or measuring tape.
Geographical Grids
Lines on a map that help us find and
describe locations
Most common grids are lines of latitude and
longitude
Types of
Thematic Maps
Display Information using Thematic Maps
Thematic mapping is the process of shading a map
according to a given theme
Data displayed on the map is called a thematic variable
Shading can be based on color, patterns, symbols, …
Use values of data for comparisons and checking patterns
Large variety of thematic options to display the same data.
Common thematic map types include: Chloropleth maps, aka ranged maps
Dot density maps
Proportional symbol maps
Maps with pie or bar charts
Grided maps, aka heatmaps
Cartograms
…
Chloropleth/Ranged Maps
Involves coloring
geographic areas to
represent categories of
rates or densities
Most common type of
thematic map
Represent a single
georeferenced variable
Show variations over
discrete regions.
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All data classification methods revolve around two questions:
How many data ranges should there be?
How large is each data range?
The answers to these questions are determined in part by
the classification method selected.
Some common data classification methods are:
EQUAL-COUNT
EQUAL RANGES
NATURAL BREAKS
STANDARD DEVIATION
CUSTOM
Determining Ranges in Chloropleth Maps
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Equal-Count classification aims at
having an equal number of
cases in each range
Depending on the data
table, this will be
approximate only
In the example, 50 states and the
District Of Columbia could not be
exactly divided by 4. The result is
that the number of cases in each
range varies from 10 to 14.
Notice that the size of ranges
varies greatly, from about
1,100,000 (the smallest range) to
about 24 million (the largest).
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Equal Range classification
aims at equal-sized ranges.
In the example, every range
has an interval of about 7
million.
This method does not always
reflect the data well.
Notice that the lowest range
has 42 cases and the highest
range has only 1 case, making
for a very unbalanced map.
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Natural Breaks
classification creates
ranges based on
clusters or gaps
within the data itself.
This makes for
ranges that reflect the
data very well.
Note however, that
the number of cases
per range and the
size of ranges can
vary considerably.
Graph of U.S. 1990 Populations by State, Ascending Order.
2.3 million
4.7 million
10.8 million 29.8 million
Chart with data used in previous example
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Standard Deviation classification
creates classes 1 standard
deviation in size around the mean.
In the example, the mean value is
about 4.9 million and the standard
deviation is about 4.5 million.
Four ranges were specified, but
only three ranges are shown
because the lowest range
contains no values.
In effect, the map shows above
and below average values.
This type of classification is
popular for highlighting extremes
of data (either much smaller or
much bigger than the average.
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Custom Ranges are used to
highlight particular features
of the data (for example,
population between 1 and
3.7 million) or to modify
ranges defined by one of the
other methods.
Data ranges can be
discontinuous. This is
acceptable, as long care is
taken to ensure that no data
values fall into the gaps.
Notice that all of the examples shown
illustrate a convention in thematic mapping to
use progressively darker shades to represent
progressively higher value ranges.
On the question of how many ranges to use, research has
shown that between 4 and 6 ranges is the most effective.
Also notice that all of the examples are showing exactly the
same data. Clearly the appearance of a thematic map can
vary greatly depending on the classification method used.
light
dark
low
high
Colors and Number of Ranges
In terms of visual effectiveness, maps look better when
there is a balance between data ranges (i.e, there are
about the same number of values in each range)
The Equal Count and Natural Breaks methods are
probably the best choices for most general purposes.
Which method to use
Usage of colors on cloropleth maps
Specific color progressions should be used to depict the
data properly, in a way that facilitates interpretation
Single-hue progressions fade from a dark shade of the chosen color
to a very light shade of relatively the same hue. Common method
used to show magnitude.
Bi-polar progressions use two hues and are normally used to show a
change in data numbers from negative to positive.
Blended hue progressions use related hues to blend together the two
end point hues. This type of color progression is typically used to
show elevation changes.
Full spectral progression contains hues from blue through red.
Usually not recommended.
Point Density Maps Showing a symbol (e.g., a
dot) for each individual or
group of individuals
Used together with polygons
Useful to show densities or
values in a continuous way
Can be used to show
multiple variables (e.g.,
through multiple colors, dot
sizes, symbols, etc.)
Fails when many individuals
or groups are present
Only used when exact
location is unknown
Effect of changing the dot value
1 dot = 150 persons
1 dot = 300 persons
1 dot = 1000 persons
Effect of Changing the Dot Size
Proportional Symbol Maps
Also called graduated/graded
symbol maps
Represent classes of counts,
not individual counts
Useful for counts that lack
exact location (use regions)
Can represent continuous or
discrete (ranged) values
Can represent multiple
variables through juxtaposed
symbols at same locations
Charts in Thematic Maps
Can show numeric rates,
numeric densities, or
nominal values
For densities, the divisor
is usually the magnitude
of a geographic area
Can use various formats
for the charts
Can show multiple
variables
Pie Chart
Bar Graph
Gridded Thematic Maps (Heatmaps)
Can show numeric rates
or numeric densities
Shows continuous
variations for a single
variable
Colors can be based on
interpolated values from
centroids Lecture on spatial statistics …
Cartograms
Deform map area or map
distance with basis on the
variable being represented
Area cartograms
Distance cartograms
Shows a single variable
e.g., travel times, population, …
Usually combined with other
types of thematic mapping
techniques
Maps can mislead if you are not careful or are
unscrupulous.
Maps are models and all
maps distort the reality!
Attention to map
scale, feature
selection,
symbolization
metaphors, etc…
Creating Thematic Maps
Google Earth Thematic Mapping Engine and API
http://thematicmapping.org/engine/
R Project for Statistics
A Practical Guide to Geostatistical Mapping
http://spatial-analyst.net/book/
Questões?