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?