Figure 5.1
from Thematic Cartography showing phenomena and appropriate ways
of representing them
Typically, data are collected data at discrete locations (weather stations, cell phone towers) or aggregating over regions (blocks, counties, states). The actual phenomena being modeled is continuous.
Type of
visualization used depends both on the nature of the underlying
phenomenon and the purpose of the map. Compare these visualizations?
Visual Variables for Qualitative Phenomena:
Visual
Variables for Quantitative Phenomena
should reflect ordinal level of measurement:
Here is an
example of using colour to map life expectancy
in the US from mapoftheunitedstates.org
For the same data, the second is
from measureofamerica.org. Which one is more readable?
Here is an appropriate use of a wide variety of colors showing
data from facebook on the favorite American Football teams for
various counties.
We can also use pictographic symbols
Here are the issues:
Here its pretty easy to make rough
comparisons - its tricky to make exact comparisons as its hard
to avoid the lie factor we talked about last week.
Another issue related to the size of the states is the overlap of the icons in the northeast making it hard to read the data there.
The visualization above showing
the raw number of brew pubs may give people the wrong
impression. California has much more
land mass and more people than Illinois which has more land mass
and people than Rhode Island. It might be better to view the number of brew
pubs per million people in each state (the raw data may need to be standardized to better
show the underlying phenomena).
Comparison of choropleth, proportional symbol, isopleth,
and dot mapping:
Figure 5.10
from Thematic Cartography
Choropleth
Used to portray data
collected for units such as counties or states
Regions are shaded / colored
based on the phenomena
Good for when values change
abruptly at unit boundaries but hides variation within units.
Isopleth
(contour map)
Interpolating set of isolines between sample points of known
values
The data should be standardized
Good when data collected was from a smooth continuous phenomenon
Proportional
symbol
Scale symbols in proportion to the magnitude of the data
Symbol might be a true point (located at a data collection
point) or a conceptual point (at the center of a unit)
Dot mapping
One dot
is set equal to a certain amount of the phenomenon
Dots
should be placed where the phenomena occurs (much more accurate than other maps, but require more resources and what if the dots are overlapped)
Selecting
visual variables for choropleth maps
Figure 5.11
from Thematic Cartography
Here are some other ways of
displaying data from Information Graphics - A
Comprehensive Illustrated Reference
Here is an
image showing crime statistics compared to the average over time
for the US from CommonGIS via the Thematic Cartography and
Geovisualization book.
