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Chapter 7 Using GIS to Make a Map
affect the outcome of the choropleth map. An issue involved with choropleth
mapping is displaying the values of data that can be counted (for instance,
the total population values per state or the total number of housing sales
per county) when the sizes of the areas being mapped are very different. If
you were mapping the number of vacation homes of each state, a very large
state like California is probably going to have a larger number of homes
(12,214,549 total homes with 239,062 of them being seasonal, according to
the 2000 Census) than a smaller state like Rhode Island (439,837 homes total
with 13,002 of them seasonal). Thus, if you’re making a choropleth map of
the number of vacation homes in each state, California will show many more
vacation homes than Rhode Island, just because it has a lot more houses (due
to a larger area and population).
However, a better measure to map would be the percentage of the total number of houses that are vacation homes—in this way, the big difference in housing counts between California and Rhode Island isn’t a factor
in the map. Instead, you’re mapping a phenomenon that can be comparably measured between the two states. When you map the percentages of
the total housing that are considered vacation homes, California’s seasonal
homes only make up about 2% of the total houses, while Rhode Island has
about 3% seasonal homes. To make a choropleth map of count data, the
data should first be normalized, or have all count values brought onto the
same level. For instance, dividing the number of seasonal homes by the
total number of houses would be a way of normalizing the data. See Handson Application 7.4: Interactive Thematic Mapping Online for an online tool
that is used for creating thematic maps.
It’s now time to start making your own thematic
maps. Open your Web browser and go to http://
thematicmapping.org/engine. This is the Website
of the Thematic Mapping Engine, which allows you
to create numerous kinds of thematic maps of data
from a variety of topics. First, select an indicator (start
with something simple like Population) and choose a
year (like 2010). Choose Choropleth for the technique
and pick a set of colors. For the Classification method,
choose Quantiles. Lastly, click on the Preview button
to see a thematic map of world population displayed
in an interactive Google Earth interface in your Web
browser (alternatively, you can select the Download
option to actually download the thematic map as a
new layer in Google Earth). After the Quantile map is
set up, try the same settings but with the Equal Interval classification method to check out what kind of
map is generated.
Try some of the other indicators (such as CO2
emissions, GDP per capita, Infant mortality rate, or
Mobile phone subscribers) over a range of years.
These thematic maps can be created using graduated symbols, choropleth maps, or even 3D-style
prism maps (we’ll use the Thematic Mapping Engine
again in Chapter 14 with these types of maps). You
can specify which data classification method to use
and finally display the mapped results using Google
Earth or the online preview.
Hands-on Application 7.4
Interactive Thematic Mapping Online
normalized altering
count data values
so that they are at
the same level of
representing the data
(such as using them as
a percentage).
Chapter 7 Using GIS to Make a Map
affect the outcome of the choropleth map. An issue involved with choropleth
mapping is displaying the values of data that can be counted (for instance,
the total population values per state or the total number of housing sales
per county) when the sizes of the areas being mapped are very different. If
you were mapping the number of vacation homes of each state, a very large
state like California is probably going to have a larger number of homes
(12,214,549 total homes with 239,062 of them being seasonal, according to
the 2000 Census) than a smaller state like Rhode Island (439,837 homes total
with 13,002 of them seasonal). Thus, if you’re making a choropleth map of
the number of vacation homes in each state, California will show many more
vacation homes than Rhode Island, just because it has a lot more houses (due
to a larger area and population).
However, a better measure to map would be the percentage of the total number of houses that are vacation homes—in this way, the big difference in housing counts between California and Rhode Island isn’t a factor
in the map. Instead, you’re mapping a phenomenon that can be comparably measured between the two states. When you map the percentages of
the total housing that are considered vacation homes, California’s seasonal
homes only make up about 2% of the total houses, while Rhode Island has
about 3% seasonal homes. To make a choropleth map of count data, the
data should first be normalized, or have all count values brought onto the
same level. For instance, dividing the number of seasonal homes by the
total number of houses would be a way of normalizing the data. See Handson Application 7.4: Interactive Thematic Mapping Online for an online tool
that is used for creating thematic maps.
It’s now time to start making your own thematic
maps. Open your Web browser and go to http://
thematicmapping.org/engine. This is the Website
of the Thematic Mapping Engine, which allows you
to create numerous kinds of thematic maps of data
from a variety of topics. First, select an indicator (start
with something simple like Population) and choose a
year (like 2010). Choose Choropleth for the technique
and pick a set of colors. For the Classification method,
choose Quantiles. Lastly, click on the Preview button
to see a thematic map of world population displayed
in an interactive Google Earth interface in your Web
browser (alternatively, you can select the Download
option to actually download the thematic map as a
new layer in Google Earth). After the Quantile map is
set up, try the same settings but with the Equal Interval classification method to check out what kind of
map is generated.
Try some of the other indicators (such as CO2
emissions, GDP per capita, Infant mortality rate, or
Mobile phone subscribers) over a range of years.
These thematic maps can be created using graduated symbols, choropleth maps, or even 3D-style
prism maps (we’ll use the Thematic Mapping Engine
again in Chapter 14 with these types of maps). You
can specify which data classification method to use
and finally display the mapped results using Google
Earth or the online preview.
Hands-on Application 7.4
Interactive Thematic Mapping Online
normalized altering
count data values
so that they are at
the same level of
representing the data
(such as using them as
a percentage).