151
How Can You Perform Basic Spatial Analysis in GIS?
AVERAGEHI > 30000) OR CITY_NAME = ‘Boardman’. This would select cities that meet both the population and the income requirement and would select cities named ‘Boardman’ as well. The result of a query will be a subset of
the records that are chosen, and then those records can be used for analysis,
rather than the whole data layer. Selected records can also be exported to
their own dataset. For instance, the results of the previous query could be extracted to compose a new GIS layer, made up of only those records (and their
corresponding spatial objects) that met the query’s criteria. Once you have
your selected records or new feature layers, you can start to work with them in
GIS (see Hands-on Application 6.1: Working with Queries in GIS for an example
of doing queries with online GIS utility).
How Can You Perform Basic
Spatial Analysis in GIS?
One you have your selected data, it’s time to start doing something with it.
A simple type of analysis is the construction of a buffer around the elements
of a data layer. A buffer refers to an area of proximity set up around one or
more objects. For instance, if you wanted to know how many rental properties
were within a half-mile of a selected house, you would create a half-mile buffer around the object representing the house and then determine how many
rental properties were within the buffer. Buffers can be created for multiple
points, lines, or polygons (Figure 6.3 on page 152). Buffers are a simple tool
to utilize, but they can provide useful information when creating areas of
analysis for various events.
Exclusive Or the
operation wherein the
chosen features are all
of those that meet the
first criteria as well as
all of those that meet
the second criteria,
except for the features
that have both criteria
in common in the query.
Durham, North Carolina, has an online GIS Spatial
Data Explorer utility that allows the user to do a
variety of spatial analyses with the city’s data, including performing queries. To work with this tool,
open your Web browser and go to http://gisweb.
durhamnc.gov/gomaps/map/index.cfm. In the
upper left-hand corner are several tools (such as
zooming and panning) to get oriented to the GIS
data. Included in the tools is a “hammer” icon, which
allows you to build queries of the data. Press the
“hammer” icon and a new set of options will appear
on the right, allowing you to construct simple queries of the GIS. First, select an available layer (for instance, select 2000 Census Block Groups). Second,
select an attribute to query (for instance, choose
“Housing Units”). Third, select an operator (for instance, choose the greater than “>” symbol). Press
the “add to query” button and you’ll see the SQL
statement appear. To finish the query, type a value
(for instance, type 1000), and press the “Send the
Query” button. All records meeting your query will
be retrieved (in this case, all Census block groups
with more than 1000 housing units in them). The
selected block groups will be highlighted on the
map, while the records (and the attributes that go
along with them) will be displayed in a spreadsheet
at the bottom of the screen. Investigate some of the
other available data layers and the types of queries
that can be built with them.
Hands-on Application 6.1
Working with Queries in GIS
buffer a polygon of
spatial proximity built
around a feature.
How Can You Perform Basic Spatial Analysis in GIS?
AVERAGEHI > 30000) OR CITY_NAME = ‘Boardman’. This would select cities that meet both the population and the income requirement and would select cities named ‘Boardman’ as well. The result of a query will be a subset of
the records that are chosen, and then those records can be used for analysis,
rather than the whole data layer. Selected records can also be exported to
their own dataset. For instance, the results of the previous query could be extracted to compose a new GIS layer, made up of only those records (and their
corresponding spatial objects) that met the query’s criteria. Once you have
your selected records or new feature layers, you can start to work with them in
GIS (see Hands-on Application 6.1: Working with Queries in GIS for an example
of doing queries with online GIS utility).
How Can You Perform Basic
Spatial Analysis in GIS?
One you have your selected data, it’s time to start doing something with it.
A simple type of analysis is the construction of a buffer around the elements
of a data layer. A buffer refers to an area of proximity set up around one or
more objects. For instance, if you wanted to know how many rental properties
were within a half-mile of a selected house, you would create a half-mile buffer around the object representing the house and then determine how many
rental properties were within the buffer. Buffers can be created for multiple
points, lines, or polygons (Figure 6.3 on page 152). Buffers are a simple tool
to utilize, but they can provide useful information when creating areas of
analysis for various events.
Exclusive Or the
operation wherein the
chosen features are all
of those that meet the
first criteria as well as
all of those that meet
the second criteria,
except for the features
that have both criteria
in common in the query.
Durham, North Carolina, has an online GIS Spatial
Data Explorer utility that allows the user to do a
variety of spatial analyses with the city’s data, including performing queries. To work with this tool,
open your Web browser and go to http://gisweb.
durhamnc.gov/gomaps/map/index.cfm. In the
upper left-hand corner are several tools (such as
zooming and panning) to get oriented to the GIS
data. Included in the tools is a “hammer” icon, which
allows you to build queries of the data. Press the
“hammer” icon and a new set of options will appear
on the right, allowing you to construct simple queries of the GIS. First, select an available layer (for instance, select 2000 Census Block Groups). Second,
select an attribute to query (for instance, choose
“Housing Units”). Third, select an operator (for instance, choose the greater than “>” symbol). Press
the “add to query” button and you’ll see the SQL
statement appear. To finish the query, type a value
(for instance, type 1000), and press the “Send the
Query” button. All records meeting your query will
be retrieved (in this case, all Census block groups
with more than 1000 housing units in them). The
selected block groups will be highlighted on the
map, while the records (and the attributes that go
along with them) will be displayed in a spreadsheet
at the bottom of the screen. Investigate some of the
other available data layers and the types of queries
that can be built with them.
Hands-on Application 6.1
Working with Queries in GIS
buffer a polygon of
spatial proximity built
around a feature.