Geography 387 – Fall 2011
Lab 7
Vector Spatial Analysis
Lab 7: Data Vector Spatial Analysis
1.0 Introduction
This lab will introduce vector spatial analysis functions found in ArcMap. In particular, we
will learn about selecting based on location, buffers, and overlay. We will use a variety of
scenarios and datasets to illustrate these concepts.
2.0 Download data
Before we begin, download the lab7.zip data archive into your Lab 7 folder on your USB
thumb drive, or local computer C drive. Unzip the archive. You should have a lab7 folder
with 4 sub-folders, one for each part of this lab.
3.0 Selecting nearby features
When you analyze the relationships between map features, you might need to know which
features are within a certain distance of other features (proximity) or share the same
boundary (adjacency).
The Scenario: Suppose you are interested in buying a gas station near Interstate 40 in
Old Town, Albuquerque, New Mexico. One of your requirements is that it must be within
1,000 feet of the interstate so you can attract as many drivers as possible.
Start ArcMap and open the map document lab7_part1.mxd in the lab7_part1 folder. When
the map document opens, you will see a gas stations layer, a streets layer, a business
layer, and a zoning layer. These are all geodatabase feature classes. Set the default
geodatabase to the lab7_part1.gdb in the lab7_part1 folder from the File  Map Document
Properties window, and make sure that the box is checked to “Store relative pathnames to
datasources”. If needed, use the Full Extent button
to zoom to and center the data in
the map window. The I-40 freeway is selected (shown in light blue, also called cyan) in the
streets layer. You will use this feature to select gas stations within 1,000 feet. See the
screenshot below.
Note: If you need to reselect the I-40 freeway features, use Select by Attributes from the
streets layer Table Options, and select "STR" = 'I40' (there should be 30 selected
features).
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Go to the Selection menu, Select by Location. The Select by Location tool lets you do
simple GIS selection analysis. Keep in mind that the window provides a multitude of
criteria that form a single “selection sentence.” I will provide you with the criteria for now,
but later, you will have to try some of these criteria on your own.
Enter the criteria as shown on the screenshot below:
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This tool will select features from gas stations that are within a distance of 1,000 feet of the
selected streets. Read the criteria from top to bottom so that you understand the logic in
this spatial query.
Click OK when all your criteria are entered. Close the Select by Location window.
Question 1: (4)
a. How many gas stations were selected? Hint: Gas stations are point features and
the selected set is in light blue (cyan). (2)
b. What are the names of the selected gas stations? Hint: use the identify tool or look
in the attribute table. (2)
Unfortunately, neither of these two gas stations is for sale. You would like to make a
tempting purchase offer, but first you must know the market potential for each station.
Because you want to sell gasoline to business customers and employees as well as
freeway travelers on I-40, you decide to find out which gas station is closer to more
businesses.
Go to the Selection menu, and scroll to Select by Location. Enter the selection criteria as
shown on the screenshot below:
Be sure to uncheck STATIONS in the Target layer part of the window. You want to select
features from businesses that are within a distance of 1,320 feet (1/4 mile) from the
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selected features of gas stations. Notice that the check box next to "Use selected features"
is checked, indicating that we are only going to consider the distance from selected gas
stations (Stations). It tells you how many features are selected, although I blurred this out
in the screen shot above. Also understand that I arbitrarily chose 1/4 of a mile as a buffer
distance, but you could have put at 1 mile, 1 km, etc. by changing the buffer distance
parameters. Note that the box next to “Apply a search distance” is checked.
Click OK when all your criteria are entered as above.
To see which business were selected and examine their attributes, open the Attribute
Table for business. Notice that many of the businesses have the same address, indicating
that they are located in a shopping center. Also note that your selected gas station points
(from Question #1) also show up in the selection in the business layer. This is because the
gas station layer was originally created by selecting gas stations from the business layer.
For the question below, count the number of business points near each selected gas
station by using the Identify tool
. When you click on a highlighted business point with
this tool, you will see in the results all of the businesses at that location. If you hold down
the control key as you click on additional business points, they will be added to the existing
list of businesses in the Identify window. You can then view the attributes for any of your
business points by clicking it in the Identify window.
Note: You can also identify multiple features with the Identify tool by holding down the left
mouse button and drawing a box around the features that you want to identify. Select the
business layer in the “Identify from:” drop-down box on the Identify window to use this
option.
Question 2: (4)
a. Based on this selection of businesses, which of the gas stations you identified in
Question #1 do you think would service more customers? (2)
b. How many businesses are there around this gas station within 1/4 of a mile? (2)
ArcMap can also find features adjacent to selected features. To see how this works,
consider a situation involving parcels of land.
The Scenario: Suppose that Old Town is planning major improvements to its drainage
system. Municipal planners need answers to these questions: 1) which parcels include or
are adjacent to drainage ditches?; 2) how many of these parcels are in the city’s
jurisdiction and how many in the county’s jurisdiction?; and, 3) what is the total acreage of
parcels by jurisdiction?
Turn off all the layers in the table of contents (TOC) and turn on the Zoning layer. Notice
that drainage ditch polygons are already selected for you (cyan color). First, we’ll find out
which parcels are adjacent to drainage ditches.
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Note: If you need to reselect drainage ditch polygons, use Select by Attributes from the
zoning layer Table Options, and select "DESCRIPTIO " = 'DRAIN' (there should be 9
selected features).
You want to select features from the zoning layer that touch the boundaries of the selected
features in the zoning layer (i.e., drainage ditches). Note that you have to change the
“Spatial selection method” from its default value.
Click OK when all your criteria are entered. To see which parcels were selected, open the
Attribute Table for the zoning layer.
Question 3: (2)
How many parcels (including the ‘DRAIN’ features) will be affected by the drainage
ditch improvement project?
Now we need to determine the total acreage of these selected parcels by jurisdiction. In
the Attribute table of the zoning layer, right click on the Jurisdiction field (it is called
JURIDICTI) and select to Summarize. Enter the options exactly as you see them in the
screenshot below. Be sure to select the check box to summarize the selected records.
Save the output table in your lab7_part1.gdb geodatabase as “jurisdiction_acreage”.
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Do you see what this Summarize selection will do for you? Click OK when all your criteria
are entered. Click Yes to add the table to the map document. Open the table to answer
the following questions:
Question 4: (4)
a. How many city acres will be affected by the improvement project? (2)
b. How many county acres will be affected by the improvement project? (2)
Congratulations on your spatial analysis -- now Old Town will know how much city and
county land the new drainage improvement program will affect.
4.0 Selecting features within
The Select By Location tool also allows us to select features that fall completely within a
polygon. Conversely, you can find polygons in one layer that contain particular points,
lines, or polygons in another layer. This type of spatial relationship, features inside of other
features, is known as containment. Finding out where a feature is inside or outside a
boundary can be crucial to making decisions.
The Scenario: Your company is transferring you to the Atlanta region, and you would like
to purchase a home after you get there. Before you call a real estate agent, you would like
to become familiar with the region yourself, and possibly even identify some areas you
might like to live in. You are primarily interested in areas where population is low compared
to the rest of the region, and more importantly, in areas where your thirteen year-old
daughter can attend middle school close to home.
Open the map document lab7_part2.mxd in the lab7_part2 folder. We are done with
lab7_part1.mxd; you can save changes if you want (optional). When the lab7_part2.mxd
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opens, you will see Middle Schools, Counties, and Census tracts layers. Set the default
geodatabase to the lab7_part2.gdb in the lab7_part2 folder from the File  Map Document
Properties window, and make sure that the box is checked to “Store relative pathnames to
data sources”. If needed, use the Full Extent button to zoom to and center the data in the
map window. See the screenshot below.
First, we will find areas that have a low population density. We will search for areas that
have population less than the mean population for the area. We will use 1990 population
attribute data in the Census tracts layer to do this analysis. Open the attribute table for
Census tracts and calculate the statistics for 1990 population (“POP_90” field). Make a
note of the mean population value. Now go to the Selection menu, and pick Select by
Attribute. Remember this tool? You may have only used this tool from the attribute table of
a layer, but it is also available from the Selection menu. Enter the appropriate selection
based on the criteria stated above, applied to the Census tracts layer (i.e., find Census
tracts polygons with a 1990 population less than the mean population). Click Apply when
you are done and close this screen to view your selection. See the screenshot below.
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By looking at the map view, you can see that some of the highlighted census tracts have
middle schools. You want to find census tracts that have low population and middle
schools. Since the census tracts and middle schools are in separate layers, we need to
use a layer-on-layer selection to find census tracts that meet both of your criteria.
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Go to the Selection menu, Select by Location. Enter the criteria as follows:
You are selecting features from the currently selected features in the Census tracts layer
that completely contain the features of Middle Schools. Read through the criteria from top
to bottom to make sure understand the selection statement. Click Apply when you are
done and close this screen to view your selection. You should see census tracts that meet
both your criteria -- low population and contain a middle school (see screenshot below).
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The Scenario: Now you have just learned from a future coworker in the Atlanta region that
property taxes are lower in Cobb County than in other counties in the region. As a bonus,
you also learn that your future boss lives there, too! You now need to find out which of the
selected census tracts fall within the Cobb County to help narrow your search for a home.
Turn off the Middle schools and Census tracts layers in the TOC. Open the attributes table
for Counties and select Cobb County. Hint: you can build a query or just click the row in
the attribute table that has Cobb County. Close the attribute table and notice that the Cobb
County polygon is selected in the map (cyan color).
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Now turn on all the layers in the TOC again. Go to the Selection menu, and pick Select by
Location. Enter the selection criteria as shown in the screenshot below:
This tells ArcMap to select from the currently selected features in Census tracts only those
polygons that are completely within the selected features of Counties (i.e. Cobb County).
Click OK when you are done.
Question 5: (3)
a. How many census tracts were in your final result? (1)
b. What is the mean per capita income (PER_CAPINC) for your selected set? (2)
5.0 Buffering features
Buffering and overlay are two of the most common operations in spatial analysis. A buffer
zone is an area that is within a given distance from a map feature. Points, lines, or
polygons can be buffered. Buffers are used to identify areas surrounding geographic
features. For example, you may wish to keep septic systems over 100 meters away from
streams, locate housing within a quarter mile of existing roads, keep hiking trails away
from seasonally-flooded rivers, or make sure most of your city is within some maximum
distance from a fire station or school. When you buffer on a set of features, the output is a
set of polygons (buffering points or lines creates a new layer that contains polygon
geometry). These polygons define an inside region, an area less than the specified buffer
distance from the features of interest (e.g., less than 300 meters from a stream), and an
outside region, an area more than the specified buffer distance from the features of
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interest. The figure below shows streams buffered at 300 meters. Be sure that you
understand what areas are inside of the buffer zone and what areas are outside of the
buffer zone.
Sometimes when you buffer a stream network with a large buffer distance, the buffers from
different streams intersect and you get enclosed polygons (see figure above). These areas
are still more than the specified buffer distance from the stream (they are outside of the
buffer zone), but are isolated because you must cross a buffer zone to get to them. Some
GIS software programs identify these areas as being different from areas which are totally
outside the requested buffer.
The Scenario: Vernal pools are seasonal wetlands in shallow depressions that fill during
the rainy season and persist for several months before drying up in the summer. Some
rare and specialized plant and animal species, known as “vernal pool endemics”, are
completely dependent on these ephemeral pools. Once more widespread, much of this
habitat has been lost due to development. Most of the remaining vernal pools in Sonoma
County are found on the Santa Rosa Plain. (Community Foundation Sonoma County,
2010, Section 2, pp. 10-11. Biodiversity Action Plan: Priority Actions to Preserve
Biodiversity in Sonoma County. Retrieved Nov. 4, 2011 from
http://www.sonomalandtrust.org/pdf/Biodiversity_Action_Plan_2010_reduced.pdf)
Additional information: A two page document from the City of Santa Rosa about vernal
pools on the Santa Rosa Plain that includes pictures of some of the special status species
can be downloaded from: http://ci.santa-rosa.ca.us/doclib/documents/VernalPool.pdf
Scenario: The County of Sonoma has contracted you to identify and make a map of
parcels that are located within 2000 feet of active vernal pools, so that the parcel owners
can be notified of special permit requirements for new projects. The area of interest is
located in a portion of unincorporated Sonoma County west of Hwy 101 between Santa
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Rosa and Rohnert Park. The county has provided you with GIS layers of parcels point
locations of the vernal pools in this area.
Data disclaimer: The point locations of vernal pools used for this lab assignment are
hypothetical, and do not represent actual pools. The points are, however, located within
areas that have been identified by the California Dept. of Fish and Game as “vernal pool
complexes” that are at least 40 acres in size.
We will use buffers and Select by Location to make this map. Open the map document
lab7_part3.mxd in the lab7_part3 folder. You do not need to save lab7_part2.mxd.
Set the default geodatabase to lab7_part3.gdb, and make sure that the box is checked to
“Store relative pathnames to data sources”.
First, we will make a 2,000 foot radius buffer around the vernal pools. Open the
ArcToolbox window and navigate to Analysis Tools  Proximity  Buffer. (Alternatively,
you can find Buffer, and several other commonly used tools, under the Geoprocessing
menu on the standard toolbar.) Select vernal pools as the input feature and choose an
appropriate name for the output polygon feature class (store the output in lab7_part3.gdb).
Enter in 2000 feet for the buffer distance. Select ALL in Dissolve Type to dissolve all the
buffers together into one output feature and remove overlapping features. You can expand
the help window with Show Help >> and then click in any parameter window to see what
the different options mean. This is particularly helpful when buffering lines and polygons,
as there are more options available. See the screenshot below.
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Click OK to run the buffer tool. Turn on the output vernal pool buffer in the TOC.
This gives us circular zones around vernal pools. We will need this buffer feature class for
our final map. We now need to select those parcels that are within these zones. We can do
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this with Select by Location. As with many GIS tasks, there are different options for doing
this. One option is to select parcels that "are within a distance of” vernal pools, and set the
distance at 2000 feet. See example of this selection below.
However, there is an equivalent way, and this will allow you to learn another selection type
called “intersect”. When features share the same geographic space, they overlap, or
intersect. ArcMap can find features that intersect, whether they are in the same layer or in
different layers. Using layer-on-layer selection, you can find and select features that
intersect other features in another layer. Here we can select parcels (polygons) that
intersect the vernal buffer polygons (2000 radius polygons around vernal pools). The
results are the same as using the "within distance of" method in Select by Location, shown
above. See the screenshot below.
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Click OK.
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Geography 387 – Fall 2011
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Vector Spatial Analysis
Your selected parcels should look like the screenshot above. Notice that some parcels go
beyond the bounds of the buffer zones. This is because only a portion of any given parcel
must intersect the buffer zone for it to be selected. This is what we want in this case, since
the county wants to notify parcel owners if any portion of the parcel is within 2000 feet of a
vernal pool.
Let's export the selected set of parcels that intersect the vernal pool buffers to a new
feature class. Right-click on parcels, and navigate to Data  Export Data. Give your
output parcels a meaningful name and save to the lab7_part3.gdb geodatabase in the
lab7_part3 folder.
Note that we could have selected only those parcels that are completely within the buffer
zones by modifying the query once again, using the “Target layer(s) are completely within
the Source layer feature" option in Select by Location.
You can now clear the selected parcels. Do this by going to Selection, then Clear Selected
Features. Save the map document.
Map 1 (10)
Make a map of the parcels that intersect the 2000 ft buffers around the vernal pools.
The map should be in gray-scale colors to reduce costs of printing the map to
distribute to parcel owners. Include the 2,000-foot radius buffer circles around the
vernal pools (hint: make the fill transparent). The selected parcels that intersect the
vernal pools should be dark gray, and all other parcels should be light gray. Include
a scalebar, title, name, legend, etc. Export the map to a PDF file with 150 dpi.
6.0 Overlaying layers
Overlays are another common analytical operation. They are the primary way in which
information from two separate layers may be brought together in an analysis. Overlays are
most common for polygon data. A geometric intersection of 2 polygon layers results in a
new layer with the combined attributes of both input layers. For example, suppose we wish
to identify all the areas in a given county that are also forested. We might have a counties
layer and a forest layer, both with polygons. If we overlay these two layers, we will have a
single output layer that has both county and forest cover information. By selecting only
those polygons that are both in a desired county and that are forested, we will identify the
forested portion of the desired county.
The Scenario: This final lab exercise will use roads and water layers to find potential
campgrounds sites for the Desoto National Forest, in southern Mississippi. The
campground will have ‘drive in’ sites, which must be within a buffer zone around roads.
The final map will show locations that are both within 150 meters of a lake or 500 metersof
a river and within 300 meters of a road.
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Open a new map document in ArcMap. Set the default geodatabase to the lab7_part4.gdb
in the lab7_part4 folder. Load the Roads line feature class and the Water polygon feature
class from the lab7_part4.gdb. Use the buffer tool to make a 300 meter buffer around the
roads. For buffer parameters, use Side Type "FULL", End Type "ROUND" and Dissolve
Type "ALL". Remember, you can use the help window and click in the parameter windows
to learn more about these options. Give your output roads buffer feature class an
appropriate name, and output it to lab7_part4.gdb.
Open the Water feature class attribute table. Note that there is a column called Buff_Dist
that has 150 coded for Lake polygons and 500 coded for the River polygon. We will use
this field to produce a variable-distance buffer. Again go to the buffer tool and select
Water for the input feature. This time chose the Field option instead of a fixed distance. For
the field, chose Buff_Dist. This will make the buffer distance around each Water polygon to
be defined by the value in the Buff_Dist field.
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Click OK to run the buffer. Notice that the lakes and rivers have different buffer sizes.
We are now going to overlay the variable-distance Water buffer and the fixed-distance
Roads buffer. To overlay them, we need to do a few things.
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When overlaying layers you must set up the two files in a manner that allows the results to
show from which input layer a particular feature originated. To do this, add a field to the
water buffer layer, call it “inside_waterbuf”, and calculate a value of 1 for the water buffer
polygons (you should know how to do this by now without step-by-step instructions). Do
the same for the road buffer layer. Add a new attribute called “inside_roadbuf” and assign
it a value of 1 to indicate it is inside the road buffer polygon.
You may ask why we don’t assign the outside a value, too? Right now, there are no
features outside our water or road buffers. We can’t assign values without an underlying
feature.
The Overlay toolset in Analysis Tools in ArcToolbox contains tools to overlay multiple
feature classes. Options are to combine, erase, modify, or update spatial features in a new
feature class. New information is created when overlaying one set of features with another.
There are seven types of overlay operations; all involve joining two or more existing
feature classes into a single feature class, allowing us to identify spatial relationships
between the input features. The following table lists the tools available in the Overlay
toolset and provides a brief description of each.
Tool
Description
Erase
This tool creates a feature class from those features or portions of
features outside the erase feature class.
Identity
This tool combines the portions of features that overlap the identity
features to create a new feature class.
Intersect
This tool builds a new feature class from the intersecting features
common in both feature classes
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Geography 387 – Fall 2011
Spatial Join
Symmetrical
Difference
Lab 7
Vector Spatial Analysis
This tool builds a new feature class by joining attributes from one
feature class to another based on a spatial relationship
This tool creates a feature class from those features or portions of
features that are not common to any of the other inputs.
Union
This tool builds a new feature class by combining the features and
attributes of each feature class.
Update
This tool updates the attributes and geometry of an input feature
class or layer by the Update feature class or layer that they overlap.
The final step of this section is to overlay our two buffer layers. Go to ArcToolbox again
and navigate to Analysis Tools  Overlay  Union. Enter the following criteria, and
remember that you can drag and drop layers into the Features list from the TOC. Be sure
that the box is checked next to “Gaps Allowed”. See the screenshot below.
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Click OK. Display the new Union layer and open its attribute table. Your output should look
similar to the screenshot below.
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Question 6: (3)
a. How many records are there in the Union layer? (1)
b. Why do you think you have more records than in your input buffer layers? (2)
Use Select by Attributes on the Union layer to find those areas that are BOTH within the
roads buffer and the water buffer (hint: this is where inside_waterbuf and inside_roadbuf
both have a value of 1). Export your selected polygons to a new feature class in
lab7_part4.gdb, called acceptable_sites.
Map 2 (10)
Create a map of the acceptable camping sites (acceptable_sites). Include roads
and water features, but not their buffer layers. Include a scalebar, title, name, date,
legend, etc. If you want a background layer of the extent of the study area, you can
add the study feature class. Optional: You can also add background imagery or
other layers by clicking the side pop-up menu next to the Add Data button, then go
to Add Basemap.
7.0 Conclusions
You now have first-hand experience with the basic tools that you need for vector spatial
analysis. We have not shown you all the tools available. For example, the overlay tools
intersect, identity, update were not covered. It is up to you to explore these tools and their
many parameters. The datasets from this and previous labs are relatively small and make
great layers for experimenting with the outputs from various analytical tools. Always
remember to use the ArcGIS Desktop Help to get a more detailed explanation. Have fun
exploring!
8.0 To turn in
●
●
●
Your answers to the 6 questions in a Word document (20)
Map 1 (pdf format) (10)
Map 2 (pdf format) (10)
Submit electronic files via email to your instructor, subject "G387, Lab 7, [your last name]".
Credits: This lab was designed by Matthew Clark, Geography and Global Studies
Department, Sonoma State University. Data layers and much of the text for Sections 3, 4
and 6 were modified from materials provided by Paul Bolstad (Univ. of Minnesota) and
George Raber (Univ. of Southern Mississippi). Edited for ArcGIS 10 by Elizabeth Lotz.
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