Module 4.3: Working with Boundaries
Clip, Union, and Intersect practice
Laura
Layoung2@vt.edu Wen Andrews
Wandrews@reynolds.edu
Johnson
Summary of skills covered:
Clip and/or Select and Export
Union
Intersect
Data needed:
4.3Data folder
Equipment and Software needed:
ArcGIS Desktop
Related book exercise (if
applicable):
Data Source:
Chapter 18 and 19
Falls Church, Virginia data: layoung2@vt.edu
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Exercise Overview
In this short practice exercise, we will be using the Falls Church, VA Urban
Forestry inventory. We will clip a dataset so that it only shows trees that fall
within the parcel boundary of the selected home lot. Then we will union and
intersect tree canopy polygons to look at the difference in results calculated with
each of these methods.
Exercise Set-up
1) In the 4.3Data folder, Open the 4.3 Falls Church map file in ArcMap.
2) This map contains three layers: Tree locations (point feature class) Live
Tree Canopy (polygon feature class), and Falls Church Parcel Boundaries
(polygon feature class). Be sure your map is centered on Parcel id number
746.
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Procedure: Part 1, Clip
1) The point feature class “Tree Locations” shows all of the trees that were
present prior to development in and around lot number 746. We want to
create a new feature class that shows only trees that fall within the lot
boundaries of parcel 746. There are two ways to do this: Clip (in
ArcToolbox), or Select by Location and Export. Create a new feature class
in the SamplePlots.mdb geodatabase named Trees_Parcel746 that has
only the tree locations that fall within the lot boundaries.
2) Which method did you choose? ________________________________
3) How many trees are within the lot boundaries? ____________________
4) Save your work.
Procedure: Part 2, Union and Intersect
1) For canopy cover to “count” towards a homeowner’s 20 percent canopy
cover goal, it has to be canopy cover that falls within their lot’s
boundaries. We want to calculate the canopy cover of the Live Tree
Canopy feature class that is within the boundary of Parcel 746. Which
tool will give us the ouput we need, Union or Intersect? _____________
(If unsure, run each tool to see what outputs you get. You can always
delete one!)
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2) Be sure you have Parcel 746 Selected before running the tool, so that the
tool only processes the part of the dataset within this parcel number. Run
the tool you selected, and name the output feature class
Canopy_Parcel746.
3) When we perform a geoprocessing operation, such as Intersect or Union,
any areas, perimeters, or distances we had previously calculated are now
suspect to error. You will need to create a new field to calculate polygon
area on the Canopy_Parcel746 Feature Class.
a. Open the Attribute Table for Canopy_Parcel746.
b. Go to the drop-down menu all the way on the top left of the
table, click it, and select Add Field…. From the menu options.
c. Name the field CAN_AREA and choose a type of Double. Leave
other items as default and click OK.
d. Now right-click on the CAN_AREA field and select Calculate
Geometry…. When you get a prompt about calculating outside of
an edit session, click Yes.
e. Make sure Property is set to Area (what we want to calculate
here), Coordinate System of the data source is selected, and Units
of Square Meters is selected. Un-check Calculate Selected Records
Only. Click OK, and Yes again on the prompt about the edit
session.
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f. What is the total area of all the canopy polygons that are only
inside of the Lot 746 boundaries? (Hint – right-click on the
CAN_AREA field, go to Statistics, and look at Sum):
_________________
4) If you have extra time: What happens if you run the tool you selected in
Step 1 without first selecting Parcel 746? How is the output different?
__________________
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