Creating Spatial Feature Classes Based on Proximity, Overlay, and Attributes - Introducing Geographic Information Systems with ArcGIS - page 423

15. Create a Union of TwoStalks and ThreeBars as before, but do it with the box titled Gaps Allowed
checked. Call the new feature class Union_with_gaps. Explode the feature class into
Union_with_gaps_spp. How many polygons are created? _______.
How do you explain the different number from the “no gaps allowed”
version?_____________________________________________. Use Identify on the added
“polygon”. What happens. Save the map as Stalks_Bars_gaps_allowed.mxd in the folder Trivial_
GIS_Datasets. Close ArcMap.
You should come away from this exercise understanding of the results of combining two polygon feature
classes with Union and what makes up the attribute table. Frankly, it takes some thinking and maybe
additional experimenting to understand the concepts involved. As I mentioned earlier, you usually get many
more polygons and lots more records in the result of a Union than were in the constituent feature classes.
Thinking this all out, throwing in also the ideas of single-part and multipart polygons, is not conceptually trivial.
Make a New Feature Class from a Subset of
Polygons: Extract
As you saw, the Union of the feature classes produced many output polygons from a few input polygons. As
you also saw, those polygons could be identified by their attributes. Frequently what you want to do after an
overlay is to select a subset of the output polygons and make a new feature class. For example, to solve the
Wildcat Boat problem, you would want to extract the polygons that met the requirements of soil suitability,
land cover, and so on. To do this, you build a query in what is called the Structured Query Language (SQL).
There is a lot of similarity between using SQL and what you did above in selecting polygons. The difference
is that the result of the SQL process is a new feature class that contains the selected features.
Suppose now that we want to make a new feature class named BOTH_TOP_and_LEFT, consisting of those
polygons on the top plus those polygons at the left.
16. Start ArcMap by opening Stalks_Bars_no_gaps.mxd. Make the only entry in the T/C Stalks_
Bars_spp by removing any others. From ArcToolbox: Analysis Tools > Extract > Select. Start the
Select tool. Read the Help panel. Then read the several different Help discussions, obtained
by clicking various places on the Select window—especially Expression. Look at the Tool Help.
When you begin to “build a query,” it is assumed that all features (and therefore records) are
selected. This is different from the previous assumption you have been using that none
are selected. You begin by selecting a subset—that is, reducing the number of records
selected, by reselecting some of them.
17. Browse to the Input Feature class Stalks_Bars_spp in
___IGIS-Arc_ YourInitials \Trivial_GIS_Datasets\Overlay_Exercise\Trivial_Feature_Classes
Call the Output Feature class
ALL_TOP_POLYS_plus_ALL_LEFT_POLYS