Creating Spatial Feature Classes Based on Proximity, Overlay, and Attributes - Introducing Geographic Information Systems with ArcGIS - page 410

You find the tools to perform these actions in ArcToolbox: Analysis Tools (for work with feature classes in
general).

Union combines feature classes in such a way that the extent of the derived dataset includes the extents
of all the constituent datasets. The idea is that, in the union of P and Q to make R, say, the territory
covered by R is the territory covered by P and also by Q, including any overlapping territory. In logic,
the union of two sets (A and B) is the set of all objects in A or B or both. If A is all red squirrels and B is all
squirrels with fuzzy tails, then A union B is the set of all squirrels that are either red or have fuzzy tails or
both. The union is equivalent to the logical inclusive OR. (The logical exclusive OR—XOR—would consist
of those squirrels that were red or had fuzzy tails, but not those that were red and also had fuzzy tails.)

Intersect combines feature classes in such a way that the extent of the derived dataset includes only the
overlapping area of the constituent datasets. The idea here is that, in the intersection of S and T to make
U, say, the territory covered by U is only the territory covered by S that is also covered by T. In logic,
the intersection of two sets (A and B) is the set of all objects that are in both A and B. If A is all red
squirrels and B is all squirrels with fuzzy tails, then A intersect B is the set of all squirrels that are red
and have fuzzy tails as well. The intersection is equivalent to the logical AND.

A number of other spatial join tools are available in the tool chests:

Identity

❏

Erase

❏

Update

❏

Clip

❏

Split

❏

Symmetrical Difference (the union with the intersection removed)

❏

Further, some of these tools are not confined to polygons. For example, you can use Intersect to
determine, from two feature classes consisting of points, which points are coincident (or lie within a
certain distance of each other). The same can be true with two sets of lines. Or to find the points that
intersect lines.

You will work with some of these in the Step-by-Step section of this chapter. Now we turn to what you
can do once you have applied buffers and spatial joins.

Deriving Feature Classes by Selecting Attributes:
Extraction
You have perhaps buffered some points, lines, and polygons, and have the results as feature classes. You
have overlaid these buffers on some other polygon feature classes. What you now have is a feature
class that has dozens, hundreds, maybe even thousands of polygons. That feature class also has an
attribute table with a list of fields as long as your arm, since it consists of almost all the attributes of the
constituents tables. Now it is time to make a feature class that consists only of those polygons that meet
your requirements. You do this by writing a query that will be processed by the Select tool, within the