Geographic and Attribute Data: Selection, Input, and Editing - Introducing Geographic Information Systems with ArcGIS - page 323

has a hole in it in which the smaller square fits. The larger square is a complete square; the
smaller square is also a complete square. Verify this by looking at the table and examining
areas and perimeters. Fill in the following.
Large polygon perimeter: _______________ Large polygon area: _______________
Small polygon perimeter: _______________ Small polygon area: _______________
So you can see by the areas and perimeters that each of these squares is independent of the other, although
they, in part, cover the same area of the Earth. We couldn't, for example, grow corn on one and wheat on the
other. In general, in a given feature class we want disjoint polygons, so this is a bit of a challenge. Not having
disjoint polygons is a problem that occurs whenever we digitize polygons inside polygons.
Using “Clip” to Remove Overlaps from the Feature Class
There are at least two different ways to solve the problem of the non-planar features. The first one is to
use the small square as a “cookie cutter” to clip away the duplicated areas of the large square that is
under it.
10. Start ArcMap with a blank map. Add as data Small_Squares_1. Open its attribute table and
dock it or reduce its size so that you can see the map. Make the color of the polygons Hollow
or No Color, so you can see the boundaries when you are editing. Sometimes the large square
blocks out the small square.
11. Start editing. Click the Edit tool. Click the map somewhere away from either square. Click the
center of the small square, and notice one square or the other is selected in the table. Now
press the N key on the keyboard. This selects the Next feature, namely, the other square.
Repeated pressings of the N key alternates the selections. If the N key doesn't seem to work
well for you, use the attribute table and select the features with it. Or use the tiny toolbar that
may show up, whose drop-down menu lets you select the square you want. However you do it,
end up with the small square selected .
12. Drop the Editor menu down and click Clip.
This tool lets you create a new area, derived from the large square. What will remain from the large square
is either (a) that area that intersects the small square or (b) what is left of the large square after an area
equivalent to the small square is taken out. To return to our cookie-cutter analogy, we can either keep the
cookie (the small square) or keep the dough around the cookie.
13. Click Preserve the area that intersects and click OK. Notice that the large square disappears
and, as you can verify from the attribute table, you are left with two small squares. Not what
you want. You are looking for two non-overlapping polygons. Choose (from the Main Menu)
Edit > Undo Clip to return to the previous configuration. Again make sure the small square is
selected. Clip again, but this time select Discard the area that intersects—that is, throw away
the “cookie” part of the big square that is under the selected small square. Now look at the
attribute table. Record the values in the following.
Large polygon perimeter: _______________ Large polygon area: _______________
Small polygon perimeter: _______________ Small polygon area: _______________