Geographic and Attribute Data: Selection, Input, and Editing - Introducing Geographic Information Systems with ArcGIS - page 325

18. Open the attribute table of Small_Squares_2. (The topology has no attribute table.) Dock or reduce
its size so that you can see most of the rest of the screen easily. Experiment with selecting the two
features. It should be clear that these are still non-planar (independent - not nested) polygons.

19. Use Customize > Toolbars > Topology to bring the Topology toolbar onto the screen. It is
inactive because you are not yet editing. Click Start Editing and notice that the toolbar becomes
active. Find the Fix Topology Error Tool and press the button. Click the pink area. Right-click and
pick Show Rule Description to see what rule is being violated. Click OK.

20. Right-click again and notice that you have several choices. The ones of main interest are as follows:

❏ Subtract

❏ Merge

❏ Create Feature

To find out what these do, press F1 to get the ArcGIS Desktop Help. Make it full screen. Navigate
in the Contents to Professional Library > Data Management > Editing Data > Editing topology >
Editing geodatabase topology > Geodatabase topology rules and topology error fixes. Like the
descriptions of topological errors that you saw in Chapter 4, there is a lot of material here. Under
Polygon Error Fixes, read the potential fixes for the Topology rule: Must Not Overlap. Dismiss Help.

Use a second way to get help for the three possibilities. Right-click the pink square. Highlight
the word Subtract by placing the cursor over it. Press Shift-F1. (In 10.1 mouse over the word
Subtract.) Read the information box that appears. Press Esc to close the box. Repeat for
Merge. Repeat for Create Feature.

It turns out that you want Create Feature. What will happen is that the original small square will be
discarded, as will the part of the large square that it overlapped. In their places, a new polygon will be
generated that is congruent to the area of overlap (i.e., the equivalent of the small square).

21. Right-click again on the pink square and choose Create Feature. Note the change in the display.
Press Ctrl-Z to undo the change you just made. Use Create Feature again and this time observe
the change in the attribute table. Write the results in the following:

Large polygon perimeter: ____ Large polygon area: ____

Small polygon perimeter: ____ Small polygon area: ____

Note the difference between the large polygon now and what you wrote earlier. Clearly you now
have two nested, planar, polygons.

22. Click Save Edits and click Stop Editing. Dismiss the Topology toolbar. Dismiss ArcMap.

Learning all about editing in ArcMap is a course by itself. The Editor has many tools, particularly computer-
aided design (CAD) tools, which are quite sophisticated. The Esri manual on the subject is a tome of
almost 500 pages. So the best we can do here is give you a insight into a few operations that you will
surely need if you do spatial analysis, and a taste of what editing in ArcMap is like. In the next few steps,
you will simply splash around in the Editor. Actually learning to swim with the editor is something you will
have to do on your own or in another course.