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Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition by Michael D. Kennedy

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Experimenting with Editor’s Union
Basically, a union of two polygons, say A and B, is a polygon whose area includes both the area of A, and the area of B (which, of course, includes any area that is common to both A and B).
____ 24. Make a second feature, say a rectangle, that partly overlaps the triangle.
____ 25. Select both polygons. (Use either the attribute table with Ctrl-click or the Edit tool with Shift-click.) Using Union in the Editor menu, make a third polygon.
____ 26. Prove to yourself that a new polygon has been created by (a) looking at the attribute table, and (b) dragging it with the Edit tool. You could, of course, make this third polygon effectively replace the first two, by deleting them, but don’t. Instead, select Stop Editing without saving edits.
____ 27. Start editing. Make a second polygon that lies outside the triangle. Again perform the union operations. Observe the result. Stop editing without saving edits.
____ 28. Start editing. Make a second polygon that lies inside the triangle. Again perform the union operations. Observe the result. Stop editing without saving edits. Then start editing again.

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