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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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Making Polygons of the Digitized Lines
The next task is to make polygons based on the lines you have digitized.
____ 19. In ArcCatalog, be sure you can see the contents of the feature dataset UTM_Zone_2. (This may require that you highlight the entry and press F5 to Refresh the Catalog Tree.) Open ArcToolbox. Expand Data Management Tools > Features and then pounce on Feature to Polygon. From the T/C, drag North_Islands_Lines into the input area. Name your feature class North_Islands_Polygons and make sure it is being saved to the UTM_Zone_2 Feature dataset within the Islands geodatabase. Click OK twice and, when the process is complete, examine the results in the Preview tab. (If not all the polygons show up you may have failed to close one or more lines in your digitizing. Lines to polygon only works on completely closed figures. You will need to go back and fix the problem. Zoom up on each island to see if there are any gaps in the lines surrounding the features. Snapping to the sketch plays an important role in making sure that lines close completely.)
____ 20. Restart ArcMap. Add North_Island_Polygons. Use the Identify tool to make sure you have proper GIS polygons. Open the attribute table and inspect it. How many entries are there? _______________. Dismiss the attribute table and the identify window.

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