5. CREATING POLYGONS
5a. ArcCatalog steps
5. Creating polygons
By now you have digitized all your lines and points from your geologic map, and created a good bit of geospatial data. Here’s an example of what your map might look like:
You’ll notice that I still have the topology layer “geo_top” turned on, and there are a few errors. That’s okay, these errors are loose ends of lines (faults and fold axes), and can be made exceptions to the rules. Otherwise, the lines form closed polygons and are ready for the next step.
Before we go and actually create polygons from our lines, go to File –> Save after you’ve closed the Editor toolbar. This next step of making polygons is done in ArcCatalog, and since we’ll be manipulating a feature dataset that is currently open in ArcMap, we’ll need to close the ArcMap project.
5a. Steps in ArcCatalog
Setting up a new polygon feature class
As I just mentioned, this next step is done in ArcCatalog. Go ahead and close your ArcMap project and return to the “contents window” for your “geology” feature dataset in ArcCatalog. Here are the steps for creating polygons:
1. You will need to open up the proper tool for creating polygons from your lines- appropriately named the “Feature to Polygon” tool. To easily locate this tool (instead of having to search endlessly through ArcToolbox to find it), go to Geoprocessing –> Search For Tools and type “Feature to Polygon” into the search bar in the new window that opens.
2. Chances are, the “Feature to Polygon” tool is the first on the list- if not, make sure that you select the “Feature to Polygon (Data Management)” option from your search results. Click to open the tool.
3. Next, you need to choose the “Input Features” that you wish to create polygons from. This should be your line shapefile, so simply click on your line shapefile in the ArcCatalog window and drag the icon into the “Input Features” box in the “Feature to Polygon” tool window.
4. Enter in a name for this new feature class in the “Output Feature Class” box. Since you’re making polygons that will represent the different geologic/ stratigraphic units present on your map, I’d suggest the name “units”. Just remember to follow the good ol’ Rule of Names. Now you’ll have a window that looks similar to the figure below:
5. Don’t worry about the “XY Tolerance” or other options available through this tool.
6. Click “OK” to have the “Feature to Polygon” tool do its job. After a few seconds, a new feature class (your polygons) will appear in the “Contents” tab in the ArcCatalog window.
7. Because you will undoubtedly have multiple types of polygons for multiple different geologic units, we’re going to need to set up a way to name these different polygons. Double-click on the “units” feature class to open its “Feature Class Properties” window.
8. Just like when we were creating different types of lines for your “contacts” or “lines” feature class, go to the “Fields” tab and add in a new “Field Name” of “name” or “unit” that has a “Data Type” of “Short Integer”. This time, set “Allow hull values” to “YES” and the “Default Value” to zero. Once this is done, click “OK”.
9. Re-open the “Feature Class Properties” window for your units. Go to the “Subtypes” tab and create a different subtype for each geologic unit. Use the unit abbreviation, that way when we go to label our units only the abbreviation pops up. You can add in the full unit name later.
10. Once you’re done here, click “Apply”, and then “OK” to finish.