3. CREATING DATA (First steps)
3c. Creating topology
3c. Creating a Topology class
Topology is a fancy word basically meaning “geometric relationships”. A very neat feature that ArcCatalog can do is use your line feature class and create polygons from those lines. Helpful when creating a geologic map and defining different map units. However, lines bounding polygons need to close in order for ArcCatalog to properly create the polygons.
By creating a topology in Arc, you assign rules for the geometric relationships in your line feature class. Arc will automatically look for topological “errors”- places where you might have erroneous lines or polygons that do not close. You’ll have to fix them yourself, of course, but at least Arc will tell you where and what needs to be done. More on this when we’re editing our feature classes. For now, here’s how to set up your topology:
1. In ArcCatalog, right-click in the contents window of your Feature Dataset. Select New –> Topology.
2. The “New Topology” wizard will pop up. Click “Next”.
3. Name your topology class. By now you should be familiar with the Rule of Names. Don’t worry about the “cluster tolerance”, the default is defined by your feature class and is usually just fine. Click “Next”.
4. You want your lines to be governed by the topological rules you’re about to set up. Check the box next to your line feature class and click “Next”.
5. You can ignore this next window. Click “Next”.
6. Here is where you’ll assign rules to your topology. Click on the button “Add Rule”. Fortunately, this “Add Rule” window that opens up gives you a brief description both visually and written of what the various rules mean/ represent.
Make sure that the first drop-down menu is set to your line feature class (by default it usually is, but check just to make sure). Select “Must Not Have Dangles” from the drop-down “Rule:” menu. Click “OK”. You’ll want to add another two rules, so repeat this step for the rule “Must Not Intersect Or Touch Interior” and “Must Not Overlap”.
As you’re selecting these rules, be sure to read what they mean so you’re not just adding things into a “Black Box”.
7. Once you’ve added these two rules, click “Next”.
8. This is just a summary window. Click “Finish”. ArcCatalog will create your topology, and you’ll get an error message saying that “No features were found within the extent of the topology”. Because you really haven’t created any features just yet, this is fine. Ignore it for now, and click “OK”.