Exercise 4a: Editing shared features with a map topology

Complexity:

Intermediate

Data Requirement:

ArcGIS Tutorial Data for Desktop

Data Path:

\ArcGIS\ArcTutor\Editing\Topology

About map topology

Many vector datasets contain features that share geometry. For example, polygon features often form a continuous fabric and are sometimes coincident with lines. Watershed units and hydrologic regions have common edges along boundaries, and lake polygons might share borders with land-cover polygons and shorelines. The Topology toolbar contains tools for working with topologically related features.

A map topology creates topological relationships between the parts of features that are coincident. You can simultaneously edit shared features with the topology tools when you create a map topology.

ArcGIS for Desktop Basic users can create and edit a map topology, which is the type used in this exercise. ArcGIS for Desktop Standard and ArcGIS for Desktop Advanced users also can edit geodatabase topology, which defines a set of rules about the relationships between feature classes in a feature dataset. You will edit a geodatabase topology in another exercise.

Creating a map topology

Prerequisite:

Start ArcMap and display the Editor, Snapping, and Topology toolbars.

In this exercise, you will update multiple watershed features in two layers by creating a map topology.

1. Click the Open button on the Standard toolbar.
2. Navigate to the MapTopology.mxd map document located in the \Editing\Topology directory where you installed the tutorial data. (C:\ArcGIS\ArcTutor is the default location.)
3. Click the map and click Open.
4. If you still have the map document open from the previous exercise and are prompted to close it, you can do so without saving your changes.
The map opens to the following view:



This map contains two feature layers. Hydrologic region contains polygon features representing three large hydrologic regions in the southwestern United States. Note that part of the Great Basin regional watershed has been omitted from the tutorial dataset. Hydrologic unit contains polygon features representing smaller watersheds within these regions. You can see the features in the Hydrologic unit layer because the Hydrologic region features are partly transparent.

The regional data was derived by dissolving the smaller hydrologic units, so the boundaries of the features in Hydrologic region are already coincident with the boundaries of the smaller watersheds. In this exercise, you will create a map topology to allow you to edit the vertices that make up a shared edge at the intersection of multiple features.

5. Click the Editor menu on the Editor toolbar and click Start Editing.
Before you create the map topology, zoom in to the area that you want to edit. Zooming in to an area reduces the number of features that the map topology analyzes when building the topology cache.
6. Click Bookmarks and click 3 Region Divide.

   The map zooms to the bookmarked area. Now you can see labels for the smaller watersheds.

7. Click Select Topology on the Topology toolbar. The Select Topology dialog box appears.
On the Select Topology dialog box, you can choose the layers that will participate in the topology and set a cluster tolerance. The cluster tolerance defines how close together parts of features must be before they are considered coincident. If your map has a geodatabase topology in it (and you have an ArcGIS for Desktop Standard or ArcGIS for Desktop Advanced license), you could also choose to edit the geodatabase topology instead of the map topology.
8. Click the Select All button. You want all the features on the map from both layers to participate in the map topology.

   

In the Options section, you can review the cluster tolerance. In this case, the dataset is in the universal transverse Mercator coordinate system, and the cluster tolerance units are meters. Accept the default cluster tolerance, which is the minimum possible value.
9. Click OK.

Finding shared features

Now you will start editing the map topology using the Topology Edit tool to select an edge and determine which features share it. You can use the Shared Features window to investigate which features share a given topology edge and control whether edits that you make to a given topology element will be shared by certain features.

1. Click the Topology Edit tool on the Topology toolbar.
2. Click the edge that is shared by the East Fork Sevier. Utah. polygon (#16030002) and Kanab. Arizona, Utah. polygon (#15010003).
The edge is selected and changes color. This edge is also shared by the larger regional polygons. To check this, you will use the Shared Features window.
3. Click Shared Features on the Topology toolbar.
The names of both layers in the map topology, Hydrologic region and Hydrologic unit, are listed with check marks on this window. The checks mean that the selected topology element is shared by features in these layers and are affected by any edits you make to the shared edge. Next, you will see which features share this edge.



4. Click Great Basin Region under Hydrologic region.

   The Great Basin region flashes on the map.

5. Click East Fork Sevier. Utah. under Hydrologic unit.

   The East Fork Sevier unit flashes on the map.

6. Close the Shared Features window.

Editing a shared edge in a map topology

Now that you have seen that the features you need to update share this edge, you'll update the boundary of the watersheds to better fit the terrain.

1. Turn on the Hillshaded terrain layer from table of contents.

   

This is a small area of hillshaded terrain extracted from the National Elevation Dataset Shaded Relief Image Service, published by the United States Geological Survey. You will use this image, and the guidelines that have been added to it, to update your watershed data.
2. Press and hold the Z key and drag a box around the selected edge. The pointer becomes the Zoom In tool.
The watershed data that you have is derived from the medium-resolution National Hydrography Dataset, published by the U.S. Geological Survey and the United States Environmental Protection Agency. This data was compiled at a scale of 1:100,000. The National Elevation Dataset hillshade is derived from 1:24,000-scale digital elevation model data. You will use the higher-resolution hillshade data to improve the watershed boundaries.
3. Double-click the edge. Now you can see the vertices (in green) that define the shape of this edge.

   

4. Move the pointer over the second vertex from the eastern end of the edge. When the pointer changes to a box with four arrows, click the vertex, drag it toward the northwest, then drop it on the blue guideline.

   

You could continue reshaping this edge vertex by vertex, but there is a faster way to update it.
5. Click once on the map, off the edge, to deselect it. Then click the edge again to reselect it.

Reshaping a shared edge in a map topology

Now you'll use an edit sketch to reshape the shared edge. You'll need to use the Reshape Edge tool and snap to the watershed edges.
1. Ensure edge snapping is enabled. If it is not, click Edge Snapping on the Snapping toolbar.
2. Click the Reshape Edge tool on the Topology toolbar.
3. Move the pointer over the edge where the selected topology edge and the blue guideline begin to diverge.

   

4. Click the edge to begin an edit sketch.
5. Continue adding vertices along the guideline. You can hold down the SPACEBAR key to turn off snapping temporarily if you are having difficulty placing the reshape line where you want it along the blue line.
6. Make sure that the last vertex you add to the sketch snaps to the edge near the vertex you moved.
7. Right-click anywhere on the map and click Finish Sketch.
The edge looks like this once you finish the sketch:



Tip:

Although you reshaped only one edge at a time in this case, you can also select multiple connected edges and update them together using Reshape Edge. To select a path of edges, hold down the left mouse button when using the Topology Edit tool and drag along the connected edges or use the Topology Edit Trace tool.

There are other locations in the dataset where you can reshape and modify edges to update the features to match the guidelines on the hillshaded terrain. You can continue editing the topology or stop editing if you are finished.
8. Click the Editor menu on the Editor toolbar and click Stop Editing.
9. Click Yes to save your edits.
10. Close ArcMap if you are done working with the tutorial. You do not need to save the map document.
11. To continue to the next exercise, click Exercise 4b: Using geodatabase topology to fix line errors.

In this exercise, you learned how to create a map topology and how to use the Topology Edit tool to edit multiple features that share edges. The map topology allowed you to maintain the common boundary between the features while simultaneously editing different layers.

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