Editing shared features with a map Topology

Exercise 4a: Editing shared features with a map Topology

About map topology

Many vector datasets contain features that share geometry. Features can share edges—for example, line segments—or nodes, the points at the ends of segments. For example, watershed polygons might have common edges along ridgelines, and lake polygons might share their shoreline edges with land-cover polygons. Three watersheds might share a single node at a mountain peak, and three river-reach features might share a node at a confluence. 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 specify the feature classes that you want to participate in the map topology. You can also choose the distance, or cluster tolerance, that defines how close together edges and vertices must be in order to be considered coincident. You can simultaneously edit shared edges and nodes with the Topology Edit tool when you create a map topology.

ArcView users can create and edit a map topology, which is the type used in this exercise. ArcEditor and ArcInfo 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 feature classes by creating a map topology.

Steps:

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 classes. Hydro_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. Hydro_units contains polygon features representing smaller watersheds within these regions. You can see the features in the Hydro_units feature class because the Hydro_region features are partly transparent.

The regional data was derived by dissolving the smaller hydrologic units, so the boundaries of the features in Hydro_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 and move a node that defines the intersection of multiple features.

5. Click the Editor menu on the Editor toolbar and click Start Editing.

6. Close the Create Features window. You will not need it in this exercise. 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.

7. Click Bookmarks and click 3 Region Divide. The map zooms to the bookmarked area. Now you can see labels for the smaller watersheds.

8. Click the Map Topology button on the Topology toolbar.

The Map Topology dialog box appears. You can select the feature classes that will participate in the topology and choose a cluster tolerance. The cluster tolerance defines how close together parts of features must be before they are considered coincident.

9. Click Select All. You want all the features on the map from both feature classes to participate in the map topology. The default cluster tolerance is the minimum possible cluster tolerance and is given in coordinate system units. In this case, the dataset is in the universal transverse Mercator coordinate system, and the units are meters. Accept the default cluster tolerance.

10. 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 Show Shared Features dialog box to investigate which features share a given topology edge or node and control whether edits that you make to a given topology element will be shared by certain features.

Steps:

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 Show Shared Features command.

3. Click Show Shared Features on the Topology toolbar. The names of both feature classes in the map topology, Hydro_region and Hydro_units, are listed with check marks on this dialog box. The checks mean that the selected topology element is shared by features in these feature classes and are affected by any edits you make to the shared edge.

Next, you will see which features share this edge.

  

4. Double-click Hydro_units. The plus sign changes to a minus, and two more branches expand below Hydro_units. Each of these represents a hydrologic unit feature that shares this edge.

5. Click East Fork Sevier. Utah. (51). Feature number 51 in the Hydro_units feature class, the East Fork Sevier hydrologic unit, flashes on the map.

6. Double-click Hydro_region and click Great Basin Region (1). Feature number 1 in the Hydro_region feature class, the Great Basin region, flashes on the map.

7. Close the Shared Features dialog box.

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.

Steps:

1. Check Hillshaded_terrain.sid in the ArcMap table of contents to turn on the image. 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. The pointer becomes the Zoom In tool.

3. While pressing the Z key, drag a box around the selected edge. 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.

4. Double-click the edge. Now you can see the vertices (in green) that define the shape of this edge.

 

  

5. 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.

6. 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

Steps:

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:

Moving a shared node in a map topology

Now that you've adjusted the edge shared by the watershed boundaries, another problem with the existing data needs to be fixed. The node at the east end of the edge is the point where the Great Basin, Upper Colorado, and Lower Colorado region watersheds come together. You'll move this shared node by a specified number of meters.

Steps:

1. Click the Topology Edit tool on the Topology toolbar.

2. Click once on the map, off the edge, to deselect it.

3. Press and hold the N key. This temporarily limits the selectable topology elements to nodes.

4. Drag a box around the node while holding down the N key.

The node is selected. Now you'll move it to the correct location.

5. Right-click and click Move. You will move this node 460 meters in the x direction (east) and 410 meters in the y direction (north).

6. Type 460 and 410 in the x and y boxes, respectively, and press ENTER.

The node is moved to the new location, and all the features that share it in the map topology are updated. You can also move the node by dragging it as you move the vertex of the topology edge.

7. Click the Editor menu on the Editor toolbar and click Stop Editing.

8. Click Yes to save your edits.

9. Close ArcMap if you are done working with the tutorial. You do not need to save the map document.

10. To continue to the next exercise, 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 and nodes. The map topology allowed you to maintain the common boundary between the features while simultaneously editing four, then six, features in two different feature classes. The Topology Edit tool and the topology editing tasks can also be used to edit the edges and nodes in a geodatabase topology.