Exercise 2c: Editing polygon features

Complexity: Intermediate Data Requirement: ArcGIS Tutorial Data Setup Data Path: \ArcGIS\ArcTutor\Editing

About editing polygons

In the previous exercise, the Buffer command created a feature that is the extent of the original feature plus the buffer distance. Since this feature should just be the buffer, you need to remove the shape of the original inner feature from the current buffer feature. You can use the Editor menu > Clip command to cut a hole in the polygon feature.

You will also use the Cut Polygons tool to split a polygon by an overlapping line feature.

Cutting a hole in a polygon

Prerequisite:

The Exercise2.mxd is open and you are in an edit session.

The new feature is drawn on top of the existing one. To use Clip, you need to select the underlying existing feature. The Edit tool has special capabilities to help you select the correct feature from overlapping ones.

Steps:

1. Click the Edit tool on the Editor toolbar.
2. Click the center of the buffer feature. Since there are multiple selectable features where you clicked, the selection chip appears. Click the arrow to the right of the icon to view a list of the features from which you can select. Features are listed in the selection chip by their display expression, which is set on the Layer Properties > Display tab.
3. Rest your pointer over a feature in the list to flash it on the map. Click the Isolated Mesa Tops feature to select it. You will use this feature to clip a hole in the Buffer zone polygon.
4. You can check that the correct feature is selected by clicking the List By Selection button in the table of contents and noting that only the Isolated Mesa Tops is listed in the Research areas layer in the Selected category.
The Editor > Clip command only clips polygon features that are within a buffer distance of a selected feature—in this case, the Isolated Mesa Tops research area.
5. Click the Editor menu and click Clip.
6. Ensure the Buffer Distance is 0. This way, you will be clipping to the exact border of the selected feature rather than at a distance from it.
7. Click Discard the area that intersects. This removes the overlapping area from the feature that is being clipped.
8. Click OK. The overlapping area is clipped and now the original Research areas feature is visible through the hole in the buffer feature.
9. Click the List By Selection button in the table of contents, if you are not already listing layers this way, then click each feature on the map and note that the selected feature changes in the list in the table of contents. The 1 to the right of the selection icons indicates that there is one selected feature.
Since the buffer feature has a hole in it, its geometry is represented in ArcGIS as a multipart polygon. Multipart features either contain holes in them or are composed of more than one physical part that only references one set of attributes. For example, the individual islands that make up Hawaii are often represented as a multipart polygon feature. You can view the list of parts in a feature by double-clicking it with the Edit tool and opening the Edit Sketch Properties window.

Cutting a polygon

The neighboring research area needs to be divided into two polygons based on the river that runs through the middle. You can use the Cut Polygons tool to split the polygon.

To use the Cut Polygons tool, you need to select the polygon, then digitize a line where you want to cut the polygon. To change the shape of the line used to cut the polygon, click a construction method type on the Editor toolbar or on the Feature Construction mini toolbar. Segments can be created using a variety of methods, for example, as straight lines, with curves, or traced from the shapes of other features.

If you are cutting a polygon along a simple line, you can click to draw the line using the Straight Segment construction method. However, in this case, the river feature you want to use to cut with is long and curved, so it will be easier to trace around the border to create the line.

Steps:

1. Click the Edit tool on the Editor toolbar.
2. Click the Goose Creek research area, the blue polygon just to the west of the polygons you were previously editing. You may need to zoom in or pan to this feature so you can see it better.
3. In the table of contents, click the gray layer icon to the left of the Streams layer to make the streams visible again so you can trace along them. When you do this, the layer icon becomes colored .
4. Click the Snapping menu on the Snapping toolbar and click Intersection Snapping . This turns on snapping to intersections between features, which will help you ensure that the line used to cut the polygon starts and stops at the intersection of the polygon and line edges.
5. Click the Cut Polygons tool on the Editor toolbar.
6. Click Trace on the Editor toolbar palette.
7. Snap to the intersection of the polygon edge and the stream line near the buffer polygons, then click to start tracing the line through the polygon. Follow along the stream line to trace it.
8. Once you have traced all the way across the polygon, snap to the intersection of the polygon and line at the northern edge of the polygon, and click the map to place vertices.
9. Right-click anywhere on the map and click Finish Sketch.
10. You are finishing the sketch used to cut the polygon. The polygons flash on the map as the cut is made and the new features are selected. If an error occurs, ensure that you have the correct feature selected, try the trace again, then make sure your line goes completely across the polygon. It may help to zoom in when you start and end the trace.
11. Click the Edit tool on the Editor toolbar.
12. Click each new feature and notice that you now have two polygons.
13. Click the Editor menu on the Editor toolbar and click Save Edits.
14. Click the Editor menu on the Editor toolbar and click Stop Editing.
15. To continue to the next exercise, click Exercise 2d: Editing vertices and segments.

In this exercise, you learned how to clip polygons and split them by tracing along an overlapping line feature.