5.2. Lesson: 地物のトポロジ

オーバーラップやギャップなどのエラーを最小限に抑えるため、トポロジはベクターデータレイヤーの有用な側面です。

たとえば、2つの地物が境界を共有し、トポロジを使用してその境界を編集する場合、はじめの地物を編集し、そしてもう1つの地物を編集して、それらが合うように慎重に境界を描く必要はありません。それらの共有された境界を編集でき、両方の地物は同時に変化します。

このレッスンの目標: 例を用いてトポロジを理解します。

5.2.1. Follow Along: スナップ

Snapping makes topological editing easier. This will allow your mouse cursor to snap to other objects while you digitize. To set snapping options:

1. Navigate to the menu entry Project ► Snapping Options....

2. Set up your Snapping options dialog to activate the landuse layer with Type vertex and tolerance 12 pixels:

   

3. Make sure that the box in the Avoid overlap column is checked.

4. Leave the dialog.

5. Select the landuse layer and enter edit mode ()

6. Check (under View ► Toolbars) that the Advanced Digitizing toolbar is enabled.

7. このエリアにズームします (必要に応じてレイヤーとラベルを有効にします):

   

8. Digitize this new (fictional) area:

   

9. When prompted, give it an OGC_FID of 999, but feel free to leave the other values unchanged.

   If you are careful while digitizing, and allow the cursor to snap to the vertices of adjoining areas, you'll notice that there won't be any gaps between your new area and the existing adjacent areas.

10. Note the ^undo and ^redo tools in the Advanced Digitizing toolbar.

5.2.2. Follow Along: トポロジ的地物を修正

Topology features can sometimes need to be updated. In our study area, an area has been turned into forest, so the landuse layer need an update. We will therefore expand and join some forest features in this area:

Instead of creating new polygons to join the forest areas, we are going to use the Vertex Tool to edit and join existing polygons.

1. Enter edit mode (if it is not active already)

2. Select the ^{Vertex Tool} tool.

3. Choose an area of forest, select a vertex, and move it to an adjoining vertex so that the two forest features meet:

   

4. Click on the other vertices and snap them into place.

   トポロジとして正しい境界線はこのようになります:

   

   Go ahead and join a few more areas using the Vertex Tool.

   You can also use the ^{Add Polygon Feature} tool to fill the gap between the two forest polygons. If you have enabled Avoid overlap, you don't have to add every single vertex - they will be added automatically if your new polygon overlaps the existing ones.

   If you are using our example data, you should have a forest area looking something like this:

あなたが結合した森林エリアがより多くても、より少なくても、違うエリアであったとしても気にしないで下さい。

5.2.3. Follow Along: ツール: 地物の簡素化

Continuing on the same layer, we will test the ^{Simplify Feature} tool:

1. それをクリックしてアクティブにします。

2. Click on one of the areas which you joined using either the Vertex Tool or Add Feature tool. You will see this dialog:

   

3. Modify the Tolerance and watch what happens:

   

   This allows you to reduce the number of vertices.

4. OK をクリックします

The advantage of this tool is that it provides you with a simple and intuitive interface for generalization. But notice that the tool ruins topology. The simplified polygon no longer shares boundaries with its adjacent polygons, as it should. So this tool is better suited for stand-alone features.

次へ進む前に、最後の変更を元に戻すことでポリゴンを元の状態に戻します。

5.2.4. Try Yourself ツール: リングの追加

The ^{Add Ring} tool allows you to add an interior ring to a polygon feature (cut a hole in the polygon), as long as the hole is completely contained within the polygon (touching the boundary is OK). For example, if you have digitized the outer boundaries of South Africa and you need to add a hole for Lesotho, you would use this tool.

If you experiment with the tool, you may notice that the snapping options can prevent you from creating a ring inside a polygon. So you are advised to turn off snapping before cutting a hole.

1. Disable snapping for the landuse layer using the ^{Enable Snapping} button (or use the shortcut s).

2. Use the ^{Add Ring} tool to create a hole in the middle of a polygon geometry.

3. Draw a polygon over the target feature, as if you were using the ^{Add polygon} tool.

4. When you right-click, the hole will be visible.

5. Remove the hole you just created using the ^{Delete Ring} tool.

   注釈

   Click inside the hole to delete it.

結果をチェックする

5.2.5. Try Yourself ツール: 部分の追加

The ^{Add Part} tool allows you to add a new part to a feature, that is not directly connected to the main feature. For example, if you have digitized the boundaries of mainland South Africa, but you haven't yet added the Prince Edward Islands, you would use this tool to create them.

1. Select the polygon to which you wish to add the part by using the ^{Select Features by area or single click} tool.

2. Use the Add Part tool to add an outlying area.

3. Delete the part you just created using the ^{Delete Part} tool.

   注釈

   Click inside the part to delete it.

結果をチェックする

5.2.6. Follow Along: ツール: 地物の変形

The ^{Reshape Features} tool is used to extend a polygon feature or cut away a part of it (along the boundary).

Extending:

1. Select the polygon using the ^{Select Features by area or single click} tool.

2. Left-click inside the polygon to start drawing.

3. Draw a shape outside the polygon. The last vertex should be back inside the polygon.

4. Right-click to finish the shape:

   

   これは次のような結果をもたらします:

   

Cut away a part:

1. Select the polygon using the ^{Select Features by area or single click} tool.

2. ポリゴンの外側をクリックします。

3. Draw a shape inside the polygon. The last vertex must be back outside the polygon.

4. Right-click outside the polygon:

   

   以上の結果:

   

5.2.7. Try Yourself ツール: 地物の分割

The ^{Split Features} tool is similar to the ^{Reshape Features} tool, except that it does not delete either of the two parts. Instead, it keeps them both.

We will use the tool to split a corner from a polygon.

1. First, select the landuse layer and re-enable snapping for it.

2. Select the ^{Split Features} tool and click on a vertex to begin drawing a line.

3. Draw the bounding line.

4. Click a vertex on the "opposite" side of the polygon you wish to split and right-click to complete the line:

   

5. At this point, it may seem as if nothing has happened. But remember that the landuse layer is rendered without border lines, so the new division line will not be shown.

6. Use the ^{Select Features by area or single click} tool to select the part you just split out; the new feature will now be highlighted:

   

5.2.8. Try Yourself ツール: 地物のマージ

Now we will re-join the feature you just split out to the remaining part of the polygon:

1. Experiment with the ^{Merge Selected Features} and ^{Merge Attributes of Selected Features} tools.

2. 相違点に注目してください。

結果をチェックする

5.2.9. In Conclusion

トポロジ編集はトポロジの観点からの正しさを維持しながら迅速かつ容易にオブジェクトの作成や変更ができる強力なツールです。

5.2.10. What's Next?

Now you know how to digitize the shape of the objects easily, but adding attributes is still a bit of a headache! Next we will show you how to use forms, making attribute editing simpler and more effective.