Adding Features - Advanced Polygon Editing

📝 Advanced Polygon Editing

This feature allows you to select polygons from any visible overlays and create new foreground geometries based on spatial relationships between them. This works with file-based OGR overlays, spatial database overlays, or combinations of both.

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🔹 Available Advanced Polygon Operations

You can access these from the Add menu:

• Polygon Union

• Polygon Intersection

• Polygon Difference

• Union from Area Overlay

Selecting two polygons from visible overlays and applying one of these operations will create a new polygon in the foreground overlay based on the chosen spatial operation.

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📌 Operation Details

1. Spatial Union

Purpose: Combines the total area of two polygons into a single new polygon, without altering the originals.

📋 Steps:

• Select Foreground Overlay
  Choose the foreground overlay where the new merged polygon will be stored.

• Select Two Polygons

  • Click on the first polygon.

  • Click on the second polygon to confirm selection.

• Result:

  • A new polygon is created (red outline) representing the combined area of both polygons.

  • The original polygons in their respective overlays remain unchanged.

2. Spatial Intersection

Purpose: Creates a new polygon containing only the overlapping (common) area between two polygons, without changing the originals.

📋 Steps:

• Select Foreground Overlay
  Choose the foreground overlay where the new intersected polygon will be stored.

• Select Two Polygons

  • Click on the first polygon.

  • Click on the second polygon to confirm selection.

• Result:

  • A new polygon is created (red outline) showing only the shared area between the two selected polygons.

  • The original polygons in their respective overlays remain unchanged.

3. Spatial Difference ("Cookie Cutting")

Purpose: Creates a new polygon containing only the portion of the first polygon that does not overlap with the second polygon.

📋 Steps:

• Select Foreground Overlay
  Choose the foreground overlay where the new “cookie-cut” polygon will be stored.

• Select Two Polygons (Order Matters!)

  • First click on the polygon you want to keep part of (source polygon — usually shown in orange).

  • Then click on the polygon you want to cut away from the first (cutting polygon — usually shown with a blue outline).

• Result:

  • A new polygon is created (red outline) showing only the area from the first polygon that does not intersect the second polygon.

  • The original polygons remain unchanged.

⚠ Important: If you select polygons in the wrong order, you’ll remove the wrong area. Always select the “keep” polygon first, then the “cut” polygon.

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🛠️ Edit Polygon Options (Right-Click Menu)

When editing polygons, right-click to access:

• Merge Polygon

• Intersect Polygon

• Cut with Polygon

Process:

1. Left-click inside the polygon to select (must overlap or touch current polygon).

2. On the confirmation dialog, click the green tick to confirm.

3. Repeat to add more polygons for the current spatial operation.

4. To change the operation, repeat the selection with the new spatial operation.

Example:

• Merge: The base polygon is extended by the area of the added polygon(s).

• Cut: The base polygon is reduced by subtracting the overlapping area.

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🔄 Union from Area Overlay

This option lets you create or edit a polygon, then merge polygons from a selected background overlay before saving.

Adding a New Polygon and Performing Union:

1. Select your foreground overlay.

2. Click Add Polygon button.

3. Draw or snap a polygon, then click Finish (don’t save yet).

4. Select Union from Area overlay from the Add menu.

5. Choose a background overlay in the popup and click OK.

6. The union of your polygon with polygons from the background layer will be constructed and displayed.

7. Click Save Graphic Record.

Creating a Union from an Existing Polygon and Background Layer:

1. Select foreground overlay with the existing polygon.

2. Click Add Polygon, snap to the existing polygon, then Finish (don’t save).

3. Select Union from Area overlay from the Add menu.

4. Choose the background overlay and click OK.

5. Click inside polygons from the background overlay to select them (confirm each with the green tick).

6. The merged polygon will be displayed.

7. Click Save Graphic Record.

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⚠️ Important Notes

• After saving, you can still:

  • Use Merge Polygon to add missing polygons.

  • Use Cut with Polygon to remove unwanted parts.

• If the source polygon fails the GeometryIsValid test, a warning will appear.

• During union, any invalid intersecting polygons in the background overlay are ignored.