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Once you have selected to Edit Features for your Polygon or Multi-Polygon Layer and selected the New feature button, either the Create or Modify mode then you will see the new Feature Builder switch under the Feature Editing heading. It will also be enabled if you select an existing Polygon feature using the Select feature button and clicking on the existing polygon on the map.
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button outlined in red below.
Once you have switched Feature Builder on you then click on the map.
The first map click will create a geometry from the underlying OS MasterMap feature. Clicking on neighbouring features will enlarge the geometry to include the selected polygons. Clicking on features that overlap the current geometry will remove them shrinking it or creating a hole.
If your Layer doesn't support Multi-Polygons and you click on non-adjacent features, you may end up with a shape that contains multiple parts and the Save button will be disabled.
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6. The feature will be displayed on the map in the layer's style
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