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Is there a method where I can detect if a polygon is inside the concave area of another (in this case a bay), then output the bay itself?

It's hard to explain with words so I'll demonstrate below:

Polygon within bay

This polygon falls within a bay area and I want QGIS to make this bay area a new shapefile like below:

enter image description here So I want the area of coastline encompassed by the thick black outline above

Is it possible that this can be done? The output would be part of a model where I am trying to measure the shortest line distance between two polygons, taking into account the shape of the coastline using the v.net.visibility GRASS tool, but simply running this model taking the whole of the British Isles coastline into account takes up too much processing power.

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This could work but it depends on the shape of your polygons:

  1. Convex hull the grey polygons
  2. Difference output with grey polygons, to create bay polygons (orange in the screenshot)
  3. Singlepart these to make each bay an invidivual polygon
  4. Extract bays that contain purple polygons

enter image description here

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