I'm using QGIS to construct some fixed width buffers from a coastline shapefile. Is there a way to easily handle overlaps/ median lines? (I'm not sure if there's a particular term or phrase I should be searching for here.)

More detail:

I have a shapefile for several islands. I need to create a 12nm territorial sea coastal zone. This is straightforward with the buffer function.

However, if the distance between two islands is less than 24nm (12 + 12), the boundary should follow the median line.

Is there a way to calculate this in QGIS so that, instead of a merged polygon, I can generate two polygons (one around each island) with a boundary along the mid point.

For bonus points, I need to generate a second smaller coastal zone. This lies 4nm from the coast, or 1/3 of the distance to the territorial sea boundary described above. Is there a way to construct a buffer with a proportional distance?

I hope my explanations make sense. Please let me know if anything is unclear and I'll add more detail.

I'm using QGIS to construct some fixed width buffers from a coastline shapefile. Is there a way to easily handle overlaps/ median lines? (I'm not sure if there's a particular term or phrase I should be searching for here.)

I have a shapefile for several islands. I need to create a 12nm territorial sea coastal zone. This is straightforward with the buffer function.

However, if the distance between two islands is less than 24nm (12 + 12), the boundary should follow the median line.

Is there a way to calculate this in QGIS so that, instead of a merged polygon, I can generate two polygons (one around each island) with a boundary along the mid point.

For bonus points, I need to generate a second smaller coastal zone. This lies 4nm from the coast, or 1/3 of the distance to the territorial sea boundary described above. Is there a way to construct a buffer with a proportional distance?