How can I achieve something similar to the below image? (Apologies for the crudely drawn diagram) Either in QGIS or PostGIS.

I have a series of station grounds and I want to retain the inner boundaries, but I want the coastline to buffer out 1 mile (distance for example, could be any distance). Also I want to split an estuary equidistant between polygons.


  • some sort of buffer of coast touching stations and an intersection with the coastal buffer should do it.
    – Ian Turton
    Oct 31, 2018 at 15:30

2 Answers 2


For the coastline buffer I would select the coastal states and buffer those as normal, then use Difference to delete every part of the buffer which overlaps a state.

Unsure about splitting the estuary, I'll think about it.

  • 1
    This sort of worked. The problem is, that it leaves you with overlapping polygons for the buffers. You can remove the overlap but then it still gives the larger overlap to one 'state' than another. Somehow I need to figure out how to find the difference between the two and split it evenly.
    – Dave C
    Nov 1, 2018 at 13:54
  • @D.Chapman . I don't know how to evening split them automatically but maybe manually splitting the feature and assigning state names to each buffer might be the quickest option? Nov 1, 2018 at 14:09

I think if instead of doing a 1 mile buffer you do a number of much smaller buffers (e.g. 100 of 1/100 of a mile) and delete any overlapping bits before doing the next buffer, you'll effectively "grow" your polygons out slowly. This will fill in the estuary (at 1/100 mile resolution) and create a 1 mile offshore region.

I'll see if I can implement this, but it will have to be in R at first because that's easier for me to program in than Python/QGIS.

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