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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.

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In case anyone is as confused, nm does not stand for SI nanometers here, but nautical miles. >< – lynxlynxlynx Aug 21 '12 at 16:24
yes, sorry. I should probably follow convention rather than my own personal shorthand :) – Andy Harvey Aug 23 '12 at 4:27

The QGIS buffer tool by default does not merge overlapping buffers, but there is the option to allow you to do so.

More difficult is to define what is the "median line" between two islands: it is define by the shoreline of one of the two islands? It is an interesting problem, but it don't think is trivial to solve it.

QGIS also allows you to create buffers using the distance defined in a attribute of the vector you are buffering. I believe you can use the Sextante modeler (available in the Sextante plugin) to introduce some kind of logic.

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thanks giovanni. the median line is equidistant between both shorelines, bit I agree this is non-trivial to apply in practice. i will have a look at sextante, thanks for the suggestion – Andy Harvey Aug 23 '12 at 4:29

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