Topic: Buffering Boundaries

I'm sure you'll be delighted to learn that I have recently recreated all the LERC boundaries.  This was because as they have changed over time, I have collected new boundaries from different sources.  This led to small discrepancies between neighbouring boundaries, probably only a few metres across, but imperfections nonetheless.  Basically the jigsaw didn't quite fit together properly.

The recreated boundaries have all been generated from the same source, which is OS Boundary Line.  The whole set fit together perfectly, so when they are used in the Atlases, there is no chance of any records falling in a gap or an overlap.

The issue I have now though, is that as some LERCs will want a marine buffer placing on their boundaries.  If I do this for some, the I need to do it for all the coastal LERCs.  Producing a buffer all around a boundary is straightforward enough, but obviously the buffer only needs to go out to sea.  I think I can achieve this by individually creating buffered boundaries, and then removing the inland buffered bit by clipping against the boundary of adjacent LERCs.  A bit tedious, but doable.

The big problem arises when with adjacent LERCs up and down the coast.  To avoid overlaps, we want the buffer to stretch halfway across the water to the other LERC.  For example, the Lincolnshire buffered boundary should meet the north and east Yorkshire one halfway across in the Humber estuary.  Both buffers will then extend to the full 19Km (is that right, or should it be 22km, i.e. 12 nautical miles?) into the North Sea.

So, does anyone know how I can achieve this in QGIS?  I feel this is a logical problem rather than a technical one, i.e. using a collection of the existing QGIS functions in the right order rather than creating or programming a new fancy one.

Many thanks,

Tom Hunt - ALERC National Coordinator

Re: Buffering Boundaries

Hi Tom,

That's an interesting problem for us to puzzle over during the weekend.

I suspect that the solution will involve something like an intersection with voronoi polygons. These generate boundaries that are equidistant between neighbouring points.

http://djjr-courses.wikidot.com/soc128: … i-polygons

Hope that helps

Mike

Mike Beard
Natural Course Project Officer
Greater Manchester Local Records Centre

Re: Buffering Boundaries

Thanks Mike.

I have had one go at using this method already, and it didn't yield the result I was after.  Having said that, I didn't really investigate it properly.  I think there are possibly one or two extra steps to add in somewhere along the line.

I should add that there is someone out there who knows how to do it as it has been done before, though not by me.

Regards,

Tom Hunt - ALERC National Coordinator