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How do I create a fixed distance buffer in QGIS so only one side of the buffer (inside) is processed? I have a big shapefile to buffer and can't be expected to edit the nodes that lie outside each polygon. If I type a negative value in the buffer window, the entire inside of the polygon fills, even with a distance of 1.

The goal is to create a border to a polygon which is styled to the inside of that polygon. If there is another way about doing this do let me know.

Projection is British National Grid (EPSG: 27700)

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    What projection? What software? Does an external buffer with 1 work normally? – HeikkiVesanto May 3 '17 at 10:05
  • Brritish National Grid (EPSG: 27700) Although projection won't matter in this scenario if I'm not mistaken - accuracy is not a priority. Sorry I should have mentioned I'm using QGIS. – James B May 3 '17 at 10:33
  • Please Edit the question in response to requests for clarification. It's not fair to those who would answer to need to mine the comments for critical information. Since you are describing the correct function of a negative buffer, you may want to perform a difference operation of this result with the source polygon, or to convert polygon to polyline and buffer the right side only. – Vince May 3 '17 at 10:59
  • Thanks Vince, will perform a difference operation to follow. – James B May 3 '17 at 11:15
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What you are experiencing is the expected behaviour of a negative buffer. The negative value is the distance from the boundary, so a value of 1 will be one unit of the layer's CRS from the edge. So, if your CRS units are meters, the buffer polygons would be almost identical to the originals, just 1m smaller. If your units are degrees, you will get some 'interesting' results.

So, you could use your described approach and set the negative value but style it to hide the interior of the original polygon. Or you could create a proper doughnut by following the described procedure and then differencing the original polygons and the interior buffer.

  • Many thanks Mappa, I was under the impression the centre would be white space and the polygon would follow the perimeter. I had to zoom in to see it work. – James B May 3 '17 at 11:13

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