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I have a road centreline dataset which I have buffered using the road width. I used the standard QGIS buffer tool. I realise for my analysis I actually need to split the road buffer in half and essentially create a 'right' and 'left' side of the road.

My idea is to use the QGIS processing toolbox buffer and select the 'don't make caps' option to create a square buffer. Then I should be able to use the original centreline to split the buffer.

My questions are, if anyone has done something like this:

A. Is there a plugin that can already do this? Or if not, B. What would be the best way to run this process over 30,000 polylines? And, C. How could I label each side of the buffer? (keeping in mind the labeling could be as arbitrary and 'side A' and 'side B'.)

I'm also open to using ArcGIS or any other tool if it means I could get this done faster.

2

I know two and a half ways to create single sided buffers directly with open source tools.

  1. OpenJUMP buffer tool can do left side/right side buffers (advanced options).
  2. Spatialite 4.1 has SQL function SingleSidedBuffer http://www.gaia-gis.it/gaia-sins/spatialite-sql-latest.html
  3. PostGIS perhaps does not have single sided buffer yet but is has OffsetCurve that can be utilised.
  • Thank you, OpenJUMP worked. Unfortunately saving the shapefiles in OpenJUMP and dropping them back into QGIS caused QGIS to crash. In the end I saved the result to PostGIS and QGIS read it perfectly. – jase81 Jun 9 '14 at 5:27
  • In that case there must be a bug with shapefile handling either in OpenJUMP or in QGIS. Could you share a small sample which makes QGIS to crash? – user30184 Jun 9 '14 at 5:31
  • The output from OpenJUMP whether in shapefile or PostGIS has no SRID. I tried creating buffers around my line data in EPSG 4326, 3587 and 2193. QGIS was unable to "Save as" a new shapefile, until I brought the data into QGIS from PostGIS. My guess is it was an issue in the shapefile saving module from OpenJUMP. Either that or user error on my part, not knowing what needed to be done in OpenJUMP (with regard to SRID) prior to exporting a shapefile. – jase81 Jun 9 '14 at 5:44
  • Ok, OpenJUMP is really mostly unaware of projections. With shapefiles you can only copy your original .prj file and save it with the same basename as the shape saved by OpenJUMP. Before saving into PostGIS you must set the SRID with menu item Layer - Set SRID. It may be enough but I am not sure, the PostGIS table (new) driver is pretty new in OpenJUMP. – user30184 Jun 9 '14 at 5:52
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Not open Source, but the ArcGIS Buffer tool has an option for left or right side.

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