Is there a possibility to determine the longest side of a polygon with QGIS? As I need to do that for hundreds of polygons I would prefer an automized method (like a plugin).

So far my idea was:

  • transforming a polygon into a line
  • cut it on the vertices
  • measure the length
  • here I became stuck, how to become the longest line without checking the whole bunch of lines one by one?

looking forward to your answer


If you can use PostGIS, try using the ST_LongestLine function like this:

SELECT ST_LongestLine(geom, geom)
FROM mypolytable;

This will find the longest line that can be drawn within each polygon geometry. Note that I'm using this function that references the same geometry column geom twice.

  • Thanks for your answer. The function you mentioned returns the 2-dimensional longest line points of two geometries but Im looking for the longest line of a single polygon. Am i wrong? Anybody else an idea?? – ragavendra Apr 24 '13 at 15:53
  • Two geometries is actually one geometry when you reference the same geometry twice, as I have described. Did you try it? – Mike T Apr 24 '13 at 23:40

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