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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

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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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