I don't know how the Natural Breaks (Jenks) algorithm works in QGis graduated symbolism, and I've deliberately not researched it in order to ask this question, as my point is based on what seems to be intuitively wrong, rather than what may be technically correct (but not useful or expected).

So! If you have a range of values and constrain them to a few classes, Jenks does a nice job of splitting up the data:A

But if you add further classes, Jenks just repeats the lowest value, often: B

Since I don't know (yet - I will now research it) how Jenks works, it'd be remiss of me to say the algorithm is coded "wrongly", however the implementation is peculiar and unexpected. Possibly if it auto-constrained itself to the maximum number of independent classes? Or pooled identical-valued colour bands?

  • What version of QGIS? – Nathan W Sep 24 '13 at 22:06
  • 2.1.0 I believe, linux & windows 7. Linux may be slightly older, there's a problem with ubuntu software centre updating at the moment... – dez93_2000 Sep 25 '13 at 4:16
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    There appears to be a SERIOUS error with the Jenks calculation in 2.0.1 (Dufour). Every time you hit classify the ranges appear to recalculate almost at random. The differences are not negligible and effect whole numbers (not just trailing decimal places) .... I'd say avoid using Jenks for the time being! (p.s. this is on a Windows 7, 64 bit machine) – Andrew Tice Sep 25 '13 at 5:47
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    UPDATE - I've just checked with an earlier master build (1.9) and this error doesn't occur. I've used the same file in both 1.9 and 2.01 for the test.... – Andrew Tice Sep 25 '13 at 5:51
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    @AndrewTice Did you report/fix this issue? Would be good to know if this thread can be closed. – underdark Apr 26 '14 at 22:16

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