5

When styling a polygon layer using the Graduated styling and the Natural Breaks (Jenks) mode, QGIS is classifying the data with different values.

So every time I click on "Classify" the values are different.

Why?

EDIT 1:

Using QGIS 3.4.4 on Windows 10

  • I can't reproduce this on Ubuntu 18.04, 3.4.4-Madeira - can you add operating system and qgis version – Ian Turton Feb 11 '19 at 16:54
  • See Edit 1 for the info – TurboGraphxBeige Feb 11 '19 at 16:57
  • Using the same dataset, ArcMap complained that the number of features was too big and that I could increase the sample size if I want to. So maybe QGIS is taking a different sample of the data everytime I click on "Classify"? – TurboGraphxBeige Feb 11 '19 at 17:02
  • I can reproduce with QGIS 2.18 if there are "many" features. In my test, it occurred when more than 3001 features were to be classified in 5 classes. – JGH Feb 11 '19 at 17:03
  • Testing with 21426 polygon I got the same behavior (different value after each classification), the number of classes didn't matter. QGIS 3.4.1 on window 7 – J.R Feb 11 '19 at 17:11
9

Looking at the code, QGIS will take the exact values if there are up to 3000 features. Otherwise, it will take the largest between 3000 random features or a random 10% of the data.

  • And if you have for example 4000 features, does QGIS uses 10% of 4000 features (so 400) or it uses up to 3000 features? – TurboGraphxBeige Feb 11 '19 at 17:17
  • 1
    it will take the largest between 3000 features or a random 10% of the data – J.R Feb 11 '19 at 17:19
  • Ah! I read it wrong the first time. – TurboGraphxBeige Feb 11 '19 at 17:20
  • @GobTron I have clarified it. It will take 3000 random features – JGH Feb 11 '19 at 17:25

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