Classes in Natural Braeks (Jenks) Symbology

[qgib]

Author Name: Diego Mo (Diego Mo)
Original Redmine Issue: 6625
Affected QGIS version: 1.8.0
Redmine category:symbology

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Symbology of a vector layer (shapefile in the test case)--> Graduated Renderer --> Natural Breaks (Jenks) mode.
It seems that Quantum GIS (version 1.8 on WIN XP, using new simbology) propose different intervals (using the same column and number of classes) each time you classify features.

[qgib]

Author Name: Regis Haubourg (@haubourg)

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Yes, confirmed here, and it looks like a n / n-1 array bounds error.
Needs to be confirmed again in 1.9 when classification regression will be solved (#16237)

[qgib]

Author Name: Nyall Dawson (@nyalldawson)

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I can't replicate this in current master -- do you have a test dataset which shows the issue?

[qgib]

Author Name: Diego Mo (Diego Mo)

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As test case you can use toponomastica_10000_geo.shp (field ROTAZIONE) that you can download from
http://www.comune.torino.it/geoportale/dati/carta_sintesi_shp.zip
The pubblication page is http://www.comune.torino.it/geoportale/ser_professionali_2.htm
The problem seems to happen both in Qgis 1.8 and Master(release 2a4d79d)

[qgib]

Author Name: Nyall Dawson (@nyalldawson)

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Actually after looking into it this is the intended behaviour -- in order to speed up the calculation the natural breaks classifier takes a random sample from a layer which has a large number of rows (> 1000 rows). When a layer has more than 1000 rows, a random sample of either 1000 values or 10% of the dataset (whichever is larger) is used to calculate the breaks.

[qgib]

Author Name: Nyall Dawson (@nyalldawson)

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• status_id was changed from Open to Closed