Negative values are classified incorrectly using new symbology

[qgib]

Author Name: Wes Meltzer (Wes Meltzer)
Original Redmine Issue: 6096
Affected QGIS version: master
Redmine category:symbology

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In the new symbology mode, at least in 1.8.0, several classification modes render negative real values as though they were positive values. This occurs with Quantile, Natural Breaks and Standard Deviation classifications. It also fails completely to classify percentages stored as reals (-1 to 1) in Equal Interval and Pretty Breaks modes.

I have import data (from either CSV or XLS) and join it to a vector layer. The data is attached, although with identifying information stripped out because it's embargoed.

The values range from -0.0791 to 0.0165 and are stored as reals. The classifications are a mess; for instance, here's the classification in Quantile mode, in order from lightest color to darkest for 7 categories:

0.0000 - -0.0342
-0.0342 - -0.0185
-0.0185 - 0.0000
0.0000 - 0.0000
0.0000 - 0.0000
0.0000 - 0.0000
0.0000 - 0.0165

I don't know why it includes three categories of 0-0, but it leads to misleading categorization. The data has a MIN() of -0.0791 so it should be something like

-0.0791 - -0.0345
-0.0342 - -0.0185
-0.0185 - 0.0000
0.0000 - 0.0165

off the top of my head, although of course that's only 4 categories. There are 56 data points, so it's not for lack of trying.

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• sample_zip_code_data.xls (Wes Meltzer)
• Screen_shot_2012-07-25_at_11.47.29_AM.png (Wes Meltzer)
• zipcodes_new.zip (Wes Meltzer)

[qgib]

Author Name: Giovanni Manghi (@gioman)

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Can you check if this is a regression since 1.7.4?

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• status_id was changed from Open to Feedback
• category_id was configured as Symbology

[qgib]

Author Name: Wes Meltzer (Wes Meltzer)

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Giovanni Manghi wrote:

Can you check if this is a regression since 1.7.4?

Short answer: no. I never upgraded from 1.7.1 to 1.7.4 (lazy) but this issue is present in 1.7.1. It's hard to believe I never had a data set consisting of mostly negative reals with 1.7!

Note: I discovered one issue that is a red herring. The weirdness with the excess 0.00 values listed above is actually a problem with the dataset the database reporter generated, which only showed up in quantiles mode. Too many null values. This is not an issue with the Quantiles categorization. The negative numbers issue remains if I change the mode, however.

Here is the Jenks categorization for 1.7.1 and 1.8.0:

0.0000 - -0.0714
-0.0714 - -0.0420
-0.0420 - -0.0282
-0.0282 - -0.0185
-0.0185 - -0.0083
-0.0083 - 0.0030
0.0030 - 0.0165

which is still erroneous without manually changing the order of processing. Top line should be -0.0791 - -0.0714.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• status_id was changed from Feedback to Open
• fixed_version_id was configured as Version 2.0.0

[qgib]

Author Name: Nyall Dawson (@nyalldawson)

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By my testing this is fixed by pull request 453:

#453

[qgib]

Author Name: Nathan Woodrow (@NathanW2)

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Fixed in changeset "7c0746b091498dbe741c34396f7da5bff51030da".

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• status_id was changed from Open to Closed