area calculation in Field Calculator is depending on Output field type

[qgib]

Author Name: Richard Duivenvoorde (@rduivenvoorde)
Original Redmine Issue: 14082
Affected QGIS version: master
Redmine category:vectors

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Not sure how much this is related to #21270

But when I use the Field calculator to create a virtual field with the area of some polygons, my first try resulted in 80% NULL values.
On further testing the results seem ok. Only difference I did was changing the Output field type from Whole number (integer) to Decimal number (real)

To test, (see screendump for output with both options)

• open attached shp file with the 12 provinces of The Netherlands
• set project crs to epsg:28992 + OTF, and data crs is also epsg:28992 (Amersfoort)
• now open the Field calculator and create a new field / create virtual field and call it 'area'
• as Expression use $area
• click OK: as you can see only 3 polygons have an area value, rest has NULL
• now open the Field calculator again and create a new field / create virtual field and call it 'area2'
• BUT change the default output fieldtype to Decimal !!
• as Expression use $area
• click OK: now all provinces have an area value!

?? what goes wrong here. Or at least how should a normal user find out this behaviour?

Maybe after fixing, also change the default value of output to Real/Float?

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• area_calculations.png (Richard Duivenvoorde)
• qgisprov.zip (Richard Duivenvoorde)

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Related issue(s): #20739 (relates), #21270 (relates)
Redmine related issue(s): 12622, 13209

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[qgib]

Author Name: Richard Duivenvoorde (@rduivenvoorde)

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Update: I found out that it is apparently a integer overflow problem. That is, only the 3 smallest provinces have a value...

And if I change the expression to $area/(1000*1000) (so from meters to square km), all give a valid result EVEN when I set the output type to integers. Off course because then the values are smaller.

But I also checked the error messages, but did not see anything.

Proposal:

• give a clear/descent error message when a integer overflow takes place during calculation (popup?)
• make floats the default when you create an expression (or text), so this problem does not occur for innocent users like me :-)

[qgib]

Author Name: Giovanni Manghi (@gioman)

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Hi Richard,
related also to #20739 ?

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• status_id was changed from Open to Feedback

[qgib]

Author Name: Giovanni Manghi (@gioman)

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It is still true on the latest master and from what I see is not only related to virtual fields.

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• status_id was changed from Feedback to Open
• category_id was changed from Virtual Fields to Vectors

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• easy_fix was configured as 0
• regression was configured as 0

[qgib]

Author Name: Giovanni Manghi (@gioman)

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End of life notice: QGIS 2.18 LTR

Source:
http://blog.qgis.org/2019/03/09/end-of-life-notice-qgis-2-18-ltr/

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• resolution was changed from to end of life
• status_id was changed from Open to Closed