I cut a part out of one feature and added it to another feature in a polygon shapefile. My problem now is that both features has the same values in the area-, perim-, p2a-, width- and length-columns? The features are very much different in size, how do I get QGIS to sort of "re-calculate" the areas etc?
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1related question: http://gis.stackexchange.com/questions/23355/how-to-calculate-polygon-areas-in-qgis. Some of those columns are not direct calculations, therefore you must specify how to calculate them in the field calculator.– Alexandre NetoCommented Jun 18, 2013 at 10:20
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Thanks, @alexandre. But I'm still a bit confused, how do I calculate P2A, width and length in the field calculator?– SoSCommented Jun 18, 2013 at 11:11
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That makes two of us :-). In order to be able to help, more information about what each columns represents and how you gonna use it further is needed. Maybe you can get that information from your original data provider.– Alexandre NetoCommented Jun 18, 2013 at 11:22
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1perimeter should be the length of a polygon feature (eg. the border), area is self-explaining and i assume that p2a represents a perimeter to area ratio (which you can also calculate with the Fieldcalculator). The rest makes no sense for me with polygons– CurlewCommented Jun 18, 2013 at 11:29
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2 Answers
Use the field calculator (Layer>Open attribute table>Field Calculator/Ctrl+I or right click shapefile>Open attribute table>Field Calculator/Ctrl+I). There is an operator "area" that will calculate the area of each row in the table. All units will be calculated in the units of the projection, so you probably want to project it to a projection that uses feet or metres before doing that, rather than lat/lon.
answered Jun 18, 2013 at 20:09
The fields in the attribute table are only static values, even if you created them by using the geometry section of the field calculator. There is no way to re-calculate them automatically if you altered the polygon size, or changed the CRS of the polygon layer.
The only way I see is that you have to delete those "Geometry" fields, and re-create them by using the functions in the geometry section of the field calculator.
