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I have two polygon files (A being my area of interest and B being a categorised land cover layer). Both layers were in the same projection (projected and not geographic). Using QGIS 2.0, I used the Intersect tool (Vector > Geoprocessing tools > Intersect) to extract and pair the information from my land cover layer (layer B) to my area of interest (layer A).

I then opened the attribute table for my newly created layer and enabled editing. A new column was made (using a decimal field type) and I then used the Field Calculator to calculate the area for each of my table rows. The "preview output" at the bottom of the Field calculator showed a realistic number. However, when I clicked ok the resulting output was generally nonsensical with illogical values produced. I then toggled editing and saved the values.

Then using the Group Stats plugin, I used my intersected layer but used the plugin to calculate the area of my intersected attributes and it produced the correct answer.

Apologies for the long-winded description but can someone tell me why the area calculation using the Field Calculator failed to produce the correct answers? As I said, both layers were projected and matched the projection projection so I don't see this being a projection issue.

  • Can you please post examples of both the Field calc output and the Group Stats plugin? the answer may lie in the formatting of the values. – bjmarra Dec 5 '13 at 23:13
  • Thanks - apologies for the delay. As an example. In Field calc, I get an area of -2641416.088 using "$area" but under Group Stats, I get 208876 for the same category. Not all of the results under the Field Calc output are negative but all are incorrect. Does this help? – gyrovague Dec 9 '13 at 15:09
  • Again, under the Preview of Field Calc, it suggests that I'm going to get the right answer until I click ok and then I get the nonsensical values – gyrovague Dec 9 '13 at 15:13
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Not sure if this will help, but there may be a connection...

http://osgeo-org.1560.x6.nabble.com/Field-calculator-issue-td5091278.html

  • That is interesting - so if I switch off the OTFP then the area calculation works and I get the correct output. Do you know if this will be the final solution? – gyrovague Dec 9 '13 at 15:16

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