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.