I'm using QGIS 2.4.0 and I've noticed some problems with the derived area and the $area field. Firstly, I have a polygon which is a filled ring, inside another polygon. The derived area, and therefore my $area field, is saying 600sqm, but the actual measurement made with the area measure tool, is 300sqm. Why the discrepancy?
Also, why am I having to re-write the area field every time I make any changes to the vector layer. It's a real pain as I'm having to present areas in Hecatre so the formula involve both conversion and rounding. To have to re-write it each time I make any adjustment is really slowing me down. Any plugins that might have an auto-updating area field?