I've just succeeded in getting the area of a polygon using field calculator. The polygon was drawn on a Google hybrid aerial photo. There are no units given in the calculation and I assume the area figure is based on latitude / longitude. How do I go about changing the projection in order to get an area figure in hectares?
You have to look up what CRS is used for the polygon layer with
Rightclick -> Set CRS for layer.
If it is in EPSG:3857 (the same as for openlayers plugin background), it has no real unit. Google mercator units (although called "metres") fit only at the equator to real metres.
To get a real metres layer, choose the UTM projection for your part of the world, and
- reproject your layer into it with
Rightclick -> Save as ...
- create a new layer in that CRS and draw the polygon again.
Note that simply changing the CRS with
Set CRS for layer does not reproject your data to the desired CRS.