great program. Plus goggle 3d warehouse allows you to search for models that other people have used and you can just drop them into your plan. So if you want an olive tree you just search for one and there you go a 3d model is there for you to use. So you can quite quickly build up a 3D model of a piece of Land, and it's all to scale.
I fiddled about about with this way of making contours for a while. But it turns out there is a free rudy script that does it for you
Ruby library depot where you can set your contour lines at whatever distance you want.
Be warned that the google earth data can leave a lot to be desired when compared with the actual land. The land I am using it for has traditional Spanish terracing on it which can leave the contour lines as a tangled mess in parts. But none the less useful in giving a idea of whats going on.