Map Calculator - Save the Rain
A very good Map Mashup that calculates the potential yearly yield in corn, soya beans, rice and wheat.
Search a location:
Zoom and/or Pan to a clear roof top.
Digitise/Draw the outline of the roof.
The Map calculates the sun/mean rainfall of the location and produces a bar of produce that can grown with the rainfall footprint of the roof.
A Built in Video Tutorial (below) shows how the tool works.