Kalin of the Safecast organization has been working hard at building data maps from measurements taken by their mobile data collectors. The current map set includes over a million data points, and even has a few background measurements in various places around the world to serve as a comparison. These baseline measurements were taken with the same hardware that Safecast normally drives around Japan with, so there should be no adjustments needed due to differing equipment.
The zoom above is right over the Fukushima power plant (click the image for a full size image), and tells a very interesting story: Radiation does not fly nor fall how we all thought. It is not a perfect sphere. Obviously, we suspected that wind and rain would play some role in how the radiation distributed, but I think even the scientists were surprised by the patterns that are emerging.
High levels of radition have been detected much further than the 50 Km safety zone. In fact, there are many areas outside the exclusion zone with HIGHER readings than inside the zone.
Terrain, weather and surface material plays a much larger role on radiation distribution than once thought.
Wind affected the initial release of radiation.
Rain water allowed much of that deposited surface radiation to essentially float around.
Terrain affected how that floating radiation would flow overland.
Finally, the actual surface material the radiation lands on affects it's "stickiness" to the material.