Based on data released on 28 March 2011 by MEXT showing cumulative and average hourly radiation dose at 10 monitoring posts around Fukushima 1 NPP.
The circles in the map are centred on the approximate location of the monitoring posts. The sizes of the circles represent the relative magnitude of the cumulative doses recorded at those posts. The colour of each circle represents the average hourly dose, with red indicating an average dose of 20 µSv/hr or more, amber an average dose of between 10 and 20 µSv/hr, and green an average dose of less than 10 µSv/hr.
Note that the source data is based on readings at the various posts over durations ranging from 1 day to 4 days, and hence the data and the representation of it below (especially the cumulative dose) are not directly comparable.