Ironically, the Japanese Communist Party (JCP) is about the only credible "opposition" party left in Japan in the middle of the kindergarten-level squabble within the DPJ (many of whom are ex-LDP politicians anyway) and the LDP. Its representatives in the Tokyo Metropolitan Assembly, with the help from experts, did the survey of the air radiation levels in various parts of Tokyo. I believe it is by far the most detailed survey done by any entity.
(The following information from the JCP's press release, in Japanese, in May 25, 2011)
They divided Tokyo into 10 kilometers-by-10 kilometers grids and measured the air radiation level at total 128 locations. In the eastern Tokyo where the high radiation levels have been detected, they used 5 km by 5 km grids to measure 55 locations. 8 locations in Shinjuku-ku were measured, and 23 locations around Toyosu (landfill where the Tokyo metropolitan government is moving the wholesale fish market, by the way), as both Shinjuku and Toyosu have been reported to have higher radiation.
Findings:
According to the official number from the Tokyo Metropolitan government measured at 18 meters off the ground at the Tokyo Metropolitan Institute of Public Health [in Hyakunin-cho, Shinjuku], the average radiation level for the month of May was 0.068 to 0.062 microsievert/hour. The ground level (1 meter from the ground) showed the similar number, according to the government. However, when we measured the radiation at 1 meter from the ground, the radiation level similar to the official level was only observed in very narrow, limited areas in Ota-ku, Suginami-ku, and Machida City. The areas with relatively high level of radiation were Ome City, Akiruno City, Nerima-ku, at 0.09 microsievert/hr, the Tokyo Bay area of Edogawa-ku and Koto-ku at 0.1 microsievert/hr, and the highest level of radiation was observed in Adachi-ku and Katsushika-ku, at 0.2 to 0.3 microsievert/hr. (Map 1)
The eastern Tokyo - Adachi-ku, Katsushika-ku, and Edogawa-ku - has areas of high air radiation between 0.18 to 0.39 microsievert/hr. The high level of radiation in Toyosu landfill is thought to be the continuation of the high level in the eastern Tokyo. The ground-level measurement (0 meter) done in one location in this area registered 0.618 microsievert/hr. (Map 2)
There is a possibility that the radiation level in areas east of the line that connects Koto-ku and Nerima-ku is at the level that will result in 1 millisievert/year. (Map 3)
There are huge variations in radiation within a very narrow (3.5 kilometer radius) area within Shinjuku-ku, from 0.066 to 0.116 microsievert/hr. (Map 4)
The radiation on the surface of the grass and around trees was higher than other surroundings.
The survey was done between May 6 and 25, using ALOKA PDR-101 pocket survey meter. The air radiation at 1 meter from the ground was measured, and in some locations the radiation on the ground surface was also measured. The measurement was repeated 10 times at 10-second intervals, and the numbers were averaged.
These are the maps that summarize the JCP's findings:
Map 1:
Map 2:
Map3:
Map 4: