Why does official reported radiation levels tend to be lower?

 Citizen's pressure on government to set additional monitoring post, which is 1.5 m high from the ground, is increasing in Japan.

Some citizens who are seriously concern about the quantity of radiation in their daily life get geiger counter through the Internet so that they can measure radiation level individually.

Amateur inspectors including me measure how much radioactive substances they are exposed to. As more people have that counter, they begin to find that individually measured data are twice as much as that of officially reported data.  Then they tenaciously upload their data to their blogs or web sites.

A critical incident was developed by one politician. He was so suspicious about the government's data that he bought a geiger counter and  checked how much is the radiation level around him. He measured radiation level with his counter as much as he could. For example he measured in the lawn near the Imperial palace and in the intersection of Kasumigaseki, which is the very center of Japan. After he found that his counter detected twice as much as that of the offical data, he aggresively commented about his investigation in the famous weekly magazine, which ignited explosive citizen's complaint about underestimated public data.

Consequently Japanese government acknowledged that they needed to have more offical detectors and they decided to add more monitoring points at average human height.

To tell the truth, government didn't try to underestimate their data.The reason they use such a tall monitoring post is the following reason:
 While the world were polarized during cold war era, many countries such as China, Russia, Soveit Union, and U.S. executed several nuclear tests in the air.

As a result, Japanese government became scared about nuclear contamination and decided to built monitoring posts so that they could measure the radiation level from the outside Japan. To detect the number precisely, they needed to build tall monitoring posts because if they were built low, monitor sensed natural radiations. Natural radiations are prone to stay long on the ground, so the detectors may catch unnecessary radiation and they can't measure correctly.

So far, the average data is double compared with official ones. In my opinion, so far we are safe in relation to the radiation data in the air.

If government had given more correct and precise information in advance , people might not have been so skeptical about nuclear accident trouble.