The Geospatial Information Authority of Japan (国土地理院, Kokudo Chiri-in), or GSI, is the national institution responsible for surveying and mapping the national land of Japan. The former name of the organization from 1949 until March 2010 was Geographical Survey Institute. It is an extraordinary organ of the Ministry of Land, Infrastructure, Transport and Tourism. Its main offices are situated in Tsukuba City of Ibaraki Prefecture.
It also runs a museum, situated in Tsukuba, the Science Museum of Map and Survey.
The Authority is represented on the national Coordinating Committee for Earthquake Prediction.
The Japanese water height reference point (日本水準原点, Nihon Suijun Genten) is installed in a small building in front of the National Diet Building in Nagatacho Chiyoda, Tokyo. The building is called the Japanese water height reference point storehouse (日本水準原点庫, Nihon Suijun Genten Hyoko). Construction of the building started on August 1890 and it was completed on December 24, 1891. It serves as a reference point for elevations in Japan (vertical datum). The building cannot be entered, but there is a stone base with a description outside.
Elevations of Japan are determined with reference to the mean sea level of Tokyo Bay (elevation 0 m). This is called mean sea level of Tokyo Bay (東京湾平均海面, Tōkyō-wan heikin kaimen) or Tokyo Peil (TP for short, "Tokyo level"), where the word Peil comes from the Dutch language. The stone base monument of the datum has a crystal scale with a 0 which indicates 24.3900 m (80.020 ft) above the mean sea level of Tokyo Bay since October 21, 2011.
Since it is difficult to refer to the altitude in Tokyo for remote islands, 37 islands have their own zero point. The heights of Okinawa z. B. are measured from the middle water level of Nakagusuku Bay ( 中 城 湾 , Nakagususku-wan ) and that of Miyake from the Ako Bay.