The Maritime Safety Agency plans to fully revise Japan's nautical charts by the summer of 2002 because they are oriented 400-500 meters too far southeast agency officials said Wednesday.

The current charts are based on a single point in Tokyo's Minato Ward with a geodetic position that was determined in 1918.

However, recent surveys conducted using the global positioning system, which relies on U.S. satellites, have found that datum to be inaccurate.

According to the current datum, the point is at 139 degrees, 44 minutes and 40 seconds east longitude and 35 degrees, 39 minutes and 17 seconds north latitude.

The GPS-based datum, however, puts the point about 12 seconds, both of east longitude and north latitude, away.

As a result, the new positions of major Japanese ports will appear to shift northwest by 404 meters for Otaru in Hokkaido, 465 meters for Tokyo, 450 meters for Nagoya, 443 meters for Kobe, 423 meters for Moji in Fukuoka Prefecture, and 488 meters for Naha, Okinawa Prefecture.

Most ships use both the current charts and the GPS system when they navigate. Nonetheless, there are fears that if the erroneous charts are not revised soon, they may cause ships to collide or run aground in narrow waterways or small bays.

There are 871 charts covering the waters surrounding Japan. The first ones to be revised will cover areas with heavy traffic, such as Tokyo Bay, Ise Bay and Osaka Bay.

Authorities in the United States amended their nautical charts in the 1980s using GPS surveys.

The Geographical Survey Institute, an office of the Construction Ministry, will revise its maps of Japanese land areas over the next several years using GPS data.