Making a Magnetic Deviation Map with Azimuth Survey Data of Ino Tadataka

At the beginning of the 19 century Ino Tadataka surveyed almost all of Japan’s coastline and its main roads, and mapped them. He made ca. 70,000 magnetic azimuth measurements to perspective objectives such as mountain tops, isolated islands, and offshore rocks. Some of them are indicated on 1/216,000 maps with perspective lines and additional numeric characters. Differences between magnetic azimuth angles described on Ino’s map and true azimuth angles measured on current maps show the magnetic deviation at sites. A total of 555 sites and values were obtained across Japan, which were divided over 26 regions. Using 26 region-presentative values chosen statistically to mitigate errors, a magnetic deviation map of Japan at the beginning of 19 century was compiled.