Restorement of early 19^th century’s Geomagnetic declinations in Japan, from Tadataka Inoh’s survey azimuth ledger

Abstract. The Santou-Houi-Ki is a national treasure of Japan recorded by Japanese cartographic surveyor Tadataka Inoh in 1800 to 1816, consist of 67 volumes survey ledger to produce the first survey map of Japan, called “Coastal Area Map of great Japan,” or Inoh map (1 : 36000, 1 : 216000, 1 : 432000).

In the Santou-Houi-Ki estimate 200,000 magnetic compass survey azimuth data by accuracy of 0°05′ unit were written, with the name or short description of magnetic compass survey reference point and target points. Inoh’s team carried out the survey did not apply the correction of magnetic declination. Because before his survey, Inoh tried to observe magnetic declination in Edo (Tokyo) was nearly zero. Inoh conducted the survey on the assumption that the influence of magnetic declination to survey map of Japan is at least. The surveyed region extends from North eastern coast of Hokkaido Island to Yakushima Island in western Japan. We start analysis, check the outline of reference point and target points from the name or the description of reference point and target points, the survey azimuth recorded in Santou-Houi-Ki, and Inoh’s survey diary, Inoh’s maps, modern survey map, today’s survey map or local source books. We execute interdisciplinary and simultaneous analysis of geomagnetic declination, real azimuth, precise position of the survey execution point and target points (latitude and longitude less than 0.1 second). And add the historical importance of each precise position of survey reference point. Calculate backward the precise position of survey reference point, where the value of magnetic declination, subtracting the magnetic survey azimuth from the true azimuth to any target points is similar or approximate. We cannot read the precise content of national treasure Santou-Houi-Ki without this interdisciplinary simultaneous analysis. We tried to analysis at 227 points in Japanese archipelago, the distribution of magnetic declination and precise position of the survey reference points and target points in early 19 century’s Japan are gradually elucidated. When we compare the analysis of declination from Santou-Houi-Ki to Noaa’s Historical Declination Viewer, NOAA’s progress of declination west is always 5 to 6 years late than the analysis of Santou-Houi-Ki. And differences are more remarkable in western Japan. We must supply the analysis data from Santou-Houi-Ki to NOAA.