Ino Tadataka

A surveyor in the mid-Edo period. Commonly known as Kageyu, his pen name was Toga. Born in Ozeki Village, Yamabe County, Kazusa Province (Kujukuri Town, Chiba Prefecture). After his mother's death, he moved to his father's family home, the Jinbo family in Kozutsumi, Musa County, and at the age of 18, he was adopted into the Ino family in Sawara, Shimousa Province. Although the family was a distinguished one, he revived the sake brewing industry, which was on the verge of collapse due to debt, built up a fortune through rice brokerage, and devoted himself to his hometown as a village headman and village guardian. From a young age, he loved learning, and was familiar with mathematics, geography, and astronomy. After retiring at the age of 50, he went to Edo and studied astronomy under Takahashi Yoshitoki, who was 19 years younger than him. At the time, the number of miles per degree of latitude needed to create an accurate calendar had not been determined, which was a problem in astronomy. To solve this problem, Tadataka attempted to survey a long distance from north to south, and because of the need to defend the northern border, he applied for and received permission to survey the southeastern coast of Ezo, where it was easier to obtain permission from the shogunate. In 1800 (Kansei 12), he achieved the results he had hoped for, but the project then expanded to survey the entire country, and by the time it was completed in 1816 (Bunka 13), it had become a huge project involving 10 surveys, a total of 3,736 travel days, 43,708 km of land survey distance, and 150,000 compass measurements. The traditional method of combining the road line method and the intersection method is by no means new, but it can be said to be a victory of perseverance and hard work, with meticulous attention and strictness in setting a sufficient number of measurement points. The length of one degree of the meridian that Tadataka obtained was 28 ri 2 bu (110.75 km), which is only about one thousandth of an error from modern measurements.

Tadataka made maps for each survey, but died at his home in Hatchobori, Edo, without seeing the final results. Edited by his disciples, the "Dai Nihon Enkai Yochi Zenzu" (Complete Map of the Japanese Coastline) was completed in 1821 (Bunsei 4). The large map was made up of 214 sheets with a scale of 1 cho (about 109.1 meters) per minute (about 3 millimeters, 1:36,000), the medium maps were made up of 8 sheets with a scale of 1/6 of that, or 1:216,000, and the small maps were made up of 3 sheets with a scale of 1/2, or 1:432,000. These maps, along with the "Kigan Jissokuroku" (Record of Coastal Surveys), were presented to the shogunate. The projection method was the same as the Sanson projection, and the earth was treated as a sphere, so the Tohoku region and Hokkaido are slightly shifted to the east. However, this was Japan's first scientifically measured map, and it is an achievement worth being proud of, even when compared to Western maps of the time. Although it was an official map, its origins lie in Tadataka's academic spirit of inquiry, and it was a project supported by the enthusiasm of those below that led to this brilliant success.

This map was not widely used during the Edo period, but after the Meiji Restoration it was used as the base map for military, educational, and administrative maps issued by the new government, and its influence continued until the end of the Meiji period. Thanks to this survey map, which Siebold obtained through Takahashi Kageyasu, the correct shape of Japan was now depicted on foreign maps.

[Hiroshi Ishiyama]

"Otani Ryokichi, editor: Ino Tadataka (1917, Iwanami Shoten)" ▽ "Hoyanagi Mutsumi, editor: Ino Tadataka's Scientific Achievements: The Path to Modernizing Japanese Cartography (1974/Reprinted in 1997, Kokin Shoin)"

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

江戸中期の測量家。通称勘解由(かげゆ)。号は東河。上総(かずさ)国山辺郡小関村（千葉県九十九里町）に生まれる。母の死後、父の実家であった武射(むさ)郡小堤の神保(じんぼ)家に移り、18歳で下総(しもうさ)国佐原の伊能家へ婿養子に入る。名家ではあったが負債で没落しかけていた酒造業を再興し、米の仲買いなどで産を築き、名主や村方後見(むらかたこうけん)として郷土のために尽くした。若いときから学問を好み、数学、地理、天文書に親しみ、50歳で隠居すると江戸に出て、19歳年下の高橋至時(よしとき)について天文学を学んだ。当時、正確な暦をつくるうえに必要な緯度1度の里程数が定まっておらず、天文学上の課題になっていた。この解決のために忠敬は長い南北距離の測量を企て、北辺防備の必要から幕府の許可を得やすい蝦夷地(えぞち)南東沿岸の測量を出願して官許を得た。1800年（寛政12）期待したとおりの成果を収めたが、その後全国の測量へと発展し、1816年（文化13）に終了するまでに、10次にわたり、延べ旅行日数3736日、陸上測量距離4万3708キロメートル、方位測定回数15万回という大事業となった。道線法と交会法を併用する伝統的な方法はけっして新しくはないが、細心な注意と測定点を十分に多く設ける厳密性を図った、根気と努力の勝利といえよう。忠敬の得た子午線1度の長さは28里2分（110.75キロメートル）で、現代の測定値と約1000分の1の誤差しかない。

　忠敬は一期測量するごとに地図をつくったが、最終の成果をみずに江戸・八丁堀の自宅で死去した。弟子たちの編集によって『大日本沿海輿地全図(よちぜんず)』が完成されたのは1821年（文政4）である。大図は1町（約109.1メートル）を1分（約3ミリメートル、3万6000分の1）で214枚、中図はその6分の1、21万6000分の1の縮尺で8枚、小図はさらに2分の1、43万2000分の1の縮尺で3枚からできている。これに『沿岸実測録』を添えて幕府に献上された。図法はサンソン図法と一致し、地球を球として扱ったため、東北地方や北海道はやや東へずれている。しかし、これは日本初の科学的実測図であり、当時の西欧の地図と比べても誇るに足る業績といえる。官製の地図とはいっても、忠敬の学問的探究心がその出発で、下からの熱望に支えられた事業だからこそ、この輝かしい成果を得たのである。

　この地図は江戸時代には一般に活用されることはなかったが、明治維新後、新政府によって発行された軍事、教育、行政用の地図に基本図として使われ、影響は明治末期にまで及んだ。シーボルトが高橋景保(かげやす)を介して得たこの測量図によって、外国製の地図にも日本の正しい形が描かれるようになった。

［石山　洋］

『大谷亮吉編著『伊能忠敬』（1917・岩波書店）』▽『保柳睦美編著『伊能忠敬の科学的業績――日本地図作製の近代化への道』（1974／復刻新装版・1997・古今書院）』

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