How were the Inō maps measured and made?

From commerce to astronomy

Distance alone cannot position a winding coastline: bearings are also needed, and astronomical latitude supplied an additional check on accumulated measurements. Tadataka passed on his household in 1794, studied in Edo from the following year and began surveying Tōhoku and southern Hokkaidō in 1800. Mapping required connecting different observations as well as travelling far.

Distance and bearing relate successive points, but errors can accumulate. Astronomical observations supplied an additional positional reference such as latitude. Neither observing stars alone nor counting footsteps alone established all positions; the methods had complementary roles.

Measure, record and connect

Instruments, diaries and mapped results preserve different stages of teamwork. Measurements needed identification and comparison before becoming lines and shapes. Looking only at the final map hides that labour; examining the surviving materials together connects field procedures with compilation.

Tadataka did not participate personally in the Izu Islands expedition of 1815–1816. Teamwork and records continued beyond his presence. Distinguishing his route from the project’s coverage recognises collaboration without diminishing his contribution. A map’s familiar personal name does not identify one person as every worker.

Completion and survival after his death

Tadataka died in 1818 before completion in 1821. Compilation continued after fieldwork. Large, medium and small maps differ in scale and detail. Producing another scale is an editorial task, not simply displaying the same sheet at a smaller size.

The National Diet Library holds 43 sheets of an early Meiji copy of the 214-sheet large map. Survey coverage, the complete compilation and one institution’s surviving holdings are different matters. Dates of original compilation and later copying must be distinguished, extending the history into preservation and access.

Large, medium and small compilations had 214, eight and three sheets. Scale changes the relationship between detail and overview; survey subject, compilation and copying dates are also distinct.

Eastern coastal maps were submitted in 1804 before later regional surveys. Intermediate products and final compilation show staged expansion and continuity of records, not one journey inferred backward from the completed map.

Before and after

1. 1795

   Tadataka studies under Yoshitoki in Edo.

2. 1800

   The first survey begins.

3. 1818

   Tadataka dies.

4. 1821

   The completed maps are presented to the shogunate.

Key terms

Bearing

The direction of a route or target expressed as an angle.

Latitude

A measure of north–south position that can be checked astronomically.

Scale

The ratio between real distance and mapped distance.

What supports the account?

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