The true north and magnetic north are not drawn in the maps of 1:36000 made by INO Tadataka at the beginning of 19th century. In this paper, the author developed method of estimation for the geographic meridian and magnetic meridian for digitized Ino maps. Correlation index between longitudes in the actual digital map of the Geospatial Information Authority of Japan and coordinates in the rotated map image becomes the maximum when the north of rotated map matches to the true north. In this way we were able to fix the north in Ino map of 1:36000.

Ino had believed that the magnetic north was equivalent to true north anywhere through surveying and making maps of Japanese Islands. Therefore, north-south direction in his maps of 1:216000 is magnetic meridian which crosses parallels drawn at right angles. Notwithstanding lack of parallels in the map of 1:36000, we were able to fix the magnetic north comparing with the parallel of the map of 1:216000. This method is the same as true north fixing.

Author carried out this study for clarifying topographical characteristics and verifying scale of natural phenomenon in the places indicated by the “Shizensaigaidenshohi” (monument of natural disaster). The followings are the major results.

1) Clarification of topographic characteristics where a “Shizensaigaidenshouhi” stands or indicates.

Most of the place where “Shizensaigaidenshouhi” (monument of natural disaster, simply referred to as “the monument” in this article) stands or indicates, locates not only in very low areas (e.g. flood plain), but also in a little bit higher areas (e.g. natural levee). This is because the monument building was not reasoned by heavy rainfall or flood itself, but by the damage of lives or properties which exist in higher areas.

2) Verification of scale of natural phenomenon which brought disaster where a “Shizensaigaidenshouhi” stands or indicates.

Past flood depth inferred by the description of the subject monuments was slightly low or equal to that estimated by modern hazard map. In early and middle of the 20th century, many flood disaster happened and the relevant monuments were built thereafter, but in the late 20th century, the number of severe floods which recorded in the monuments decreased. However, frequency of heavy rainfall is increasing recently. This suggests flood disasters likely to be recorded in the monuments would occur again in the near future.

These results show that location of a “Shizensaigaidenshouhi” stands or indicates prefers a little bit higher areas as people lives more in such areas compered to lower land and also show that past flood depth inferred by the description of the subject monuments was compatible with that estimated by modern hazard map.

Map symbol “Shizensaigaidenshohi” is recently introduced to the basic topographic maps, and the new topographic maps therefore provide people with new map reading experience, which might raise people's awareness of disaster prevention as existence of “Shizensaigaidenshohi” is deeply relating to topography.

Since the beginning of the COVID-19 pandemic in 2019, disease maps have attracted a lot of public attention. Nevertheless, thematic maps have been rather neglected in the history of cartography as pointed by Robinson (1982). Koch (2011, 2017) has traced the history of disease maps in Western countries, but little is known about their history in Japan. This study addressed this gap by tracing the development of disease mapping in modern Japan in comparison to its history in Western countries. Documents pertaining to an outbreak of cholera in Meiji era indicated that the first disease map of Japan was published in 1879, after the development of Japanese health statistics.

However, since the topographic map of the country was incomplete at that time, the base map was not accurate. In 1885, a disease map of Japan was published using choropleth map based on a surveymap. Concerning large-scale map within the city, a dot map of cholera cases of central Tokyo was developed in 1896. The Taisho era saw an uprise in the publication of a wide range of disease maps that presented distributions of infectious agents or prevention measures for epidemics. Thus, disease maps in modern Japan were developed a little later than those in Western countries, paralleling the development of health statistics and survey maps.

The “Sokuchi-Genzu” owned by the General Library of the University of Tokyo is the “Shita-zu”, which are namely the manuscripts of the first real surveyed maps by Inoh Tadataka(1745-1818). It is composed of 93 sheets which are 1 large scale(1:36000), 36 medium scale (1:216000) and 56 small scale(1:432000).

On the each Sokuchi-Genzu, the surveyed points are plotted by pin holes and the surveyed lines are drawn by tying them. Furthermore, the place names and the astronomical observed points are recorded. On the medium scale Sokuchi-Genzu, the survey results during several days are plotted and the distances of directions of south-north and east-west between the end points of each survey line were measured on the correspondent large scale map mapsheet and were scaled down one sixth. The drawn areas on some of medium scale Sokuchi-Genzu corespond to the contents of some of Shita-zu owned by Inoh Tadataka Memorial Museum, which are the manuscripts of the Large scale maps.

On the other hand, the map sheet line of directions of south-north and east-west are drawn on the small scale Sokuchi-Genzu. These lines coincide with the map frameworks of the Large scale maps. The length of each framework lines and the distances between the cross points of each survey line and framework lines were scaled down one twelfth by using the reduce measure from the large scale map sheets.

These distances recorded on both scale SokuchiGenzu are expressed by the Japnese measuring system, namely “Shaku-Sun-Bu-Rin”=“30.3cm-3.03cm-3.03mm-0.303mm”. The numerical values under Rin are described by the multiple figures of 1/12. These were read by using the reduce measure on the large scale．

Since Ohtani Ryokichi's voluminous “INOH Tadataka” (1917), it has been commonly accepted that the small scale map were reduced from the medium scale map by reduction in half. However, as the framework of small scale Sokuchi-Genzu corresponds to that of the large scale map, it is said that the small scale maps were prepared from the

large scale mapsby scaling down one twelfth.. The medium scale maps were prepared by reduction from also the large scale maps, so that the medium scale maps and the small scale maps were prepared separately.

The General Library of the University of Tokyo owns the manuscript maps named "Sokuchi-Genzu", which were drawn for the first surveyed map of Japan by Inoh Tadataka (1745-1818). "Sokuchi-Genzu" is based on 93 sheets of the "Shitazu", the manuscript maps. The scales of "Sokuchi-Genzu" are classified into large(1:36000), medium(1:216000) and small(1:432000), which correspond to the scales of Inoh’s Map of Japan. On the "Sokuchi-Genzu" the results of Inoh's survey are plotted by needle points. The "Sokuchi-Genzu" is very important material for the research of Inoh's Map.