Abstract
Old Japanese topographic maps made before 1960's were based on polyhedric projection. It is necessary to know the characteristics of the projection so as to use these old topographic maps for measuring positions on the map and overlaying time series geographic information. The principle of the polyhedric projection is considered to be rather simple and they are explained in many books. However, explanations in those books differ mutually and they cause confusion.
This paper clarifies the method of the polyhedric projection used for the Japanese topographic maps according to old authoritative documents. This makes it possible to accurately measure the geographic position expressed on the old maps.
Based on this actual method of projection, five kinds of explanations about the polyhedric projection are examined. These explanations describe principle of the polyhedric projection as (1) gnomonic projection and (2) orthographic projection on a plane that passes through four points on the Earth defining the corner of the map, (3) polyconic projection, (4) secant conic projection, and (5) sinusoidal projection. It is concluded that these explanation except for the sinusoidal projection have no detectable differences within the small size of one map sheet which is the extent the polyhedric projection is applied. In this sense these different explanations apply enough to the polyhedric projection. But from the viewpoint of the conceptual conformity, it is concluded that the explanations as polyconic and equidistant conic projections are most suitable to describe the actual characteristics of the polyhedric projection.
