Vol.1 No.2 2008

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Research paper : Creation of seamless geological map of Japan at the scale of 1:200,000 and its distribution through the web (K. Wakita et al.)−76 Synthesiology - English edition Vol.1 No.2 (2008) Every geological map has unique geological divisions or local formation names to describe the geological information of the area. However, a common legend must be applicable to all formations and rocks in Japan. Consequently, only two criteria - lithology and age of rocks - were employed in the legend. Similar legend system had already been developed and adopted for the first time in the third edition of the 1:1,000,000 scale geological maps of Japan (1992)[10]. It was the only nationwide legend created from the most precise geological model in 2004 when the project started. Applying this legend (basic version) as standard, we published the seamless 1:200,000 scale map on the web as the first stage. However, the legend for the 1:1,000,000 scale maps was too rough for the 1:200,000 scale maps to provide sufficient information, due to far greater quantity and quality of information in the latter. To solve this problem, improved unified legend with more detailed geological division was developed in the next stage (Figure 4).A criterion for nationally consistent petrological and stratigraphical divisions for the larger scale map was reviewed and discussed in five categories (sedimentary rocks, accretionary prism, volcanic rocks, plutonic rocks, and metamorphic rocks). Metamorphic rocks were further classified into three categories: 1) metamorphic conditions, 2) age of metamorphic rocks, and 3) original lithofacies[11]. Accretionary prism was subdivided into two categories for rock types in each structural unit. Legend for sedimentary rocks was reviewed according to detailed subdivision of geological ages based mainly on regional stratigraphic correlation. Besides geological age, classification by chemical composition was carried out on volcanic and plutonic rocks. Holocene sediments in densely populated plain regions were subdivided by sedimentary environment including dune, wetland, alluvial fan, natural levee, lake, and artificially altered land.Though the above classification was ideal up-to-date legend for the harmonized 1:200,000 maps, not all regional maps could adopt the above classification, as they were compiled during varying periods. Listing all the legends used in each map and comparing them with the ideal legend, 384 geological divisions were adopted as optimum combination for the new geological map. The number of divisions nearly doubled from 194 divisions in the 1:1,000,000 maps. The new legend allowed the seamless geological map to provide more detailed information. Figure 5 is an example of the map of Shikoku region.3.3 Application of unified legend and establishment of single attribute tableAfter replacing the former legend with the new legend, reassembly of attribute data in digital geological maps was carried out with the aid of geographic information system (GIS)Term 6. Polychromatic analog maps were digitized, and original classification was given to each geological province, followed by permutation by correlating with the unified legend. Adopting vectorsTerm 7 to digitized geological data enabled independent calculation within each area as well as automatic slimmer, and resulted in both rapid data compilation and cost reduction, while maintaining the location accuracy of geological boundaries. As stated above, correlation of each classification in original maps to the most recent maps was very complicated and specific task, because the geological models of the maps differed by time of their production. Compilation of geological maps required cooperation of several researchers specializing in different research fields such as volcanology and sedimentology, or formation of different geologic ages. They usually had their own fields of specialty in certain locality where they conduct research. Consequently, one or more geologists who were specialized in the region participated in the correlating procedure. For example, one geologist handled volcanic rocks in northern Kyushu while another worked on compiling sedimentary rocks in southern Tohoku region. Each specialist examined the old and new data, and correlated them with the aid of other references including relatively recent geological maps of neighboring areas or maps of different scales and the latest paleontological and/or radiometric data. More precise correlation was possible when sufficient data were available to make regional stratigraphic correlation tables and/or columnar sections. Some of the recent 1:200,000 maps were drawn with legend showing almost equivalent geologic ages to the basic version, and these allowed easier correlation.3.4 Boundary smoothing and compilationThe next procedure was to maintain continuity of geological boundaries and fault lines between neighboring maps. This involved adjustment of precision and interpretations of the maps, by making old map data consistent with latest geological and topographic information (Figure 6). It also involved reviewing geological classifications and boundaries based on the latest geological model.Combining neighboring maps is widely used procedure to create integrated geologic map. Wide-area geological map is usually made from larger scale maps. For example, the 1:200,000 geological map is made by combining 1:50,000 maps. However, no attempt has ever been done to create integrated map from original maps of the same scale. In compiling a smaller scale map from larger scale maps, one of the most common methods is reducing the large scale maps using a copy machine and joining the boundaries by hand. However, this is applicable only in cases where the accuracy of locality data may be sacrificed.To create continuous geological map in same scale as the (4)−

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