湖底地形調査の現状と課題 : 国土地理院の湖沼調査を例に  [in Japanese] Bathymetry and Landform classification of Lakes in Japan : Based on the Lake Survey by Geographical Survey Institute  [in Japanese]

国土地理院では,第四次基本測量長期計画に基づき,面積1km^2以上の約100湖沼,面積約2,300km^2を対象に,湖底地形等を測量する湖沼調査を行ってきた。これまでに60湖沼,面積で94%の湖沼について調査を終え,調査成果を1:10,000湖沼図として刊行してきた。この調査について,今後次のような改良が望まれる。1)空中写真測量,サイドスキャンソナー画像の利用等面的な測量技術の導入やレーザー測深などの新技術の開発;2)人工的に改変された湖沼の改測;3)小規模な湖沼のための効率的な測量方法の開発。また,湖底地形分類についても研究を進めてきた。これを一層有用なものとするために,1)湖底における最小地形単位の把握とそれに必要な調査精度の確保;2)湖底地形分類における,純学術目的と,応用目的の峻別;などの問題を解決する必要がある。さらに,長期的には,湖沼と沿岸海域を合わせた総合的な水域地形分類の体系を構築することが期待される。

GSI has surveyed principal lakes in Japan which have an area of more than 1 km^2, and has published Lake Charts at a scale of 1:10,000 (Fig. 2) according to the Fourth Long-range Plan of Basic Survey. Echo sounders have been used for the lake surveys and the charts have been compiled from the depth data obtained on the course lines of the surveying ships. The surveyed area is 2,168 km^2 (94% of the Plan), and 115 charts have been published which cover 60 lakes (60% of the Plan). For more effective surveys, the following improvements are indispensable. (1) Application of new methods as follows. (a) Use of aerial photographs for stereo plotting in very shallow areas of lakes (Fig. 3). (b) Use of side scan sonar images for mapping of areas with high undulation (Fig. 4). (C) Use of air-bone laser radar for lake depth mapping. (2) Revising the artificially deformed area of lakes. (3) Development of new methods for efficient surveys in small lakes, where costs are high using existing methods. GSI has also studied land form classification of lake areas for the use of lakes, development and conservation. Some experimental maps have been compiled. These include lake Biwako (Fig. 5) by Kotani, staff of GSI, and lake Ogawarako (Fig. 6) by GSI and others. Based on its experience, GSI has started a Basic Survey of Coastal Areas and published Land Condition Maps of Coastal Areas (Fig. 7). Dr. Oya and others have compiled Geomorphologic Land Classification Maps in and around lake Ogawarako, lake Abasiriko, lake Notoroko and lake Kasumigaura-Kitaura (Fig. 8). These maps are useful for understanding lake environments and for regional planning. However, the following problems are noticed; (1) The existing lake survey method makes it difficult to recognize landform units of the smallest order and to compile precise landform classification maps. (2) No clear distinction is made between pure scientific landform classification maps and applied landform classification maps. It is expected that a system of landform classification will be eslablished for the whole coastal underwater area, consisting of lake areas, bay areas and inland sea areas.