日本財団 図書館


Animation display of ocean bottom topography near Japan
Yutaka Nagata and Toru Suzuki
Marine Information Research Center, Japan Hydrographic Association
Mishima bldg. 5F, 7-15-4 Ginza, Chuo-ku, Tokyo 104-0061, Japan
and
Akira Asada
Institute of Industrial Science, University of Tokyo.
4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan
ABSTRACT
 Bottom topography of ocean is generally more complicated and more spectacular than landscape. The knowledge of ocean bottom topography is essential for seismologists, tsunami researchers, modelers of oceanic circulations and other scientists of solid earth physics. However, seawater is too opaque for us to view ocean bottom scenery, and it is hard to grasp virtual image of ocean bottom configuration. We developed two kinds of animation displays of ocean bottom topography by using new digital bathymetry dataset J-EGG500 of JODC. For several coastal areas, we use much more detailed dataset prepared by one of the authors, Akira Asada. One of the displays is walkthrough animation in which varying scenery looking from the front glass of a submersible/flying boat is shown. The navigation routs are fixed, but we set 16 routs for the seas around Japan. Another display is to show bird's eye views from 46 spots. The view direction can be changed horizontally for 360 degrees and vertically for 180 degrees by manipulating mouse. The whole software is supplied in a CD-ROM, and can be operated by usual personal computer.
 
Keywords: animation display, ocean bottom topography, walkthrough animation, bird eye's view,
1. Introduction
 Accurate knowledge of bottom topography is essential for researches on ocean in various fields. Configuration of trenches and ridges, alignment of seamounts and so on gives insight of ocean floor movement, and is important information source for plate tectonics. We cannot forecast run-up height distribution of tsunamis without the knowledge of bottom topography. Bottom configuration restricts nature of oceanic currents, and physical oceanographers have a keen interest on it. Good fisheries grounds are usually found over or near shallows and sea mounts, and fisheries scientist and fishermen want to have detailed bottom topography.
 However, seawater is too opaque for us directly to view ocean bottom scenery, and it is hard to know intuitively bathymetric distribution in general. The Marine Information Research Center (MIRC) was established in May, 1997 in the Japan Hydrographic Association under the financial support of the Nippon foundation, in order to help activities of the Japan Oceanographic Data Center (JODC) by designing quality-control software and by developing new techniques on marine data management, to produce high quality data products for various users and to popularize oceanographic knowledge by using its product. As one of our efforts on popularization of oceanographic knowledge, we developed two kinds of animation displays of ocean bottom topography near Japan. One is walk through animation, in which you can see ocean-bottom-scape from a front glass of a submarine/flying boat. Another one is to show bird's eye views from various spots. The scenery of ocean bottom is much more spectacle than landscapes, and the computer graphics that we designed are enjoyable both for scientists and for popular people.
2. The bottom topographic data used
 JODC compiled digitized bathymetry dataset, J-EGG500 (JODC-Expert Grid Data for Geography) of 500m x 500m grid in 1999. This is the most complete dataset in present stage for the seas in the vicinity of Japan (Fig. 1). We used this dataset in our animation displays, but it is too coarse to display the detailed topography in coastal areas and bays. So, for several coastal sub-domains, we used much more detailed 150m x 150 m grid data, which were provided by one of the authors, Akira Asada. We used more detailed data of 50m x 50m grid for the case of Sagami Bay, and of 90m x 90m grid for the case of Tokyo Bay. As to the height distribution over lands, we used 50 m x 50 m dataset provided by the Geographical Survey Institute, Japan.
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Figure 1. Opening display of the animation software. The area with bottom topography shows the domain, in which digitized bathymetry dataset, J-EGG500 is given. The round button on the map indicates the spot, from which bird's eye view is available (the buttons would be erased in the final version of our software, as entry for bird's eye window will not set in the opening window). By clicking the list of the names of sub-domains or by clicking on the suitable portion of the map, you get the second display with a map of sub-domain.
3. Walkthrough animation
 The opening display of our animation software is shown in Fig. 1. The area with bottom topography shows the coverage of the used dataset, J-EGG500. We divided areas into 16 sub-domains: Hokkaido and Northeast Honshu, Japan Trench off Choshi, Izu-Mariana Ridge and Trench, the Izu Islands, Sagami Bay, Suruga Bay, Enhgunada and Kumanonada, Ise Bay, Osaka Bay and Kii Channel, the Seto Inland Sea, Bungo Channel and Hyuganada, Kagoshima Bay, the Okinawa and Nansei Islands, the Tsushima Strait, the Oki Islands and Wakasa Bay, and Toyama Bay and Sado Island. If you click on the area corresponding to the sub-domain above mentioned, or click the list of area names shown top of the display you can move to the second display, the map of the sub-domain. The second display includes an explanation of the characteristic topographic feature of the area (Japanese only in the present state). There are list of 16 sub-domains: if you click some domain name, you can move to the other area. The buttons are also given to start the walkthrough animation or to get bird's eye view. There are several round buttons in the map, indicating the spots where you can see bird's eye views. This will be explained in the next section.
 One example of the sub-domain map is shown in Fig. 2 for Sagami Bay area, which is located to the west of Tokyo Bay, and is widely opened to the North Pacific. Several location names are shown in the map both in Japanese and in English (English words appear only here in the present state). The line starting at the upper-left corner and ending at the mouth of Tokyo Bay indicates the navigation rout of our submarine/flying boat. The starting point is near the top of Mt. Fuji, and the boat submerges just after it comes over Sagami Bay. The boat moves southward, and goes around Oshima Island (One of the famous Japanese recreation spots with active volcano, Mt. Mihara). Then, it reaches the mouth of Tokyo Bay after by navigating along the axis of the Tokyo Submarine Canyon.
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Figure 2. Sub-domain map for Sagami Bay area. The line with arrow indicates the navigation rout of the submergible/flying boat. Round buttons indicate the spots, from where bird's eye views are available. Two segments attached to each of the round buttons shows range of view in the initial stage.
 If you click the starting button of the walkthrough animation, you can see varying scenery looking from the front glass of the submersible/flying boat. However, It is recommended to keep the rout in memory, as we have no oral explanation during the voyage. You can enjoy the scenery of Mt. Fuji when you moving northward to the west of Oshima Island,
Another example is shown in Fig. 3 for Suruga Bay area. Suruga Bay is located just west of Sagami Bay. A deep canyon, Suruga Trough extends northwards along the bay axis from offshore, and reaches almost the bay head. We set rather simple navigation rout in this case: the boat starts from the center of Sagami Bay, and goes southwestward along the coast of lzu Peninsula. Off the tip of the lzu Peninsula, it turns to west, and crosses Suruga Trough, After taking a circular path, it navigates northward along Suruga Trough, The rout is just pointing to Mt. Fuji, and the boat comes water surface near the bay head to fly over Mt Fuji. You can see the crater of Mt. Fuji in the last scene of the animation. One of the examples of the scenery from the front glass of the boat is shown in Fig. 4, when the boat is navigating northwards along the axis of Suruga Trough. You can see the configuration of Suruga Trough and Mt. Fuji faraway through the canyon. Vertical scale is taken as several times larger than horizontal scale in our animation, in order to illustrate the scenery more understandable. So, the shape of Mt. Fuji is a little sharpened than our real image.
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Figure 3. Same as in Fig. 2, except for Suruga Bay area.
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Figure 4. View from the front glass of the boat, when it navigates along the axis of the Suruga Trough (see Fig. 3) Mt. Fuji is seen through the canyon.
4. Bird's view animation
 The bird's eye view would give more concrete image on ocean bottom topography, In order to see bird's eye views, one of the round buttons shown in the map should be clicked, If you click the button in Sagami Trough of Fig. 3, you will get the bird's eye view as shown in Fig. 5. The height of the observing points can be selected whether a little above the sea surface or the higher level. The underwater level was not chosen for observing points as it does not give spectacular scenery in general. The higher level is selected in the case of Fig. 5.
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Figure 5. Bird's eye view from the southern observation point of Suruga Bay (see Fig, 3 for its position and viewing direction).
 The two segments attached to the round button indicate the range of vision in the initial stage. Eye direction can be changed both horizontally for 360 degrees and vertically for 180 degrees by maneuvering the mouse.
 The bird's eye view well demonstrates the characteristic nature of Sagami Bay: How deep the Sagami Trough is, and how the trough invades near to the coast.
 The number of observing points is 46 in total, and these are shown in Fig. I with round buttons, (In the final version of the software, these buttons in Fig, I will be eliminated, as we need to click the button from the sub-domain map to get bird's eye view, We believe that almost all of the important and speculative sceneries can be seen from one of the given observing spots.
5. Concluding Remarks
 The ocean-bottom-scape is much more interesting and spectacular than landscape if we can see it, The animation of ocean bottom topography gives it, by removing seawater or by replacing sea water with transparent liquid, We hope that our bottom topography animation of two kinds can give good sea bottom images for scientists and for public people, The total software described above will be installed in a single CD-ROM The bathymetric dataset will be not included The displays will be given in full color, though only black-and-white pictures are given here.
 The design of the software is now in the final stage. However, several improvements or corrections would be necessary. For example, some of the names of the locations in Fig. 2 are erroneous (especially for English names), The final version will be given by the end of June 2001.








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