Abstract
A video was produced showing the landscape genesis of the 'Pellenz', an area in the western part of Germany, which was formed by various volcanic events. A general project description was given at the Onuma Workshop of ISPRS (Boehler, Scherer, Siebold, 1999). Together with a presentation of the final film product, a more detailed description of the mathematical modeling of the volcanic events, such as the development of maars and scoria cones as well as pyroclastic and lava flows can now be presented at the Ayutthaya International Workshop. Information from topographic and geological maps was used together with data from mining and exploration to create digital terrain models of the landscape for the last 300.000 years. For the volcanic events, the topographic changes for every vertex of the used raster DEM had to be modeled for every increment, i.e. for every one of the many thousand single frames of the video. Simultaneously, the changes affecting the texture overlays had to be modeled accordingly.
The Landscape
A video showing the landscape genesis of the Pellenz area in the last 300.000 years has been developed at i3mainz recently. In a first period of volcanic activities, between about 250.000 and 200.000 B.P., many maars and scoria cones with lava flows changed the appearance of this landscape. A long period without volcanic activities followed where the landscape was changed only by erosion and loess deposits. About 13.000 B.P., a huge volcanic explosion of the Mt. St. Helens type occurred. An estimated 8 cubic kilometers of material was thrown out within a few days, causing heavy fall-out. Several pyroclastic surges, consisting of hot gases and ashes, crashed down into the neighboring valleys. The area and its geological history is well investigated and published (Meyer 1994, Schmincke 1988, Ippach 1999, 2000).
The Video Project
The basic requirements for digital landscape modeling were DEMs and texture overlays for every single frame. Those were animated, rendered and combined for a digital video showing the landscape development without any gap for the last 300.000 years. A general outline of the project has been presented at the Onuma Workshop of ISPRS, Commission V (Boehler, Scherer, Siebold 1999). Meanwhile, a film has been produced, combining the digital video with film sequences of comparable volcanic events in the last decades. A sound track was added, too. The film is displayed several times a day at the Rauschermuehle Information Center in the Pellenz area and will also be shown in the poster session of this workshop. Figure 1 gives a general idea of the whole process. No off-the-shelf software is available for modeling the build-up of volcanic cones and maars or the flow of lava and pyroclastic surges. Own software was developed using C as programming language.
For landscape animation, a complete digital terrain model (DTM) is precondition. It consists of a digital elevation model (DEM) describing the landscape's morphology and textural information illustrating the landscapes appearance, depending on vegetation (depending again on climate). Therefore, solutions had to be found and developed for modeling both of the components (DEM and image data).
The following sections describe the modeling process (according to the leftmost column in Fig. 1) for maars and volcano cones as well as for lava and pyroclastic flows in more detail.
Fig. 1: Production workflow for the whole project. Modeling, animation and rendering was accomplished at i3mainz, the final film was produced by 'Institut fuer den wissenschaftlichen Film', Goettingen.
Dem Modeling
Initial DEM
A 40m raster DEM is available from the local land surveying authorities. The same raster was used for the whole project. So any removal or addition of material had to be modeled changing the elevation values of the 40 m raster vertices.
Before any modeling of volcanic events could be started, the initial DEM of the Rhenish Massif had to be reconstructed as it appeared most likely 300.000 years ago. This was accomplished starting from a DEM around the year 1900 which was computed from vectorized contour lines from the first topographic maps (scaled 1:25.000) available for the area. The present DEM could not be used because the topography of the landscape was considerably changed in the 20th century by intensive mining of pumice and basalt with industrial methods.
To obtain the initial elevation model, all volcanic deposits had to be removed mathematically. Since the thickness of the deposits is well known for most regions from mining activities or investigative drilling, the elevation changes for the DEM raster points could be interpolated and considered. For some areas, where geological data were insufficient, contour lines of the initial landscape were drawn manually by geologists and again converted into DEM raster information.
Maars and Volcanic Cones
Quite often, maars are the initial stage for the build-up of a volcanic cone. They result from explosions being triggered by underground water coming into contact with hot magma. In contrast to volcanic cones, maars consist of a low circular embankment only that surrounds a deep crater, often filled with water. Nevertheless, volcanic cones and maars can be modeled in the same way. So, if the term 'volcano' is used in the following, it refers to both, volcanoes and maars.
Input data To model the build-up of a volcanic cone, first of all we need the extension of the volcano (outer limit of its projection to the horizontal plane). This borderline polygon is digitized from a geological map, spacing the vertices individually to record the shape as close as necessary. Secondly, the location of the center of the eruption and its altitude are also taken from the geological map. As a third input, we need one single profile representing a typical cross-section (from the center to the outer edge) of a standard cone (cf. Fig. 2). Finally, it has to be decided how many frames the video sequence of the whole event shall comprise. All input data have to be made available as ASCII files in an ESRI® compatible file format since ARC/INFO®, Release 7.2.1, was used for partial solutions (vectorizing scanned maps, building polygon topology or generating and interpolating DEMs) wherever possible.
Modeling To model the intermediate locations of the cone's borderlines during built-up, a linear interpolation is carried out between each vertex of the borderline polygon and the center point, both known from the input data (Fig. 3). The increment depends on the number of frames assigned for the build-up of the cone. The series of concentric polygons documenting the horizontal growth is saved as one single ESRI file.
As a next step, the standard profile is transformed horizontally between the center point and each vertex of the present borderline polygon (dotted lines in Fig. 3). As a result, we obtain an individual profile for every vertex of the border line. All profiles originate in the center and run outwards in a star-shaped figure.
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| Figure 2 Standard profile (1-2-3-4) for a typical |
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| Figure 3 Interpolation of volcano growth polygons cone (1 = center point). based on vertices of borderline polygon of maximum extension. |
The vertical extent is computed as the difference between the altitude of the highest elevation of the volcano taken from the 1900 map and the lowest DEM vertex of the pre-volcano terrain within the horizontal final extent of the volcano. This difference is divided by the number of frames assigned to the event. Thus, we have the increment the volcano grows along the vertical axis during each frame. A linear interpolation by time can be performed using this increment. All profiles are now scaled in the vertical as well. Thus, the maximum vertical extent of every profile of a frame is identical and represents the current height of the volcano for this frame. Consequently, we have to deal with two different scale factors for the horizontal and the vertical transformation of every profile.
In the next step, the volcanic cone has to be prepared for integration into the digital (raster) elevation model. So far, the volcano is just described by a number of profiles. As mentioned above, the raster width used in the project is 40 m; the program could handle any other width, too, however. Affected are all points of the grid that lie within the box defined by the extreme points of the volcano's horizontal extent (of course, the same coordinate system for all data is a precondition). For every grid point G inside this box the two neighboring profiles of the star-shaped figure have to be found (Fig. 4). They are used to compute a new profile running through the grid point under consideration. The distances of profile points 2, 3 and 4 from center point 1 on the new profile are interpolated using the angles between this profile and the neighbor profiles. The height of the grid point can finally be interpolated on the new profile.
When this procedure is repeated for all grid points within the chosen box, a grid based height model of the volcano is available for the box area. These heights now have to be added to the pre-volcano elevations of the general elevation model to obtain new absolute heights. If the old terrain was very uneven, however, the computed cone would be distorted accordingly. Therefore, the volcano grid heights always are added to the horizontal plane defined by the lowest DEM-vertex within the volcano's extent, instead.
All steps described above have to be repeated for each volcano or maar and for every single frame. At the end of processing, we obtain a sequence of elevation models describing the genesis of the maars and volcanoes in this period. It has to be considered, too, that in some cases several volcanic events take place at the same time at different locations.
Figure 4 Interpolation of the height of a grid point (G) from heights of neighboring profile points (see text).
