Automatic Generation of 3D Terrain Models for Disaster Prevention

Abstract

   Japan is prone to several types of natural disaster, primarily earthquakes, typhoons and tsunamis. In 2011, Tohoku was severely hit by large-scale earthquake, and in 2016 powerful 7.0 quake has struck near Kumamoto. City officials and urban planners have drawn the maps for disaster prevention such as post-earthquake town reconstruction and large-scale earthquake countermeasure.

    3D town models based on these maps are quite effective in understanding what if this alternative plan is realized. 3D town models will be constructed on the area where the town can be escaped from tsunami and landslide. For 3D town models construction, editing and modification of 3D terrain models are necessary for land formation and site preparation, and then 3D house models are placed on these sites. However, enormous time and labor has to be consumed to create these 3D models, using 3D modelling software such as 3ds Max or SketchUp. So far, in order to automate laborious steps, we are proposing a GIS (Geographic Information System) and CG (Computer Graphics) integrated system for automatically generating 3D building models, based on polygons of a digital map.

   3D terrain models can be created from map contours or iso-lines. In most cases, the map contour lines are receded when the altitude increases. At the certain height, if a receded map contour line is crossed by own line itself, then a contour line or a contour polygon is divided into two or more contour polygons. In this way, a contour polygon will change topologically at the receding process. If the hill with two peaks is surrounded by one contour polygon, then one polygon will be divided into two polygons as the altitudes increases. In these case, the polygon has to perform crossing detection by the polygon itself since the polygon reduces its size gradually.  In this case, the straight skeleton or medial axis computation are quite useful for crossing detection and shape reduction when the polygon is shrinking. For example, the straight skeleton is defined as the union of the pieces of angular bisectors traced out by polygon vertices during a continuous shrinking process in which edges of the polygon move inward, parallel to themselves at a constant speed. The straight skeleton is unexpectedly applied to constructing general shaped roofs based on any simple building polygon, regardless of their being rectilinear or not. As shrinking process, each vertex of the polygon moves along the angular bisector of its incident edges. This situation continues until the boundary change topologically.

   In our research, the straight skeleton computation or medial axis computation are used for automatic drawing of map contour lines from key contour lines which are given altitudes. Based on the map contour polygons automatically drawn, 3D terrain models are automatically generated, and then 3D house models are put on these prepared sites. Our methodology saves the laborious steps of drawing contour lines which will be changing topologically. Depending on the building polygons, the integrated system automatically generates a 3D urban model so instantly that it meets the urgent demand to realize another alternative urban planning for disaster prevention.

Keywords:

3D Terrain Models; Automatic Generation; Land Forming; Map Contour; Automatic Drawing; Disaster Prevention Simulation; Straight Skeleton Computation