Proceedings of the Open source GIS - GRASS users conference 2002 - Trento, Italy, 11-13 September 2002 Construction and Visualization of Three Dimensional Geologic Model Using GRASS GIS Shinji Masumoto*, Venkatesh Raghavan**, Tatsuya Nemoto*, Kiyoji Shiono* * Department of Geosciences, Osaka City University, 3-3-138 sugimoto, sumiyoshi-ku, Osaka 558-8585, Japan, e-mail masumoto@sci.osaka-cu.ac.jp ** Media Center, Osaka City University, 3-3-138 sugimoto, sumiyoshi-ku, Osaka 558-8585 , Japan, e-mail raghavan@media.osaka-cu.ac.jp 1 Introduction Recently, the need of the geologic information has been rising in many fields such as environmental geology, disaster mitigation, and urban geological applications. For these fields, it is effective to provide geologic information as a three dimensional(3-D) model that can be generated and visualized in general purpose GIS software. The present work aims at introducing a basic theory, implementing methodology and algorithms for 3-D modeling and visualization of geologic model using the Open Source GRASS GIS environment. 3-D geologic model is constructed from the boundary surfaces of geologic units and the logical model of geologic structure. The algorithms for construction and visualization of the proposed model are based on the geologic function g. The geologic function g assigns a unique geologic unit to every point in the objective 3-D space. The boundary surface that divides the objective space into two subspaces, were estimated using data from field survey. The logical model showing the hierarchical relationship between these boundaries surfaces and geologic units can be automatically generated based on the stratigraphic sequence and knowledge of geologic structures. Based on these algorithms, 3-D geologic model can be constructed virtually on GRASS GIS. Applying this model, various geologic surface and section models can be visualized in GRASS GIS environment. Further, “Nviz” was used for dynamic visualization of geologic cross- sections and generation of animated image sequences. 2 Basic theory and algorithms 2.1 Geologic function and logical model of geologic structure Let a 3-D subspace Ω be a survey area and suppose that the area Ω is composed of n geologic units that are disjoint: b 1 ∪ b 2 ∪ ⋅⋅⋅ ∪ b n = Ω , b i ∩ b j = φ ( i ≠ j ) . In order to realize a 3-D geologic visualization in the GIS environment, we have introduced a concept of a geologic function g which assigns a unique geologic unit to every point in the 3-D space Ω [1] [2]. g : Ω → B, where B = {b 1 , b 2 , ..., b n } . Fundamentals of the geologic function g is explained using a simple geologic structure composed of three geologic units as shown in Fig. 1(a). Three geologic units b 1 , b 2 and b 3 are defined by two boundary surfaces S 1 , and S 2 which divide Ω into two subspaces as follows;