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The purpose of this study is to develop a terrain evaluation system for road corridor selection applicable to the intermontane basins in northern Thailand. The first stage involves database construction of the terrain factors which have a direct effect on construction costs. These factors are land cover, topography and landforms, surficial geology, soil strength, topsoil removal, difficulty of excavation, embankment height, construction materials, and drainage characteristics. Remote sensing and terrain evaluation techniques, field investigations and geotechnical laboratory tests are used to prepare maps of these terrain factors. The resulting maps are converted into digital database form as map layers by using Geographical Resources Analysis Support System (GRASS)--a raster-based Geographic Information System (GIS). / These factors are incorporated into cost models. These are constructed using local engineering cost assessments which control the selection and specification of terrain factors. Under the GRASS environment the digital map layers of different terrain attributes are converted, based on the cost models, into cost surfaces (cost per unit area). The cost surfaces are subsequently combined into an integrated terrain-cost model. / With different assigned end points and cost models, the applications of the single theme cost surfaces and the integrated terrain-cost models to least-cost route selection are provided. An existing road network can be incorporated in these applications. The quality and benefits of the database and system developed related to terrain conditions, data capture by remote sensing, GIS manipulation and modelling, and cost modelling are discussed.

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