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GIS application for mountain terrains: some considerations and options
Susmita Ghosh
Introduction
Mountains are areas of high relief having distinct changes in terrain slope and thus require a three-dimensional representation for spatial modeling. Maps and GIS in general treat the world as if it were flat (plain land); this two-dimensional view leads to incompatibility in appropriateness of GIS application between that for level land and mountains. Mountains have some very specific features that need heterogeneity. Unfortunately, it is not the practice, apart from few modifications made in GIS applications for mountain areas. The study illustrates integration of such factors in ecological zoning, land suitability classification and probability mapping, such as for land erosion. 'Agricultural suitability/capability' and 'Vulnerability to soil erosion' are two such examples.
Maps represent geographical area on the planar surface whereas due to slope differences in the mountains the actual surface area is greater, the discrepancy in area calculation leads to overestimation of population density (biotic). Watershed, having modal average slope in 20-25o ranges, has geographical area of 94 km2 as against the actual surface area of 105 km2. A large surface with very steep slope (e.g. cliff face) is reduced to negligible area on a map. Similarly, linear calculations and buffering become erroneous for mountains using simple GIS techniques. Shorter distances than actual are recorded from maps; such errors could lead to underestimation in the cost of road construction when using GIS methods. Pronounced slope and shadow effect cause much problem in interpretation of remote sensing image data. In the mountain areas, research is still needed on how to use shadow information (total shadow minus topographically caused shadow) for land cover classification. Thus, GIS has an important role in improving digital image processing in mountain areas.
Thermal differences due to aspect affects the limits of flora and fauna distribution. It is well known that the upper limit of any vegetation type is bound to be lower on the northerly aspects making them drier. Such factors were used in bio-climatic zoning in our study.
The Slope aspect can be used to our advantage in conjunction with geological structure to assess the trend of rock beds, which may be useful for planning of roads and other constructions. This approach was adopted in the finding causes of landslide zoning, which is rather recent, is being focused in various parts but none have been quite comprehensive. Inversion of temperature is another phenomenon dictating the land use in mountains. Settlements refer ridge tops and slopes than valley bottoms, as they are colder at night and foggy for longer period of the day. Accessibility is not easily defined in the hills as in the plains and this is an important factor for land use and infrastructure.
Spatial complexity of mountain regions makes extrapolation very difficult. The same locational theories of hierarchical distribution of settlements do not apply to these regions as in the plains. The complex interaction between various factors lead to this heterogeneity. Added to these are the irregularities of data and difficulties of data collection and fieldwork in these areas.
So far there are two approaches developed towards modeling the three-dimensional complexity of the mountains, these are the 'DTM approach' and the 'Landscape approach'. The mountain specialists require a truly 3-GIS. However, incorporation of the approaches into knowledge based GIS is yet to be developed. Digital Terrain Models (DTM) helps in portraying the 3-dimensionsionality of the mountains, but overlaying procedures on DTM are yet not satisfactory. Better use of aerial photography is called for in this field. Global Positioning Systems (GPS) is used with GIS for mapping new (unmapped) features. With DTM, GPS and appropriate weightage attributed to slope, aspect and elevations in GIS it is possible to improvise the present inaccuracies of GIS applications for mountainous terrains.
In recent years, the growing concern over the environmental degradation of mountain ecosystems ahs gradually placed mountain issues in environmental and political agenda (Heywood et al., 1994). An example of this growing interest was the formulation of a Mountain Agenda for the UN in 1992. Several organisations are developing regional and national scale monitoring programmes in which GIS plays a central role. Hence it is very important to assess the applicability, or degree of accuracy in such GIS applications. Heywood et al., (1994) underestimated the uniqueness of GIS application in mountain environment, stating "that there is nothing unique about the character of GIS applications in mountain areas" although they add "nevertheless, the use of GIS in mountains require some special considerations".
The primary criterion that distinguishes mountains from other land surfaces is its significant positive relief. Slope, aspect, complexity and heterogeneity of climate, vegetation, faunal and land use distribution patterns are all outcome of this primary factor, relief. The paramount effect of relief is nowhere more spectacular than in the Himalayas, and this is where our study is based. The physical characteristic that best defines mountains is their three-dimensionality. It is this three-dimensionality that poses the greatest challenge for modelling these regions using GIS, for the simple reason that most GIS and the data they incorporate still treat the world as if it were flat. GIS applications started in the West and gradually, through government or semi government organisational aid and private enterprises, spread to the underdeveloped countries. In India, particularly in the mountain areas the use of GIS has been mainly organisational. Therefore the fields of GIS application here have been land use analysis, hazard assessment, natural resource management, visualisation of terrain, ecological and hydrological modelling etc. Most of the work has been application of conventional GIS methods without much thought to the effect of relief and probable errors. The sphericity of the earth has long been recognised and assigned a role in geography but the topological nature of the surface has not received as much attention (Coffey, 1998).
This paper deals with the source of errors encountered in GIS application to mountain environment using examples from some case studies and references. The paper also goes further to evaluate some of the options for improvising GIS applications for mountain areas.
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