GIS application for mountain terrains: some considerations and options Effect of Slope factor Although for most purposes, the earth may be regarded as a uniform surface, it does have variability in its relief.. Similarly, any conceptual surface may be examined in terms of its degree of relief. Relief is commonly expressed by of gradient, which identifies the change in the vertical dimension as the horizontal dimension changes (Coffey, 1981). The difference in area calculation does not affect land use since land revenue measurements only consider horizontal land surface, and for all practical uses (construction of house, of tanks, agriculture) a quasi horizontal land is put to use. Vertical component of the land is not utilized. However, for forest cover the actual area perimeter and land surface is kept in record that does not match (is greater than) the geographical area depicted on the map. Therefore when a forestis delineated in GIS its area is underestimated. The problem becomes acute in the case of cliffes or very steeply sloping land that may be the hanitat of particular plant species. In our study area, while analysing the land use changes, it was observed that the new agricultural extensions during 1963 and 1993 were predominantly (26.5%) on the 20o - 30o average slope areas since the lower slopes were already under cultivation and higher slopes are not preferable.  Effect of topographic aspect South facing aspects in the northern hemisphere are sunnier (receiving longer period of solar radiation), and therefore warmer and drier. As a result the upper limits of occurrence of any fauna or flora are higher on the southern aspects than the northern aspects. Even the snow-line is lower on the northern aspects and the snowmelt regime too is different. The southern aspects being sunnier and warmer have less forest cover and are preferred for agricultural purpose, whereas northern aspects are more forest clad. Southern aspects are more prone to forest fires too. Such is the impact of aspect on the local moisture regime. Therefore, aspect must be considered along with edaphic, macro-climatic and infrastructural factors in land use planning for such areas. In our study area the agro-climatic regions were defined taking into consideration both elevation and aspect, and based on that the land suitability classification was carried out. Agricultural extension during 1963 to 1993 were mainly (28%) on the south-western slopes and eastern slopes of the watershed, and marginal on the northerly slopes. There was less extension on the south facing slopes due to non-availability of land. In soil fertility evaluation study slope and aspect derived from DTM have provided two of the extrapolation factors in soil carbon and nitrogen contents (Schmidt, 1991) mapping. The same geological structure has different facet according to the aspect. A north-east dipping rock strata has dip slope on the northeast aspect and anti-dip slope on the south-west aspect. This fact has significant implication on slope-cut road construction. Consequently the anti-dip slope with inward sloping rock beds provides a safer side for road construction as dip slopes would be prone to slope failure. In the Himalayas, where due to thrusting the general orientation of the rock strata is more or less uniclinal, this method is applicable. This rule was applied in our study in combination with lithology and structure, in the Garhwal Himalayas for (i) finding the major causes of landslides; and (ii) for assessment of the appropriateness of the proposed road route in a watershed. In the former study it was found that lithology and construction aggravate landslides more than the dip direction of the slopes. Whereas in the latter study where the general dip is 30o E of N at 45o to 60o slopes, and the proposed road is mostly planned on the right (west) bank of the river traversing unsafe dip slopes where slope-cut for road construction will result in rock slip and landslides. Only 20% of the proposed route lies on comparatively safe slopes (with rocks dipping away from the road cut) for road construction. Moreover the route runs across a major active landslide. Re-routing of the road on the anti-dip slopes as much as feasible and using dip slopes only where the slope is below 25o is recommended. Heterogeneity of mountain landscape: Mountain ecosystems are very heterogenous in the spatial distribution of any feature, therefore interpolation, or extrapolation of data leads to largely erroneous results. Extrapolation can be very risky without in depth understanding of the inter-relationships of the variables involved in the extrapolation, and the risks of inaccurate or improbable extrapolation increase with the number of variables and the complexity of the environment. The data ffor almost all aspects of mountain ecosystems are very heterogeneous in their length and frequency of record, spatial coverage and availability. The three-dimensional complexity of the mountain can exacerbate these problems to an extreme extent (Heywood, et al., 1994). Data sources and related problems: The main data sources for mountain regions are similar to that of the lowlands but due to its inherent heterogeneity higher resolution of remotely sensed data and greater sampling density for ground truthing is necessary. One new and reliable source of digital locational data is Global Positioning System (GPS) which is particularly helpful in representing break-line features characteristic of mountain areas. However, locking-on at least 4 satellites even in mountainous terrain is often problematic when working in confined areas, such as gorges (stocks and Heywood, 1994) so GPS may not prove to be effective in certain areas. |