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Friction surface
A friction surface which defined the costs associated with moving through different land use/cover types in the Langkawi Island was created (Table 2). This was based on the analysis of the level of difficulty of pipeline construction across the physical features as outlined in Table 1 together with the land use/cover types in the Langkawi Island. Grassland was taken as the base cost with a friction value of 1, because most of the grass areas were unmanaged and of low economic and environmental value. Subsequently, friction surface factors for other land use/cover types were also estimated relative to this base cost value. They included, in ascending order of friction surface value, mixed horticulture, paddy, and rubber. Due to poor economic returns and low yield values, many of the paddy fields have been abandoned (Langkawi District Council, 1992), so the cost of their land value was reduced to a friction surface value of 5.

Table 2 The relative costs of pipelines constructed through each of land use in the Langkawi Island.

Land use	Friction	Explanation
Waterbodies	1000	a very high cost-virtually a barrier
Inland forest	500	Passing through highlands
Mangrove forest	1000	Passing through salt marsh areas
Rubber	500	The trees felled, then removed
Paddy	5	Open space and abandoned land
Mixed horticulture	2	Less value
Grassland	1	Open space - base cost
Urban area	1000	a very high cost- virtually a barrier

Rubber is regarded as one of the commercial plantations covering 40.8% of agricultural land (Langkawi District Council, 1992), thus the compensation for cutting down rubber can be considerably higher (500 friction value). Some of the friction surfaces such as water bodies, salt-marsh mangrove and urban areas were very high (1000 friction value). This was to avoid passing through them but not totally prohibits their path in those regions (Eastman, 1997).

COST and PATHWAY functions in IDRISI allow the computation of the least-cost paths. The COST function calculates a distance surface where distance is measured as the least cost distance in moving over a friction surface. The PATHWAY function works by choosing the least-cost alternative each time it moves from one pixel to the next (Eastman, 1997).

Least-cost path
Based on the selected reservoir sites and the target areas, the proposed least-cost path for pipelines would be from the Ulu Melaka reservoir site (source cells) to Kuah town (destination cells) and from the Limbong reservoir site to Temoyong, one of the potential tourist resort areas.

Scenario 1
In the first analysis, the Ulu Melaka reservoir site, which was strategically located in the central region of the Island, was selected for the source cells and Kuah town in the south east region, was chosen for the destination cells. The cost distance analysis would need a friction surface that indicates the relative cost of moving through each cell. Using the COST function in IDRISI, a friction surface was then created for Kuah town (Fig.1).


Fig.1
Fig.1 shows different colour tones of friction surface values of costs associated with the assigned land use/cover types friction values as in Table 2. Regions with darker tones show the areas with lowest friction surface values/costs and the lightest tone indicate areas with high cost. The lowest costs were in areas near to Kuah town as well as areas with grassland, paddy, and mixed horticulture. The moderate costs were in urban and rubber areas. The highest costs were in areas of inland forest, mangroves and water bodies. Consequently, using the PATHWAY function in IDRISI, the computation of the least-cost path was completed. The PATHWAY works by choosing the least-cost alternative each time it moves from one pixel to the next (Eastman, 1997). It will begin with cells along the dam and then continue choosing the least cost alternative until it connects with the lowest point on the cost distance surface of the town centre.

Similarly, the same procedures were applied to finding pipeline routes from the Limbong reservoir to Temoyong. Fig.2 shows the result of the least-cost pipeline routes from the Ulu Melaka reservoir to Kuah town and from the Limbong reservoir to Temoyong.


Fig.2
Scenario 2
There was a possibility that rubber may provide a lower friction surface value than paddy fields in cases where some paddy fields are regarded as reserved areas with high touristic value (Langkawi District Council, 1992). Alternatively, by changing the friction surface value of rubber from 500 to 5 and paddy from 5 to 100 respectively in Table 2, a new relative cost of pipelines were developed. Again, using the COST and PATHWAY functions in IDRISI and the same explanation for their application as discussed in the above scenario 1, the least-cost for both pipeline routes for scenario 2 were computed (Fig.3).


Fig.3

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