Evaluation of Land Suitability for Crab Culture: A Methodological Study using GIS
Infrastructure: Infrastructure was considered as an important criterion for crab farming. Therefore, roads were extracted by digitising from the map 1:50,000 scale of Survey Bangladesh and Landsat TM image. This sub-model also included distance from the markets.
Weighting procedure of factors and sub-models
In order to determine potential sites for brackish water crab farming, thirty environmental and economic criteria were selected and compiled. These criteria were of two types: factors and constraints. A factor is a criterion that enhances, or detracts from, the suitability of a specific alternative under consideration. It is therefore measured on a continuous scale. On the other hand, a constraint serves to limit the alternatives under consideration. Suitability ratings for each criterion were employed, and each factor was reclassified on a scale from 1 to 4. The next step was to establish the weighting for each of the factor according to the pair wise comparison matrix of Saaty (1977). Using the weighting procedure, eight sub-models were developed. Finally, a system-oriented model was generated for brackish water crab culture. To illustrate the approach, eight sub-models were used to the final matrix (Table 1). The weighting of each sub-model was derived using the weight module, based on Saaty’s matrix. The weights were then used in the Multi-Criteria Evaluation (MCE) module to show potential sites for crab culture
Table 1: Weighting derived by the pair-wise comparison matrix for assessing crab culture potential in south-western part of Bangladesh (numbers show the rating of the row factor relative to the column).
| Factors | Land use | Infrastructure | Risk | Support | Water chemistry | Soil chemistry | Seed sources | Water sources | Weightings |
| Land use | 1 | | | | | | | | .1845 |
| Infrastructure | 1/3 | 1 | | | | | | | .0771 |
| Risk | 1/7 | 1/3 | 1 | | | | | | .0576 |
| Support | 1/4 | 1/2 | 1 | 1 | | | | | .0406 |
| Water chemistry | 1 | 4 | 5 | 5 | 1 | | | | .2056 |
| Soil chemistry | 1 | 3 | 4 | 4 | 1/2 | 1 | | | .1564 |
| Seed sources | 1/6 | 1/5 | 1 | 1/2 | 1/7 | 1/7 | 1 | | .0270 |
| Water sources | 2 | 5 | 7 | 6 | 2 | 2 | 7 | 1 | .2572 |
| Total | | | | | | | | | 1.00 |
Results of GIS Study
The results for each criterion are presented firstly for each sub model and the sub models are then used in final analysis. The reclassified surface area in each sub model are summarised in Table 2 and the corresponding spatial distributions of the areas are shown as maps (Figures 3-10).