In the previous four exercises, we have explored multi-criteria evaluation in terms of a single objective—suitability for residential
development. However, it is often the case that we need to make site selection or land allocation decisions that satisfy multiple objectives, each expressed in its own suitability map. These objectives may be complementary in terms of land use (e.g., open space preservation and market farming) or they may be conflicting (e.g., open space preservation and retail space development).
Complementary objective problems are easily addressed with MCE analyses. We simply treat each objective's suitability map as a factor in an additional MCE aggregation step. The case of conflicting or competing objectives, however, requires some mechanism for choosing between objectives when a location is found highly suitable for more than one. The Multi-Objective Land Allocation (MOLA) module in TerrSet employs a decision heuristic for this purpose. It is designed to allocate locations based upon total area thresholds as in the last part of the previous exercise. However, the module simultaneously resolves areas where multiple objectives conflict. It does so in a way to provide a best overall solution for all objectives. For details about the operation of MOLA, review the section on Decision Support found in the TerrSet Manual.
To illustrate the multi-objective problem, we will use MOLA to allocate land (up to specified area thresholds) for two competing objectives, residential development and industrial development in Westborough. As noted above, total area thresholding can be thought of as a post- aggregation constraint. In this example, there is one constraint for each objective. Town planners want to identify the best 1600 hectares for residential development as well as the best 600 hectares for industrial expansion. We will use the final suitability map, MCEFINAL, from the previous exercise, for the residential development suitability map. For the industrial objective we have already created an industrial suitability map for you called INDUSTRIAL.
We will begin by creating maps for each objective.
A
Open the module MOLA. Select the single objective allocation procedure. For the suitability image enter MCEFINAL. Select to force contiguous allocations, set the number of clusters at 1. Select to force compact allocations with the minimum span of allocations set to 3. Select to use areal requirements and enter 40000. This is in cell units and is equivalent to 1600 ha. Give an output image name as BEST1600RESID. Hit OK to run.B
We will again run MOLA with the single objective allocation procedure. For the suitability image enter INDUSTRIAL. Select to force contiguous allocations, set the number of clusters at 1. Select to force compact allocations with the minimum span of allocations set to 3. Select to use areal requirements and enter 15000. This is in cell units and is equivalent to 600 ha. Give an output image name as BEST600INDUST. Hit OK to run.EXERCISE 2-11 MCE: MULTIPLE OBJECTIVES 146
Before we continue with the MOLA process, we will first determine where conflicts in allocation would occur if we treated each of these objectives separately.
C
Open the module CROSSTAB. Enter BEST1600RESID as the first image, BEST600INDUST as the second image, and choose to create a crossclassification image called CONFLICT.The categories of CONFLICT include areas allocated to neither objective (1), areas allocated to residential objective, but not the industrial objective (2), and areas allocated to both the residential and industrial objectives (3). It is this latter class that is in conflict. (There are no areas that were selected among the best 600 hectares for industrial development that were not also selected among the best 1600 hectares for residential development.)
The image CONFLICT illustrates the nature of the multi-objective problem with conflicting and competing objectives. Since treating each objective separately produces conflicts, neither objective has been allocated its full target area. We could prioritize one solution over the other.
For example, we could use the BEST1600RESID image as a constraint in choosing areas for industry. In doing so, we would assign all the areas of conflict to residential development, then choose more (and less suitable) areas for industry to make up the difference. Such a solution is often not desirable. A compromise solution that achieves a solution that is best for the overall situation and doesn't grossly favor any objective may be more appropriate.
The MOLA procedure is designed to resolve such allocation conflicts in a way that provides a compromise solution—a best overall solution for all objectives.
D
Open the module MOLA. Select multi-objective as the allocation type and select use area requirements for the allocation. To the right of the grid, increase the number of objectives to 2. Enter the two suitability maps, MCEFINAL and INDUSTRIAL. Enter the allocation captions of Residential and Industrial for the correct suitability map. Enter .5 as the objective weight for both. Enter an areal requirement of 40000 for residential and 15000 for industrial. These are equivalent to 1600 ha and 600 ha respectively. Select to force contiguous allocation and compactness. Leave the minimum span of 3. Enter an output name as MOLAFINAL. Click OK.The MOLA procedure will run iteratively and when finished will display a log of its iterations and the final image.
1
How many iterations did MOLA take to achieve a solution?E
The MOLA log indicates the number of cells assigned to each objective. However, since we specified the area requirements in hectares, we will check the result by running the module AREA. Choose AREA from the IDRISI GIS Analysis / Database Query menu. Give MOLAFINAL as the input image, choose tabular output, and units in hectares.2
How close is the actual solution to the requested area values?The solution presented in MOLAFINAL is only one of any number of possible solutions for this allocation problem. You may wish to repeat the process using other suitability maps created earlier for residential development or new industrial suitability maps you create yourself
EXERCISE 2-11 MCE: MULTIPLE OBJECTIVES 147
using your own factors, weights, and aggregation processes. You may also wish to identify other objectives and develop suitability maps for these.