Perhaps the simplest approach for impact prediction is to use analogies or comparisons to the experienced effects of existing projects or activities. This approach can be termed a "look-alike" approach in that information gathered from similar types of projects in similar environmental settings can be used to descriptively address the anticipated impacts of a proposed project or activity. Professional judgment is necessary when analogies are used for specific impacts on the environment.
Another approach is the inventory technique. In this approach, the environmental engineer compiles an inventory of environmental resources by assembling existing data or conducting baseline monitoring. With this approach, a presumption is that the resources in the existing environment, or portions of it, will be lost as a result of the proposed project or activity. This technique can be perceived as a worst-case prediction, and for certain types of resources it represents a reasonable approach for use in environmental impact studies. Again, professional judgment is necessary to interpret information related to the existing environment and the potential consequences of a proposed project or activity. This approach can also be aided by the analogy approach.
An often used approach for impact prediction is to incorporate checklists or interaction matrices as a part of the impact study. Several types of checklists have been developed, ranging from simple listings of anticipated impacts by project type, to questionnaire checklists which incorporate a series of detailed questions and provide structure to the impact prediction activity. Some checklists include the use of scaling, rating, or ranking the impact of alternatives and incorporate relative-importance weights to the environmental factors. These checklists can aggregate the impacts of a project into a final index or score which can be used to compare alternatives. In this context, these checklists are similar to multicriteria decision-making techniques which are used in environmental planning and management.
Interaction matrices include simple x-y matrices that identify impacts and provide a basis for categorization of impact magnitude and importance. In stepped matrices, secondary and tertiary consequences of project actions are delineated. The most sophisticated types of matrices are networks or impact trees in which systematic approaches trace the consequences of a given project or activity. A key point relative to both checklist and matrix methods is that they tend to be qualitative in terms of the predicted impacts; however, they do represent useful tools for impact prediction.
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