The environmental impact statement requirement under NEPA has resulted in a proliferation of techniques for environmental impact assessment. These may be classified according to their function into the three main categories of impact identification, impact valuation, and impact comparison. For each of these categories, some problems relating to measurement techniques are identified and discussed, and a few prominent examples of techniques in the category are reviewed. Impact identification techniques must consider how the issues should be organized, how the material is to be presented, what geographical and other boundaries should be fixed to keep the analysis at a manageable size, and what descriptive criteria should be included. Sorensen's system of ‘linear graphs’ and the Saratoga Associates' matrix approach are discussed with respect to these points.
Impact evaluation techniques, in addition to idenfying the impacts of a project, place a numerical value on their significance. For some impacts, a physical measurement scale may not be appropriate without modification; for others, no objective physical scale exists. In both cases, it is necessary to resort to subjective judgements to quantify the true seriousness of an impact. These judgements should be made systematically and, where possible, in such a manner that the resulting values form an interval- or ratio-scale rather than merely an ordinal scale. The matrix procedure devised by Leopold et al. (1971) is discussed as an example of impact evaluation techniques.
Impact comparison techniques, in addition to impact evaluation, weight the relative importance of each impact and combine the impacts for an overall assessment of the project and any alternatives to it. A major problem here is the lack of comparability between different types of impact; interdependence between impacts also makes the combination of factors difficult. Again, subjective judgements are needed, and proper use of scaling techniques can provide some basis for comparison. Techniques suggested by McHarg (1968), by Peterson et al. (1974), and by Dee (1972), are reviewed in this section.
Although EIS preparation raises several of the thorniest problems that occur in planning for decisionmaking, these are problems that must be faced, not evaded. Whereas the techniques suggested by no means solve all the problems that have been raised, they do provide partial answers. People involved in environmental impact assessment can only gain by thinking systematically about exactly what tasks are to be accomplished, and by making an appropriate choice of techniques for undertaking these tasks.