Matrices relate checklists of environmental factors to a list of project components (see the following example). A number of matrices have been developed for environmental impact assessment, with some incorporating multiple dimensions.
Typical matrices, however, are two dimensional and identify cause-effect relationships between individual actions within a project and factors of the environment. As such, they allow for analysis of each aspect of a project as it affects environmental factors. Embedded in virtually all matrices is a means of measuring the magnitude as well as the significance of effects.
Matrices go beyond impact identification, (although that is certainly an important stage in the development of a matrix) to include impact measurement and interpretation. The indicators of magnitude and significance that can be built into a matrix force separate consideration of the magnitude and the significance of individual interactions. Thus, it is possible to focus on magnitude or significance, or both in analysing the effects of a proposal.
An additional feature of matrices is the way in which they communicate effects, to the public as well as to decision-makers.
Care must be taken in assigning magnitude and significance values to various interactions. This scoring is usually left to the judgment of the person(s) completing the matrix. Without very specific and documented criteria upon which to base the scores, different people can come up with very different scores, which highlights the subjectivity inherent in this method. Thus, the scoring values and techniques used must be defined and described.
Although matrices exhibit the desired characteristics of methods of impact analysis, they do not necessarily meet all aspects of the characteristics equally. For instance, only primary effects can be identified using matrices. Secondary, tertiary and cumulative effects can neither be shown nor analysed. In fact, the matrix may suggest direct cause and effect interactions that are not, in reality, so simple nor so direct. Finally, in order to differentiate between short- and long-term effects, a matrix must be designed with activities broken into different time periods.
In summary, matrices provide a useful means of identifying and measuring effects, but they are limited in the complexity of effects that can be addressed.
Typical effects matrices of the potential environmental effects of specific project examples are contained in Section 5. These cover: