4.5 Overlays

The overlay approach to impact assessment involves the use of a series of transparencies to identify, predict, assign relative significance to, and communicate impacts in a geographical reference frame larger in scale than a localized action would require. The approach has been employed for selecting highway corridors, for evaluating development options in coastal areas, and in numerous other applications.

The McHarg overlay is based on a set of transparent maps, each of which represents the spatial variation of an environmental parameter (e.g., susceptibility to erosion or recreational value). The maps are shaded to show three degrees of parameter compatibility with the proposed project. A composite picture of the overall social cost of affecting any particular area is approximated by superimposing all the transparent maps. Any number of project alternatives can be located on the final map to investigate the degree of associated impacts. The validity of the analysis is related to the type and number of parameters chosen. For a readable composite map, the number of parameters in a transparency overlay is limited to about 10 (Munn, 1979). Parameter maps present data in a summarized and easily interpreted form, but are unable to reflect the possibility of secondary impacts. They also rely heavily on cartographic skills and their effectiveness depends to a large degree on cartographic execution.

This method is easily adaptable for use with a computer programmed to perform the tasks of aggregating the predicted impacts for each geographical subdivision and of searching for the areas least affected. Automated procedures are also available for selecting sequences of unit areas for routing highways, pipelines, and other corridors. The computer method is more flexible, and has an advantage whenever the reviewer suggests that the system of weights be changed.

The overlay approach can accommodate both qualitative and quantitative data. The weakness of the overlay approach is that it is only moderately comprehensive, because there is no mechanism that requires consideration of all potential impacts. When using overlays, the burden of ensuring comprehensiveness is largely on the analyst. Also, the approach is selective because there is a limit to the number of transparencies that can be viewed together. Finally, extreme impacts with small probabilities of occurrence are not considered. However, a skilled assessor may make indications in a footnote or on a supplementary map.