The overlay method uses a series of maps depicting environmental factors, land features, and socio-economic characteristics for a given study area. A separate map is produced for each variable of concern, with categories on each map based on values relating to measures of suitability, capability, or acceptability. Shading on the maps is in proportion to the values for each map. When the individual maps are superimposed, the composite map clearly depicts the areas that would be affected the most and the least.

This method was developed in the 1960s by McHarg (1968) and has been used very successfully in many planning applications since that time. Advances in computer mapping, especially application of Geographical Information Systems (GIS), have facilitated this approach enormously.

The overlay method is quite useful, as a first cut approach, in identifying certain types of effects and in depicting the area over which those effects will be experienced. In addition, it allows for some comparison of alternatives and for identification of the effects of mitigation measures. This is accomplished by changing the values assigned to different categories and by varying the factors included in the overlay maps, in conjunction with alternative designs.

A further value of this method lies in the effectiveness in which it displays the spatial dimension of effects. Thus, overlays are very useful in identifying effects and in communicating them, at least for the area covered by the maps.

As with checklists, the relative simplicity of the method poses some limitations. While they go beyond checklists in depicting primary effects, overlays cannot extend to secondary effects, and certainly not to cumulative effects.

Because they are based on maps, the analysis that they generate is bounded by the spatial extent of the maps. Thus, it is rare that the analysis extends much beyond the boundaries of a proposed project. The cartographic capability to do this exists, but the larger the area covered, the less detail that can be mapped. There is, then, a trade-off between spatial coverage and detail of analysis.

A large amount of data is required to produce the individual maps. While much of the data is usually generally available (i.e. soil maps, topographic maps, land use), some of the data may be more difficult to obtain, particularly in a form that readily lends itself to mapping.

Thus, the data requirements seem to be high relative to the sophistication of the final product. That is, the overlay method does not provide a detailed analysis of the effects in an area despite a relatively large amount of data required for it. On the other hand, the overlay method is very useful as a means of scoping or identifying important environmental issues associated with an activity in an area.

Figure 6.1 Example of the Overlay Method