A more elaborate form of the map overlay technique involves mapping impact variables (each on a separate transparent sheet) with a consistent variable coding system. For example, areas of significant adverse impacts can be shaded with a dark tone, while areas where less severe impacts are expected can be indicated with a light tone. Additional intermediate shadings can be used to identify areas which fall between these extremes. When all overlays are combined into a composite map, the areas of darkest tone will presumably be subject to cumulative impacts, and the areas of lightest tone will experience fewer cumulative adverse effects.

This elaborate composite mapping technique has a few limitations. First, although the dark and light shaded areas may represent areas of low or high impact (both beneficial and adverse in nature), they do not indicate the importance of these impacts to the residential, commercial, industrial, recreational, and open space areas affected. Informed subjective evaluation of the maps (and the assumptions on which they were based) by airport analysts and concerned local groups can determine the potential importance of possible effects. Impact evaluation is discussed in detail later in this chapter.

A second limitation of the composite overlay mapping technique is its inability to fully describe the dynamic interrelationships between the individual components of the environment (social, economic and physical variables), and between the ways in which airport project alternatives might affect those variables. Thus, while the technique can identify where airport project-related effects might occur, it cannot anticipate how the impacts will occur or whether there will be interactions between those impacts.

Despite these limitations, the composite overlay mapping technique can provide a vivid means of organizing and displaying inventory data, and can effectively illustrate where important environmental resources may be clustered.

Illustrated on the preceeding page is a relatively simple overlay mapping technique, involving a limited number of data variables.

Photographic Reconnaissance

Photography is one of the most effective and least expensive means of presenting data regarding existing conditions in a study area. A photographic survey is especially useful for decisionmakers who lack knowledge of the study area and its natural or man-made features. Both aerial and ground-level photographs are useful. Aerial photos often substitute for base maps, especially in early planning phases, when knowledge about the overall character and pattern of an area is more important than the more precise accuracy which engineering base maps provide.