Environmental Engineering Reference
In-Depth Information
experts, are likely to be affected by the project (e.g. agriculture, woodland, noise). The
project's degree of impact on the environmental feature is shown by the intensity of
shading, darker shading representing a greater impact. The composite impact of the
project is found by superimposing the overlay maps and noting the relative intensity of
the total
Figure 4.16 Part of the Sorensen
Network. ( Source: Sorensen 1971.)
shading. Unshaded areas are those where a development project would not have a
significant impact. Figure 4.18 shows an example of this technique. Alternatively, the
same process can be carried out using GIS and assigning different importance weightings
to the impacts: this enables a sensitivity analysis to be carried out, to see whether
changing assumptions about impact importance would alter the decision.
Overlay maps are easy to use and understand and are popular. They are an excellent way
of showing the spatial distribution of impacts. They also lead intrinsically to a low-impact
decision. The overlay maps method is particularly useful for identifying optimum
corridors for developments such as electricity lines and roads, for comparisons between
alternatives, and for assessing large regional developments. However, the method is
limited in that it does not consider factors such as the likelihood of an impact, secondary
impacts or the difference between reversible and irreversible impacts. It requires the clear
classification of often indeterminate boundaries (such as between forest and field), and so
is not a true representation of conditions on the ground. It relies on the user to identify
likely impacts before it can be used.
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