109 || EIA Methodologies: Overlay Method
Overlay Method:
Overlay method is based on mapping.
Mapping involves the superimposition of several maps of environmental characteristics or themes (thematic maps) like physical, social, ecological and aesthetic aspects prepared on the same scale, to get a composite characterization of the environment of the project site.
This method is widely used in environmental planning, especially for large area developments projects like townships, industrial areas, and for linear projects, e.g., roads, railways, and cross-country pipelines.
This method is also commonly used for comparing alternatives.
Synthetic eco-environmental sensitivity using GIS and factor-overlay method:
In order to quickly obtain the calculated results using advanced GIS technology, descriptive
level of information needs to be converted into quantitative indexes and build index system.
Four basic environmental sensitivity units like soil, water, atmosphere and biodiversity conditions have to be evaluated and corresponding charts are prepared, and grade index are given.
Taking the advantage of space overlay function in advance GIS technology, a comprehensive index of eco-environmental sensitivity has to be calculated from four special sensitive charts and indexes of essential factors (soil condition, water condition, atmosphere condition, and biodiversity condition).
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