Many different criteria and factors can be used to assist decision-makers in actually reaching a decision. Very complex issues may be based on different factors, variables or criteria. A considerable amount of information may be available in digital form and may also be available in a GIS. The GIS allows the planner to overlap different pieces of information in order to generate additional information that can be of use for the decision-makers.
For example, if population data is available then this can be depicted on a map. The map may depict population densities, or depict where population growth rates are highest and so on. Whatever information is available digitally and can be referenced to a map can be depicted using the Geographic Information System (GIS).
Assuming, that GIS data are available, suitable map features can be found using different techniques:
Features that meet a particular criteria can be selected using GIS in order to visualize a subset of map features that are needed in order to be able to answer a certain planning question (e.g. all areas that are covered with grass land or shrub land, etc.).
Features that do not meet a particular criteria can be excluded (i.e. filtered) from the map being depicted (e.g. areas not located on fertile soils or have a slope greater than 30% should be excluded).
Some aspects of the map feature may be considered by the planner as being (un)suitable; perhaps because they are located close to a neighboring feature. For example, a planned factory must be located close to a river. Therefore, the areas that are deemed suitable for the planned factory can quickly be reduced to those one that are inside a buffer of 2 km left and right of the river.
Aspects that may have to be considered by the planner can also be derived by selecting a combination of criteria rather than just a single criteria. (For example, the planner may decide to depict all areas that are covered by a certain soil type, where the slope is less than 30 % and where the area is neither covered by forests or residential housing). The information generated by overlaying the information provides the planner and decision-maker with new and additional know-how that he/she would not have had if they had only considered the information individually. The overlay technique provides a vivid new insight which in turn helps in the development of the most ideal solution or a better vision of the future.