Site selection or suitability analysis is a type of analysis used in GIS to determine the best place or site for something. Potential sites used in suitability analysis can include the location of a new hospital, store or school among many others. When performing site selection analysis users must set various criteria from which the GIS software can rate the best or ideal sites. Site selection analysis can be performed with vector or raster data but one of the most widely used types of site selection, weighted site selection, uses raster data.
Weighted site selection analysis allows users to rank raster cells and assign a relative importance value to each layer (ESRI). The result is a suitability surface which ranks potential sites from 1 to 5. Sites with a value of 1 are least suitable and those with a value of 5 are most suitable. Weighted site selection is an important site selection method because it includes options for viewing next-best sites (those with a value of 4) should the ideal sites not work.
How to Use Weighted Site Selection in GIS
In order to use weighted site selection there is a standard workflow that should be followed. This workflow usually begins with defining a problem or criteria such as locating some potential sites for a new ski resort. The next step is to gather data and create raster surfaces to be used in the analysis. This step is followed by reclassifying the layers, weighting them and then overlaying the output layers with background information such as a map of topography to see the best potential sites.
Reclassification is important in weighted site selection because it is used to simplify the interpretation of raster data by changing a single input value into a new output value (ESRI). It can also be used to group ranges of cell values into a single value. For example you can assign a value of 1 to a set of values that range from 1-50 and 2 to values that range from 51-100. This simplifies weighted site selection because different types of raster data will have different values based on what they show (ESRI). By using reclassification they are all based on the same ranking scheme that can be used to compare and rank the least and most suitable sites.
Weighting layers is another critical step in weighted site selection because it allows the user to place varying levels of importance on different factors such as proximity to a major highway and sun exposure (ESRI). Weights are usually determined by a panel of experts on the subject being tested and they are based on specific criteria for the analysis. Weights are assigned as different percentages that must add up to 100%.
When to Use Weighted Site Selection in GIS
Weighted site selection or suitability analysis is best to use with raster data when a user needs to find a site based on a number of criteria such as the following problems explained by the ESRI Virtual Campus course “Using Raster Data for Site Selection”:
- When one needs to find the rankings of suitability for cells in a raster dataset
- When one needs to find next-best site options in addition to finding an ideal site
- When data has a distinct boundaries and other levels of certainty
- When the user determines where something will go based on specified criteria
- When the user wants to rank different criteria as more or less important in finding an ideal site
Weighted Site Selection Example
One example of a common weighted site selection problem is locating the best site for a new business such as a vineyard. This is a good example because vineyard operators have a number of different criteria that are needed to successfully grow grapes and these criteria can be ranked or weighted to find the best potential sites. These factors could be slope, elevation, sun exposure and distance to major freeways. In conducting a weighted site selection analysis all of these will be ranked based on their overall importance. For example, sun exposure could be weighted as 40% because grapes will not grow properly with too little or too much sun. Slope is 30% while elevation would be 20% because if the vineyard is too high, the temperatures will be too cold and there could be too much moisture for the grapes. Distance to freeways would be 10% because although important for visitors, without the right growing conditions there could be no vineyard at all.
In addition, a vineyard will have a distinct boundary, the developer will choose the final site (i.e.- the vineyard can’t move itself like an animal would) and next-best site options are necessary should the ideal site not work.
This is just one example of many potential weighted site selection projects and because of its versatility, weighted site selection is an important tool in GIS.
ESRI. (n.d.). “Using Raster Data for Site Selection.” ESRI Virtual Campus. Personal Notes. (Course Taken 2 April 2014).