One of the fundamental concepts in the disaster management is vulnerability. It refers to losses in the face of any danger. Vulnerability is discussed in different views like economic, social and physical etc. GIS-based disaster management studies mostly focus on mapping of physical losses that may occur.
There are different technics for making vulnerability maps for an area. In this field, AHP (Analytic Hierarchy Process) appears as a rational method. I prefered AHP for making vulnerability map in my M.Sc thesis.
Fundamentals of GIS-based AHP are comparing each of map layers and determining weight values. In this way decision makers could choose suitable result by defining a hierarchy for complex problems.
In this application, vulnerability analysis was performed for an area in the district of Sarıyer, Istanbul. Initially layer weights were calculated using AHP and vulnerability map generated by WLC (Weighted Linear Combination) analysis. I have developed Easy AHP plugin for QGIS and all of the analysis process were realized by using it. The analysis area and the map layers are shown below.
(Bina = Buildings; Elektrik Hattı = Power Lines; Fiber Hat = Telecom. Lines; Karayolu = Highway)
Preparing Analysis Layers (Parameters)
In AHP literature input layers are called as “parameters”. Fistly input parameters were generated from map layers.
Bina (polygon) > Bina yoğunluğu (Building density) (raster)
Elektrik Hattı (line) > Elektrik Hattına Uzaklık (Distance to Power Lines) (raster)
Fiber Hat (line) > Elektrik Hatta Uzaklık (Distance to Fiber Lines) (raster)
Karayolu (line) > Karayoluna Uzaklık (Distance to Highway) (raster)
To generate building density you can produce centroids and run kernel density analysis. For generating distance maps you can use proximity or multi-buffer tools. In this application main objects are AHP and WLC analysis. So I dont mention much about preparing map layers (parameters). Also I recommend that your map layers must be in same CRS. Otherwise yo can get incorrect resuts.
The generated parameters are raw so they could not used in WLC analysis after AHP. Because they are defined in different units (building densitiy and distance). They must be classified and converted to same units. I recommend r.reclass tool for this job. Distance to Fiber Lines, Distance to Power Lines, Distance to Highway were classified as 25m, 50m, 100m, 150m, 200m, >200m six classes. And Building Density was classified as six classes in the face of building distrubution. These four parameters are shown below.
After classification and generating parameters, we pass to next step for calculating parameter weights using EasyAHP.
Start the plugin and click Next button after reading descriptions. In this step choose the analysis parameters. Note that AHP analysis requires 3 parameters at least and not more than 15.
In the second step we fill the pairwise table. During comparion between parameters you can use 1 to 9 numbers. Greater numbers means more importance (Saaty, 1980). Also notice that cross cell are filled automatically. After filling all the cells, click Calculate button for calculating AHP indicators. Note that CR value must be less than 0,1. If not the pairwise table is considered as inconsistent (Saaty, 1980). The sample pairwise table is shown below.
After calculating AHP indicators move to next step. Now we run WLC analysis using output layer weights of the AHP for generating vulnerability map. In this step Easy AHP uses Processing Toolbox’s SAGA Raster Calculator tool. Deafultly it installed by QGIS but make sure it runs without error. Click Browse button for output location and Run the analysis. After finishing the process, result layer is added into QGIS canvas.
In this application sample vulnerability map was generated using Easy AHP that combines AHP and WLC complex analysis. The result maps are shown below.