Vulnerability Mapping Using Easy AHP

Bu sayfayı Türkçe olarak okuyabilirsiniz.

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.

Sample Application

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.

Easy AHP

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.


28 comments on “Vulnerability Mapping Using Easy AHP

  1. Really great plugin ! But I can’t run the script at the end… i’ve a problem with toolbox’s Saga provider… However I’ve check all saga options in the toolbox. (I use QGIS 2.8 Wien).

    • Hi Tedd,
      A few weeks ago i got a mail from another user about this problem. Here is my answer, maybe it will be helpfull for your condition:

      “I have tested Easy AHP on Windows7 and Linux Mint but no error message I got. The error message shows that everything went well but calculations stage (WLC) is problematic. It can not find the files, maybe it is about filenames. Please use english characters and do not put blank on files or directories. My advice is: Firstly open Easy AHP add layers and calculate weights and note results to somewhere. And open raster calculator tool (SAGA or GRASS) and run with the noted results. I can not update plugin for a few months. Thanks for the report.”

  2. Hi there!
    I’ve run this plugin for times but at the end I just am seeing this error message:
    “An error occurred during the calculation. Please make sure that Processing Toolbox SAGA provider is installed.”
    Thanks for any coming help.

  3. Hi,
    I’ve managed to use the plug in but the results seem strange to me. I have 3 raster layers (which are heatmaps created from vector points). The output seems strange because there are blank data sections in the new layer. For example, 1 layer is weighted strongly and has a high value at a certain coordinate but the output shows nothing at this coordinate. I think there is no data for this point.

    I am somewhat new to GIS and am unsure if my map layers are prepared correctly. I do have them in the same CRS but maybe there is something I am missing? It just seems strange that there is data before the analysis and then patchy spots after.

    Thank you!

    • Hi,
      This problem may be related with layers extent. If I remember correctly; one layer is smaller than the others analysis output raster layer’s extent will be same as smallest. You must check this. And you should read my answer to Tedd’s question. This may be helpful for your situation.

  4. It is amazing plugin, I have learnt AHP from bussiness context now It is support in the geography decision. However, It is still new for me. I would like to apply for agri suitability so that I try to understand the process from your sample project. I am appreciate for your detailed explaination for the assigned number in step 2 and step 3 in EasyAHP box. Looking forwards for your reply

    • Thanks for the feedback. Step 2 is about matrix comparison. You compare parameters (layers) for each other in this step. Step 3 is WLC (Weighted Linear Combination). This operation is seperated from AHP (step 2) in some papers, but others regard it a part of AHP. This is especially scientific view of this analysis.

  5. Please would you tell me if the 2nd process about WLC is: (raster1 * weighted1) + (raster2 * weighted2)……+ (rasterN * weightedN).
    that is because I have the same problem as Tedd, but I don´t understand the process in saga, so I will going to do in Arcmap with the raster calculator tool.

  6. Hi, I want to comment some problems about the weighted process. I did the process as in your example with some data. I run the AHP in Superdecisions, and as the same time I used the same weighted criteria in your plug-in. The results of weightings are very different: the values are different and the order is different.
    Easy AHP values Superdecisión values
    0.046 0.43650
    0.276 0.07204
    0.48 0.04053
    0.06 0.30263
    0.13 0.14828

    Why the results are very different, if the process is the same, and the CI value are the same??

  7. Its weird, because i have tested wight results with other samples and get same values. I recommend that you should compare Superdecisions and EasyAHP with known examples.

  8. Pingback: Tài liệu học tập học phần Bản đồ và Phân vùng chức năng

  9. I am totally new to QGIS and I am working on my college project on site selection using suitability analysis. So, i tried to learn this plugin with help of your article by imitating the steps but when I start the plugin after opening the four raster layers you provided here, no input layer or available raster layer appears in the step 1. Please guide me.

    • That’s really interesting because i tested it today without any problem. Firstly you should add raster layers (TIFF etc..) to QGIS canvas by using “Add Raster Layer” button. And than run “Easy AHP” plugin, you should see all raster layers in the selection list.

      Please make sure that you are not adding vector layers. They wont be listed in selection menu.

      • Thanks for reply. But, still the raster layers didn’t appear in the input layers list. It may happen that there’s something wrong with my inputs. I an using the same jpeg pics you have provided above in your article, and it’s QGIS 2.8.4-Wien However, if you can guess some reason as to what is causing this and tell me, it would really help, only f it’s not too much to ask.

  10. The Easy AHP is really an interesting plugin. I try to work with it but It was impossible.
    I started the plugin and added Raster Layer (Tiff. files), but the raster layers didn’t appear in the input layers list. I use QGIS 2.18 Las Palmas. Could you suggest me some solutions. Thanks for your time.

  11. Thank you for your immediate reply. But I still have the same problem. I can see in available layers list, only raster type DEM.

    • Looks like you are trying to work with raster file with multiple bands. AHP analysis work with single band raster like DEM, NDVI, etc. Because every band means different criteria for AHP (look for publications). So you need to split your raster file to bands and then try again. Also another user wrote same thing here

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