In this tutorial I will talk about the weighted sum overlay analysis in QGIS. Weighted sum overlay is usually done for the suitability analysis. It uses several weighted raster layers for the calculation. GIS uses Boolean logic to perform overlay analysis. This means it uses conditional statements on input layers to generate suitability map.

For each input layer we have to assign the weight value. This weight value is multiplied to the input layer. Then you have to sum all these weighted layers to generate the final output map.  Let me give you simple example to identify the soil erosion potential area. The inputs are terrain slope angle and vegetated/not vegetated area. Result will be calculated as “if slope >5% and vegetation=No” then erosion potential = true.

Here we are using weighted overlay analysis to find out ground water potential zones in a river basin using open source QGIS software. Usually this kind of analysis is done using ArcGIS. Unlike ArcGIS, QGIS have certain limitations but still we can do the raster overlay analysis. The inputs are as follows:

  • Geomorphology
  • Soil
  • Land use/ land cover
  • Slope
  • Drainage
  • Lineaments
  • Lithology

Some of the layers are vector and they will be converted into raster images. Then all raster layers will be multiplied by the weight value. Following table shows the weight of each layer, total weight  is 100. Our aim is to find out the ground water potential zones of the river basin.

 SL NO.

Theme

Theme Weight

1

Drainage Density

12

2

Slope (degree)

15

3

Lineament density

16

4

Geomorphology

21

5

Lithology

14

6

Soil

10

7

land use / land cover

12

 

Process 1:  Find out the Drainage Density and Lineament Density

a. Open the QGIS software and add vector layers or “Ctrl+shift+v “ (Key board shortcut)

Figure 1 vector files loaded in the QGIS

Figure 1: vector files loaded in the QGIS

b. From the main menu select “processing” and click on “toolbox” to activate the processing tools in the QGIS window

c. From the “Grass algorithms” in the “processing toolbox” select the “V to points tool” for convert line into points. Because to find out lineament and drainage density there is no line density calculation tool in the QGIS, so we can convert them into points and can find the kernel density.

Grass algorithms

 Figure 2: V to points tool from Grass algorithms

When you click on the tool a window will open

V to points

d. Set the input line layer as “drainage”

c. Set the maximum distance between the map units as “100”

e. Set the output file name and the folder and press “OK”

Figure 3: shows the points created along with the line

Figure 3: shows the points created along the line

g. Similarly generate the points for lineament also

Figure 4: Points generated with lineament lines

Figure 4: Points generated with lineament lines

h. Click on the “Grass algorithms” and select the “V Kernel tool” to find out the density map

Figure 5: “V Kernel tool box”

Figure 5:  “V Kernel tool box”

i. V kernel tool window will open, set the Input layer name as “lineament_points” and set the “output file name and folder”- press “OK”

Figure 6: Lineament density

Figure 6: Lineament density

j. Similarly find the drainage density also

Figure 7: drainage density

Figure 7: drainage density

Process 2:  Convert the Vector File into Raster

a. From the main menu select “Raster” –Select “Conversion” and click on “Rasterize (Vector to raster)

Rasterize window

Figure 8: Rasterize window

Rasterize window will open

b. Set the Input file “Gemorphology.shp”

c. Set the desired attribute field

d. Set the output file name and folder

e. Click “OK”

Geomorphology raster is created

Figure 9: Geomorphology raster is created

f. Similarly rasterize the other themes (Soil, Lithology and land use / land cover)

Raster

Figure 10: Land use Raster

Lithology Raster

Figure 11:  Lithology Raster

Soil raster

Figure 12: Soil raster

Slope map (Prepared from DEM)

Figure 13: Slope map (Prepared from DEM)

Process 3:  Ordered Weighted Averaging

a. Select “SAGA geo algorithm” from the Processing tools and click on “Grid Analysis and select Ordered weighted averaging”

( note: if you do not see the tool then make sure you switch to “Advanced Interface” button located at the bottom from “simplified interface“)

Ordered weighted averaging tool box

Figure 14: Ordered weighted averaging tool box

b. Click on the “input grids and set the themes example “Soil”

c. Set the weights as per the table we are prepared for soil its 10

d. Input rest of the numbers from the above table

d. Set the output file name and folder

e. Press “OK”

shows the Input grid selection and assigning weightshows the Input grid selection and assigning weight

Figure 15 and 16: shows the Input grid selection and assigning weight

raster overlay analysis result

Figure 17: raster overlay analysis result

We found the ground water potential zones

   Green most suitable

   Light green moderate suitable

   Brownish yellow less suitable

NOW READ:

Quick and easy to create Map – Learn basics about the QGIS