Buffer Analysis Using QGIS

Introduction

GIS provide many tools for understanding and analysis of spatial data base. GIS has capabilities to analyze feature itself as well as its surrounding area and impact of surrounding area on that earth feature. GIS functions play significant role in the data creation, edit, manipulation and analysis. There are ‘n’ number of function available in GIS based on mathematical, statistical and spatial approach such proximity analysis, buffer analysis, spatial analysis etc. 

Buffer analysis is used to delineate areas in proximity to geographic spatial feature. In this a buffer is generated around that existing geographic feature by using distance parameters and this buffer area is used to identify and analyze spatial features comes into that buffer area or outside that buffer area. 

In the buffer analysis a buffer area of specified distance from geographic feature is created then this buffer area can be used to specify the spatial features fall in that buffer area and features are beyond the created buffer area. This type of buffer area is also known as buffer zone, which can be used in variety of application for monitoring and planning

Some examples are below
  • Industrial zones in to surrounding environment 
  • HazMat transportation buffer zone 
  • Forest protecting buffer zones 
  • Commercial buffer Zones and its impact on surrounding spatial features etc. 
  • Protection zone for airport/river/canal etc. 
  • Buffer area around Highway to assess Impact of Highway on surrounding  developments 
In this article buffer analysis is discussed, which is a very significant and important process for spatial data analysis in GIS. 

Buffer can be created to all three types of spatial features entities named point, line and polygon using distance parameters. In this article, linear feature and polygon feature are used as example to create buffer in QGIS. This linear feature is represented by line entity in spatial database and river as non spatial database.
 
How to create buffer using QGIS 

1. Open polygon layer shapefile (example: State boundary/ district boundary/tehsil boundary etc.) or Line layer shapefile (example: Road/Railway/ stream network etc.) in QGIS, for this click on Menu then Layer followed by Add Vector Layer as shown below

The Data Source Manager-Vector window will pop up.

Here you have to Browse (for the polygon layer shapefile folder/Line layer shapefile folder) and select the polygon shapefile/Line shapefile. Then click on Add button and close the window. 
For example here we have opened the “ChambalSA.shp” polygon shapefile (indicating watershed boundary)

and polygon is displayed on the screen as shown below

In the same way, we have opened the “ChambalRiver.shp” line shapefile (indicating stream network)as shown below



2. Now for creation of buffer, click on Menu then Vector followed by Geoprocessing Tools and Buffer as shown below

Buffer window will pop up. Now define the Input layer for which you want to create buffer for example here we have selected Input later “ChambalRiver”. Then define the Distance and its unit, indicating for how much distance you want to create buffer. Here we have selected 3 Km distance for river buffer. Then check the Box for ‘Dissolve result’ followed by defining the output file in Buffered. Don’t forget to check the check box ‘open output file after running algorithm’. Click ‘Run’.

The output of the river network with buffer is shown here.

Note: to see the river network on buffer, you can move the layers up or down in the Layers.

In the same way we have created buffer for polygon (watershed area) shapefile “ChambalSA. shp”. Here we have selected Input later “ChambalSA. shp”. Then define the Distance and its unit, indicating for how much distance you want to create buffer. Here we have selected 2 Km distance. Then check the Box for ‘Dissolve result’ followed by defining the output file in Buffered. Don’t forget to check the check box ‘open output file after running algorithm’. Click ‘Run’. Like this you can create multi distance buffers for analysis of your study area. For example here we have created the watershed area buffer of 2 km and 5 km distance as shown below.


One can also change the symbology of  layers and for this one has to go to the Layer properties by right click on output file under Layer section and select Properties. Do the changes in Symbology window and get the output of as per choice.

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