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Time Narration
00:01 Welcome to this tutorial on Interpolation Methods in QGIS.
00:07 In this tutorial, we will learn about Interpolation methods.
00:12 Inverse Distance Weighting (IDW) and

Triangulated Irregular Network (TIN)

00:18 Here I am using

Ubuntu Linux OS version 16.04

00:24 QGIS version 2.18
00:28 To follow this tutorial you should be familiar with QGIS interface.
00:34 For the prerequisite tutorials in this series, please visit this website.
00:40 Data files required for this tutorial are provided in the Code files link.
00:46 Please download and extract the contents of the folder.
00:51 I have saved this folder on the Desktop.

Double-click on the folder to open it.

00:59 Locate Air Stations.shp in the extracted folder.
01:04 This file shows meteorological stations located in Maharashtra.
01:10 Interpolation is a method to create continuous surface from discrete points.
01:17 Two methods of interpolation are available in QGIS.
01:22 Inverse Distance Weighting (IDW) and Triangulated Irregular Network (TIN)
01:28 Interpolation Plugin is used to create interpolated raster from Point layer.
01:35 Open the QGIS interface.
01:38 Here I have opened the QGIS interface.
01:43 Enable the interpolation plugin using Plugins menu as shown here.
01:49 Check the check-box for Interpolation plugin.

Close the dialog-box.

01:56 Open the Raster menu.
01:59 Interpolation option is now added in the Raster menu.
02:04 Click on Add Vector Layer tool. Add vector layer dialog-box opens.
02:11 Click on Browse button and navigate to Code files folder.

Here we will select two files.

02:20 Select AirStations.shp file.
02:24 Hold the Ctrl key on the keyboard and click on MH_Districts.shp.
02:32 Click on the Open button.
02:35 Click on Open button on the Add vector layer dialog-box.
02:40 On the canvas we see map of Maharashtra state.
02:45 Locations of Air stations in each district are shown as point features.
02:52 Let us label these point features.
02:56 Right click on the Air Stations layer.
03:00 Click on Properties option to open Layer Properties dialog box.
03:06 Select the Labels option located on the left panel.
03:11 Click on the drop-down located at the top.
03:15 Select Show labels for this layer from the drop-down.
03:20 In the Label with drop-down, select Air underscore Pollut.

Scroll down.

03:28 Here you will find various options to modify the label style.
03:33 Choose the required style and click on OK button.
03:38 On the canvas, points with the labels will be displayed.
03:43 Open the attribute table for Air Stations dot shp layer.
03:49 In the attribute table Nitrogen Oxides levels for every station is given.
03:57 We will interpolate Air Stations layer by the Nox attribute.
04:03 Here we will use IDW method for interpolation.

Close the attribute table.

04:11 Inverse Distance Weighting method gives weights to sample points.
04:17 It is used for interpolating data such as temperature, rainfall, population etc.
04:26 Back to the QGIS interface.
04:29 Click on the Raster menu.
04:32 Click on the interpolation plugin.
04:35 Interpolation plugin dialog-box opens.
04:39 In the Input section,

Select Air Stations as option as the Vector layers.

04:46 Here, by default, Air Stations layer is already selected.
04:52 Select Interpolation attribute as NOx.
04:57 Click on the Add Button.
05:00 This will add Air Stations dot shp layer with Nitrogen Oxide attribute.
05:06 Select Points in the Typedrop-down.

Here Points is selected by default.

05:14 Go to the Output section.
05:17 Select Interpolation method as Inverse Distance Weighting.

Leave all the settings as default.

05:26 Click on the three dots button next to Output file.
05:30 Save the output as an IDW underscore Stations in the desired folder.

I will save it on Desktop.

05:40 Check, Add result to project if it is unchecked.
05:45 Click on OK button.
05:47 The map with black and white areas will be displayed on the screen.
05:53 White areas represent high levels of Nitrogen Oxides.
05:58 Black areas represent low levels of Nitrogen Oxides.
06:03 For more clarity, we will change the symbology of the layer.
06:08 Open Layer properties for the IDW layer.
06:13 From the left panel, click on the Style option.
06:17 Select Render type as Single band Pseudocolor.
06:22 Select Discrete from Interpolation drop-down.
06:26 Choose Spectral from color drop-down. Check the Invert check-box.
06:33 Click on the Classify button.
06:36 Leave all other settings as default.
06:40 Click on Apply and OK button.
06:44 Map with areas in Spectral colors is displayed on the canvas.
06:50 The red colored areas have high concentration of Nitrogen Oxides.
06:56 Blue areas have least concentration of Nitrogen Oxides.
07:01 Save the project using Save tool from the tool bar.
07:06 Give an appropriate name. Choose a convenient location.
07:12 Click on Save button.
07:15 Let us now learn about the Triangulated Irregular Network interpolation method.
07:22 TIN is used to create a surface formed by triangles.
07:28 This is based on nearest neighbor point information.
07:33 TIN method is commonly used for elevation data.
07:38 Open a new QGIS window.

Click on New tool on the tool bar.

07:45 Use the Add Vector Layer tool to load Points dot shp layer.
07:52 Open Attribute table of Points layer.
07:56 Notice the Elevation data for each point feature.
08:01 Close the Attribute table.
08:04 Again open Interpolation window from Raster menu.
08:09 In the Input section, select Points layer in the Vector layers drop-down.

Select elevation as Interpolation attribute.

08:20 Click on Add button.

This will add Points layer with elevation attribute for interpolating.

08:28 Points in Type drop-down is chosen automatically.

Leave it as such.

08:34 In the Output section, select Triangular interpolation as Interpolation method.
08:41 Save the output file as TIN-Stations and click on OK button.
08:49 Map showing triangulated interpolation appears on the canvas.
08:54 Change the symbology for this layer.
08:58 Follow the same steps as we did for IDW layer.
09:12 The map is now displayed in Spectral colors.

Areas in red represent high elevation.

09:21 Areas in blue represent low elevation.
09:25 Save the maps using the Save tool on the toolbar.
09:30 Let us summarize,

In this tutorial we have learnt about two methods of Interpolation.

09:37 Inverse Distance Weighting (IDW) and Triangulated Irregular Network (TIN)
09:43 For the assignment,

Create IDW interpolated map for Air Stations layer with SO2 attribute.

09:52 Your map should look as shown here.
09:56 This video summarizes the Spoken Tutorial project.

Please download and watch it.

10:03 The spoken tutorial project team conducts workshops and gives certificates.

For more details please write to us.

10:13 Please post your timed queries in this forum.
10:17 The Spoken Tutorial project is funded by MHRD Government of India.

This tutorial is contributed by Ambadas Maske from College of Engineering Pune, Snehalatha Kaliappan and Himanshi Karwanje from IIT Bombay.Thank you for joining.

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