QGIS/C2/Raster-Data-Styling/English-timed

From Script | Spoken-Tutorial
Jump to: navigation, search
Time Narration
00:01 Welcome to this tutorial on Raster Data Styling in QGIS.
00:07 In this tutorial, we will learn, to style a continuous raster.
00:13 To write an expression in Raster Calculator.
00:17 About raster properties.
00:20 To record this tutorial, I am using,

Ubuntu Linux OS version 16.04

00:28 QGIS version 2.18
00:32 To follow this tutorial you should be familiar with QGIS interface.
00:38 For the prerequisite tutorials please visit our website.
00:43 Download the folder given in the Code files link, located below the player.
00:49 Extract the contents of the downloaded zip file and save it in a folder.
00:56 Here is my Code files folder.
00:59 Double-click to open the folder.
01:02 In this folder you will find Population Density grid files for the entire world, for the year 2000 and 1990.
01:12 There are two files in ASCII format, with .asc file extension.
01:20 Let us open these files in QGIS.
01:24 Close the Code files folder.
01:27 Here I have opened QGIS interface.

Click on Layer menu.

01:34 From the drop-down select Add Layer.
01:38 From the sub-menu, select Add Raster Layer.
01:43 A dialog-box opens. I will navigate to the Code file folder on the Desktop.
01:52 Select the two files with .asc file extension.
01:58 Press and hold the Ctrl key, click on both the files.
02:04 Click on Open button.
02:07 Coordinate Reference System Selector opens.
02:11 In some settings, CRS will be selected automatically.
02:17 In such a situation this window may not open.
02:21 If Coordinate Reference System Selector does not open, ignore this step and proceed with the next step.
02:30 Here I will select WGS 84 EPSG 4326 from the list.
02:39 Click on OK button.
02:42 Since we are adding two layers at the same time, Coordinate Reference System Selector opens once more here.
02:51 Again select WGS 84 EPSG 4326.
02:58 Click on OK button.
03:01 On the canvas you will see a world map rendered in grayscale.
03:07 Lighter pixels indicate higher population and darker pixels indicate lower population.
03:15 In the Layers Panel, you will see both the raster layers loaded.
03:21 Each pixel in the raster has a value of population density for that grid.
03:27 To see the value of the pixel, click on Identify Features tool on the top-right corner of the toolbar.
03:35 Use the mouse wheel to zoom in the map.
03:38 Click anywhere on the raster map.
03:41 The pixel value will be displayed in the Identify Results panel.
03:48 Observe that the lighter pixel has higher value and the darker pixel has lower value.
03:57 Close the Identify Results panel.
04:00 Zoom out the map. Click on Pan Map tool and adjust the map on the canvas.
04:09 Population density pattern can be better visualized with an appropriate type of styling.
04:16 Right-click on the 1st layer in the Layers Panel.
04:21 From the context menu select Properties option.
04:26 Layer Properties dialog-box opens.
04:30 In the dialog-box select Style tab.
04:35 Under Band Rendering section, change the Render type to Singleband pseudocolor.
04:42 Let the Interpolation be Linear.
04:46 In the Color drop-down, select Spectral.
04:51 Scroll down.
04:54 Select Mode as Continuous.

Click on Classify button.

05:00 You will see 5 new color values created.
05:05 Click on Apply button and OK button at the bottom right corner of the dialog-box.
05:14 Back in the QGIS Canvas, you will see the raster map displayed in 5 classes of spectral color rendering.
05:24 Follow the same steps as shown for 1st layer and change the raster style for 2nd layer.
05:45 For our analysis, we would like to find areas with largest population change between years 1990 and 2000.
05:54 For this, we need to find the difference between each grid’s pixel values in both the layers.
06:02 For these calculations, we will use Raster Calculator tool.
06:07 Click on Raster menu from the menu bar.

From the drop-down, click on Raster calculator.

06:16 Raster Calculator dialog-box opens.
06:20 In the Raster bands section, the names of the bands are displayed.
06:26 Since each of our rasters have only 1 band, you will see only 1 entry per raster.
06:33 The raster calculator can apply mathematical operations on the raster pixels.
06:40 In this case we want to enter a simple formula to subtract the population density for the year 1990

from the population density for the year 2000.

06:52 Under Raster bands section, select the layer by double-clicking on the raster layer for the year 2000.
07:00 The expression is now added to the Raster calculator expression section.
07:06 From the Operators section, click on subtraction operator button.
07:12 Again from the Raster bands section, double-click on raster layer for the year 1990.
07:20 The formula for the calculation is now displayed in the Raster calculator expression section.
07:27 Under Result Layer section, you will see Output layer box.
07:33 In the box, type the name of your output layer as pop-change.tif.
07:41 In the Output format drop-down, select Geo TIFF.
07:47 Output CRS option is automatically selected. Leave it as such.
07:54 Check the box next to Add result to project.
08:00 Click on OK button at the bottom of the dialog-box.
08:04 You will see the new layer load in Layers Panel.
08:08 To view the map for the 3rd layer, in the Layers Panel uncheck the check boxes for pop-2000 and pop-1990 layers.
08:21 We can create a much more informative map by changing the style of this layer.
08:27 Right-click on the pop-change layer.

From the context menu, select Properties option.

08:36 Layer Properties dialog-box opens.
08:40 We want to style the layer such that, pixel values in certain ranges get the same color.
08:47 Click on Metadata tab, scroll down to Properties section.
08:55 Note the maximum and minimum values.
08:59 Maximum value is close to 6000.
09:02 Minimum value is just above -2000.
09:06 Go to the Style tab.

Under Band Rendering, select Singleband pseudocolor as the Render type.

09:14 Set the Interpolation to Discrete.
09:19 Scroll down.

Locate Add Values Manually button.

09:25 This is a green plus symbol button located next to Classify button.
09:31 Click the Add Values Manually button 4 times to create 4 unique classes.
09:39 These values show-up in the middle panel.
09:43 Here we have to change the values in each row.
09:47 Population values lower than the value entered, will be given the color of that entry.
09:54 Double-click on the first entry in the Values column to change the value.
10:00 We have observed, the minimum value in our metadata analysis is just above -2000.

Type -2000 in the first entry.

10:12 Double-click on the color-box and change the color.
10:20 Double-click on the Label column in the first row.

Type, No Data values.

10:28 Similarly fill in all the values and labels as shown here.
10:33 In the 2nd row, -10 to denote Negative change.
10:46 In the 3rd row, 10 to denote Neutral.
10:59 Finally 6000 to denote Positive change.
11:03 Because our maximum value from the meta data analysis is close to 6000.
11:23 At the bottom-right corner of the window, click on Apply button and then OK button.
11:30 Now on the canvas you will see a much more powerful visualization of the population data.
11:37 Here you can see clearly areas which has seen positive and negative population density changes.
11:46 Areas colored in blue denote the positive population change.
11:52 Areas in green show negative change.
11:56 Areas in pink have not see too much population change.
12:02 Let us summarize,
12:04 In this tutorial we have learnt,

to style a continuous raster, to write an expression in raster calculator, About raster properties.

12:17 As an assignment

Using the population data given in the Code files link, Create a new raster file which shows only negative population change.

12:28 Hint: Use Raster Calculator, write an expression to select population change less than 0.
12:36 Your completed assignment should look as shown here.
12:41 The video at the following link summarises the spoken tutorial project. Please download and watch it.
12:49 The spoken tutorial project team conducts workshops and gives certificates on passing online tests

For more details please write to us.

13:00 Please post your timed queries on this forum.
13:04 The Spoken Tutorial Project is funded by NMEICT, MHRD Government of India.

More information on this mission is available at the following link.

13:16 This tutorial is contributed by Prajwal.M from NIT Surathkal and Snehalatha from IIT Bombay.

Thank you for watching.

Contributors and Content Editors

PoojaMoolya