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

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