Welcome to this tutorial on DEM Analysis in QGIS.
In this tutorial, we will learn to,
Download DEM data from SRTM data website.
Show Hillshade of DEM.
Here I am using Ubuntu Linux OS version 16.04
QGIS version 2.18 and A working internet connection.
To follow this tutorial you should be familiar with QGIS interface.
To view the prerequisite tutorials in this series, please visit this website.
Digital Elevation Model or DEM is a raster file.
It shows elevation data for each raster cell.
DEMs are used to represent the bare earth terrain.
The terrain is usually devoid of vegetation and man-made features.
DEMs are used for calculations and analysis of an area, based on the elevation.
Let us download DEM data.
Open the given link in any web browser.
Shuttle radar topography mission (SRTM) data website opens.
SRTM data from this website can be downloaded freely.
On the Download Manager page, the elevation models are arranged into tiles.
Two options for Tile Size and Format are available.
We can choose the tile size and format by clicking the radio buttons.
Scroll down the page to the world map.
Use + sign on the left corner of the map to zoom in the world map.
Click on Maharashtra tile.
Click on Search button located on the top-left corner of the world map.
Download window opens.
Scroll down to Description heading. Click on Download SRTM link at the bottom.
A dialog-box opens, select Save File option. Click on OK button.
On my system, zip file downloads to the Downloads folder.
Extract the contents of the zip file.
Right-click and select Extract Here option.
Double-click on the extracted folder. This is a DEM dataset.
Here we see many files with different file extensions.
Close the folder.
Open the QGIS interface.
Click on, Layer menu on the menu bar.
Select Add Layer, from the sub-menu, click on Add Raster Layer option.
Data source dialog-box opens.
Navigate to the SRTM folder downloaded from the SRTM website.
From the contents of the folder, select the file with .tif extension. Click on Open button.
On the canvas you will see DEM of the terrain.
DEM contains all the 3D information about the terrain.
Each pixel on the raster image represents the average elevation at that location. This elevation is given in meters.
Dark pixels represent areas with low altitude.
Lighter pixels represent areas with high altitude.
Let us begin the DEM analysis of this map.
Click on Raster menu on the menu bar.
Click on Analysis from drop down. From the sub-menu click on DEM (Terrain models).
DEM dialog-box opens.
Input file field has DEM layer as the default selection.
Click on Select button next to Output file.
Save the results to.. Dialog-box opens.
In the dialog-box, name the file as Hillshade.tif.
I will save it on the Desktop.
Click on Save button.
Select Hillshade as Mode option.
Here by default Hillshade is already selected.
Check the check-box next to Load into canvas when finished.
Here by default it is already selected.
Leave the default settings as such.
Click on Ok button.
A pop-up box opens with message Processing Completed. Click on OK button.
Click OK button in the Qgis.bin dialog-box.
Click on Close button on the DEM dialog-box.
A new layer, Hillshade is now added in the Layers panel.
On the canvas you will see a raster map in Hillshade mode.
This map is generated using light and shadow to create a 3D image.
To make the model more pronounced, we will use Hillshade as an overlay.
Now we will change the symbology of the original DEM layer.
Right-click on the srtm layer in the Layers Panel.
From the context menu select Properties option.
Layer Properties dialog-box opens.
Select Style from the left panel.
Under Band Rendering section, change the Render type to Singleband pseudocolor.
Under Load minimum/maximum values, click on minimum/maximum radio button.
Select Linear from Interpolation drop-down.
This is a default selection here. From the Color drop-down select Spectral.
Scroll down. Select Mode as Continuous from the drop down.
Click on Classify button.
5 new color values are created.
The colors represent values of elevation of raster from lowest to highest.
Click on Apply button and OK button at the bottom right corner.
In the Layers panel, disable the Hillshade layer .
Uncheck the check-box against Hillshade layer.
Now on the canvas you will see a map in spectral colors.
The red shaded terrain is least elevated and blue is most elevated.
Enable the Hillshade layer.
Open the Layers Properties dialog-box.
Select Transparency from left panel.
Set the Global transparency to 50% by dragging the slider.
Click on Apply button and OK button.
Zoom in the map.
On the canvas now we see an enhanced topography of the landscape.
Let us summarize,
In this tutorial we have learnt to Download DEM data from SRTM data website.
Show Hillshade of DEM.
Here is the assignment.
Visualize the terrain using Slope mode for the raster map. Change the symbology for the Slope layer
Hint: Select the Mode as Slope and use it as overlay.
Your completed assignment should look as shown here.
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This tutorial is contributed by Vaishnavi Honap from college of Engineering Pune, Snehalatha Kaliappan and Himanshi Karwanje from IIT Bombay.Thank you for joining.