Tutorial QGIS – Using raster data

4. Using Raster Data

In this exercise, we will use a Digital Elevation Model (DEM), called SRTM (Shuttle Radar Topography Mission), provided by NASA and NGA (formerly NIMA).

4.1     Opening and displaying raster data

      • Open the Data Source Manager  , choose the “Raster” tab and browse to srtm_cambodia.tif
      • Double-click on the name to open the Layer Properties, and go to the Symbology tab
      • In Render type, choose “Hillshade” and Apply to view the Elevation data with shadow
      • Try also the “Singleband pseudocolor” for a color view. You can adjust the classification mode and increase the number of classes. You can choose the color ramp and invert colors.

4.2     Clipping a raster data

QGIS offers the possibility to cut a raster according to the contour of vector layers.

Exercise: Extract the SRTM on Mondolkiri province (east of Cambodia)

      • Create a shapefile layer of Mondolkiri
        1. Open the province layer: khm_admbnda_adm1_gov_20181004
        2. Select Mondolkiri province
        3. Right-click on the name of the layer / Export / Save Selected Features As…
        4. Browse to your folder and save the Mondolkiri.shp layer
      • Raster / Extraction / Clip Raster by Mask Layer…
      • Input layer = srtm_cambodia.tif
      • Mask layer = Mondolkiri.shp
      • Run

4.3     Changing the projection of a raster

The DEM srtm_cambodia.tif is in WGS84 with units in decimal degrees. To make some calculations related to distances, we will need the meter as a unit. It is therefore necessary to reproject our raster.

Exercise: Reproject the Mondlkiri DEM in the local projected CRS

      • Go to Raster Menu / Projections / Warp (Reproject)
      • Input layer = srtm_Mondkiri.tif
      • Target CRS: choose the local projected CRS
      • Browse to your folder and give a name to the Reprojected raster

4.4     Deriving information from DEM: slopes, aspect, hillshading, contours

Several tools are provided in the Analysis Menu from the Raster Menu


Exercise: Calculate slope and aspect in Mondolkiri province

      • Go to Raster Menu / Analysis / Slope…
      • Choose the Mondolkiri DEM
      • Browse to your folder and give a name to the raster of slopes
      • Run
      • The same applies to the calculation of exposure (aspect) and hillshading.

Note:    Calculating the slope in QGIS is simple. The output is also a raster of the same resolution (90 meters) as the source raster.


Exercise: Calculate the contour lines of Mondolkiri province

      • Go to Raster Menu / Extraction / Contour…
      • Choose the Mondolkiri DEM
      • Choose the interval between lines = 100 meters
      • Browse to your folder and give a name to the shapefile of contour lines
      • Run

4.5     Raster calculator

The raster calculator allows you to make calculations between several rasters or on the values of a single raster.

Exercise: extract the DEM over 500 meters high

      • Open the raster calculator: Raster / Raster calculator…
      • Select the Mondolkiri raster
      • Browse to your folder and give a name for the output layer
      • Raster Calculator Expression: Double-click the raster name in the “Raster Bands” list to enter its name in the expression then write “> 500”
      • OK

4.6     Downloading and using Sentinel-2 images

Browse and download Sentinel-2 images

We will use here the EO Browser provided by Sentinel Hub and developed by Sinergise:


Exercise: download and display a Sentinel-2 image from Phnom Penh

      • Open the EO Browser and search for Phnom Penh in the top right search box. Zoom the map to Phnom Penh
      • Choose Sentinel-2 L1C as Data source, reduce the maximum cloud coverage to 20%
      • Choose a time range and search
      • Choose one tile among the results and click on Visualize
      • Look at the different indices available to observe their distribution
        • Click on the download icon to get the weblink to download the chosen image
        • Download requires to be registered in the Copernicus Open Access Hub (https://scihub.copernicus.eu/)

Display a Sentinel-2 image in True color

      • Add the raster bands of your Sentinel-2 images: Menu Layer / Add Layer / Add Raster Layer…
      • Browse and choose the different bands to open B02 (Blue), B03 (Green) and B04 (Red)
      • Miscellaneous / Build Virtual Raster…
      • Choose the 3 input layers
      • Run
      • Open the layer properties of your virtual raster and go to Symbology tab
      • Choose the correct bands for each color and click OK

Exercise: Calculate the NDVI of this Sentinel-2 image from Phnom Penh

The normalized difference vegetation index (NDVI) is a simple index that can be calculated from optical satellite images to show the vigor of vegetation.

NDVI = (Near Infrared Band – Red Infrared Band) / (Near Infrared Band + Red Infrared Band)

      • Go to Raster / Raster Calculator
      • Browse to your folder and give a name for the NDVI
      • Write the equation of NDVI
      • OK
      • Open the layer properties of your NDVI and go to Symbology tab
      • Select Render type = Singleband pseudocolor
      • Choose a Red to Green color ramp and a method of classification
      • OK

GeoHealth Team, IRD-Espace-Dev, IPC