Tutorial QGIS – 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:

https://apps.sentinel-hub.com/eo-browser/

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