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

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
    • Open the province layer: khm_admbnda_adm1_gov_20181004
    • Select Mondolkiri province
    • Right-click on the name of the layer → Export → Save Selected Features As…
    • 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
  • Run

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.

Slope

Aspect

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.

\[ NDVI = \frac{Near\;Infrared\;Band - Red}{Near\;Infrared\;Band + Red} \]

  • 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