8.2. Lesson: Modification de la symbologie des Raster

Toutes les données raster ne sont pas des photographies aériennes. Il y a beaucoup d’autres formes de données raster et dans beaucoup de ces cas, il est essentiel de symboliser les données proprement, de sorte qu’elles deviennent bien visibles et utiles.

Objectif de cette leçon : Changer la symbologie d’une couche raster.

8.2.1. basic Try Yourself

  1. Use the Browser Panel to load the new raster dataset;

  2. Load the dataset srtm_41_19_4326.tif, found under the directory exercise_data/raster/SRTM/;

  3. Once it appears in the Layers Panel, rename it to DEM;

  4. Zoom to the extent of this layer by right-clicking on it in the Layer List and selecting Zoom to Layer.

Ce jeu de données est un Modèle Numérique d’Élévation (MNE). C’est une carte de l’élévation (altitude) du terrain, qui nous permet par exemple de voir où les montagnes et vallées sont.

While each pixel of dataset of the previous section contained color information, in a DEM file, each pixel contains elevation values.

Once it’s loaded, you’ll notice that it’s a basic stretched grayscale representation of the DEM:

../../../_images/greyscale_dem.png

QGIS a automatiquement appliqué un étirement à l’image à des fins de visualisation et nous allons en apprendre davantage sur la façon dont cela fonctionne avant de continuer.

8.2.2. basic Follow Along: Modification du style d’une couche Raster

You have basically two different options to change the raster symbology:

  1. Within the Layer Properties dialog for the DEM layer by right-clicking on the layer in the Layer tree and selecting Properties option. Then switch to the Symbology tab;

  2. By clicking on the symbology button right above the Layers Panel. This will open the Layer Styling anel where you can switch to the Symbology tab.

Choose the method you prefer to work with.

8.2.3. basic Follow Along: Singleband gray

When you load a raster file, if it is not a photo image like the ones of the previous section, the default style is set to a grayscale gradient.

Let’s explore some of the features of this renderer.

../../../_images/dem_layer_properties.png

The default Color gradient is set to Black to white, meaning that low pixel values are black and while high values are white. Try to invert this setting to White to black and see the results.

Very important is the Contrast enhancement parameter: by default it is set to Stretch to MinMax meaning that the grayscale is stretched to the minimum and maximum values.

Look at the difference with the enhancement (left) and without (right):

../../../_images/enhancement.png

But what are the minimum and maximum values that should be used for the stretch? The ones that are currently under Min / Max Value Settings. There are many ways that you can use to calculate the minimum and maximum values and use them for the stretch:

  1. User Defined: you choose both minimum and maximum values manually;

  2. Cumulative count cut: this is useful when you have few extreme low or high values. It cuts the 2% (or the value you choose) of these values;

  3. Min / max: the real minimum and maximum values of the raster;

  4. Mean +/- standard deviation: the values will be calculated according to the mean value and the standard deviation.

8.2.4. basic Follow Along: Singleband pseudocolor

Grayscales are not always great styles for raster layers. Let’s try to make the DEM layer more colorful.

  • Change the Render type to Singleband pseudocolor: if you don’t like the default colors loaded, click on Color ramp and change them;

  • Click the Classify button to generate a new color classification;

  • If it is not generated automatically click on the OK button to apply this classification to the DEM.

../../../_images/dem_pseudocolor_properties.png

Vous verrez le raster qui ressemble à ceci :

../../../_images/pseudocolor_raster.png

This is an interesting way of looking at the DEM. You’ll now see that the values of the raster are again properly displayed, with the darker colors representing valleys and the lighter ones, mountains.

8.2.5. Follow Along: Changing the transparency

Sometimes changing the transparency of the whole raster layer can help you to see other layers covered by the raster itself and better understand the study area.

To change the transparency of the whole raster switch to the Transparency tab and use the slider of the Global Opacity to lower the opacity:

../../../_images/global_transparency.png

More interesting is changing the transparency of single pixels. For example in the raster we used you can see an homogeneous color at the corners:

../../../_images/corner_values.png

To set this values as transparent, the Custom Transparency Options menu in Transparency has some useful methods:

  • By clicking on the signPlus button you can add a range of values and set the transparency percentage of each range chosen;

  • For single values the contextHelp button is more useful;

  • Click on the contextHelp button. The dialog disappearing and you can interact with the map;

  • Click on a corner of the raster file;

  • You will see that the transparency table will be automatically filled with the clicked values:

    ../../../_images/click_transparency.png

  • Click on OK to close the dialog and see the changes.

    ../../../_images/good_raster.png

    See? The corners are now 100% transparent.

8.2.6. In Conclusion

These are only the basic functions to get you started with raster symbology. QGIS also allows you many other options, such as symbolizing a layer using paletted/unique values, representing different bands with different colors in a multispectral image or making an automatic hillshade effect (useful only with DEM raster files).

8.2.7. Références

Le jeu de données SRTM a été obtenu depuis http://srtm.csi.cgiar.org/

8.2.8. What’s Next?

Maintenant que nous pouvons voir nos données correctement affichées, étudions comment nous pouvons davantage les analyser.