Not all raster data consists of aerial photographs. There are many other forms of raster data, and in many of those cases, it’s essential to symbolize the data properly so that it becomes properly visible and useful.
The goal for this lesson: To change the symbology for a raster layer.
This dataset is a Digital Elevation Model (DEM). It’s a map of the elevation (altitude) of the terrain, allowing us to see where the mountains and valleys are, for example.
Once it’s loaded, you’ll notice that it’s a basic stretched grayscale representation of the DEM. It’s seen here with the vector layers on top:
QGIS has automatically applied a stretch to the image for visualization purposes, and we will learn more about how this works as we continue.
These are the current settings that QGIS applied for us by default. Its just one way to look at a DEM, so lets explore some others.
You’ll see the raster looking like this:
This is an interesting way of looking at the DEM, but maybe we don’t want to symbolize it using these colors.
You will now see a totally gray rectangle that isn’t very useful at all.
This is because we have lost the default settings which “stretch” the color values to show them contrast.
Let’s tell QGIS to again “stretch” the color values based on the range of data in the DEM. This will make QGIS use all of the available colors (in Grayscale, this is black, white and all shades of gray in between).
But what are the minimum and maximum values that should be used for the stretch? The ones that are currently under Min and Max values are the same values that just gave us a gray rectangle before. Instead, we should be using the minimum and maximum values that are actually in the image, right? Fortunately, you can determine those values easily by loading the minimum and maximum values of the raster.
Notice how the Custom min / max values have changed to reflect the actual values in our 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:
Yes, there is. Now that you understand what needs to be done, you’ll be glad to know that there’s a tool for doing all of this easily.
Remove the current DEM from the Layers list.
Load the raster in again, renaming it to DEM as before. It’s a gray rectangle again...
Enable the tool you’ll need by enabling View ‣ Toolbars ‣ Raster. These icons will appear in the interface:
The third button from the left Local Histogram Stretch will automatically stretch the minimum and maximum values to give you the best contrast in the local area that you’re zoomed into. It’s useful for large datasets. The button on the left Local Cumulative Cut Stretch ... will stretch the minimum and maximum values to constant values across the whole image.
You can try the other buttons in this toolbar and see how they alter the stretch of the image when zoomed in to local areas or when fully zoomed out.
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 standard deviations, or representing different bands with different colors in a multispectral image.
Now that we can see our data displayed properly, let’s investigate how we can analyze it further.