Visualisation#

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Visualising vector data#

Symbology#

QGIS offers various ways to visualize vector data. In the Symbology Tab, you can select between various symbolization methods

  • Assigns one symbol to every feature of the dataset, no matter if the attributes are different.

For example, assign a hospital symbol to a layer that only contains points showing the location of hospitals.

  • Classifies features into categories using an attribute (Value).

  • A category is created for each unique value of this attribute.

  • Each category can be assigned to a different symbol.

  • This can be used for nominal as well as ordinal data.

For example, assign a different symbol for each type of building (industrial, commercial, public, residential,…)

  • Creates classes for numerical data.

  • A colour gradient can be selected to represent the distribution of the data

For example, create 6 classes of population sizes and assign a color gradient from white to red to indicate the population size in a district.

  • Create rules using an expression and assign a symbol for the features where the rule applies.

  • You can specify more accurately the features you want to symbolize.

  • You can use values from different attributes (e.g. building type and city district).

  • The expression builder helps you create rules by displaying the available values, fields, operators, etc…

For example, select a symbol for every health facility that is a hospital and has exceeded it’s capacity.

Only display the outlines of polygons

In this example, we want to change the symbology of a single layer so that only the outlines of the polygons are visible.

To change the symbology of a single layer:

  1. Open the Styling panel and navigate to the symbology tab. By default, the symbology will be set to Single Symbol. This means that the same colours and contours will be applied to all the features in that layer.

  2. Click on Simple Fill

  3. Click on the arrow to the right of Fill Colour

  4. Check the Transparent Fill option.

../../_images/en_30.30.2_vector_layer_styling_transparent.png

Change the styling for multiple overlayed layers

In this exercise, we will apply the same style to all features in a layer, but we will change multiple layers and overlay them so each is visible in a different style. We have the polygons for 3 administrative levels.

../../_images/en_30.30.2_changing_layer_style_1.png

Fig. 20 Order the layers and navigate to the styling panel of the topmost layer#

  1. Add the Adm0, Adm1 and Adm2 shapefiles to your Session 2 project.

  2. Order the layers so they are all visible: Put the Adm2 layer at the bottom, then the Adm1 then Adm0. At first, this might look weird because Adm0 will cover everything.

  3. Change the symbology of the Adm0 layer by opening the styling panel and navigating to the Symbology tab.

../../_images/en_30.30.2_changing_layer_style_2.png

Fig. 21 Change the Fill type#

  1. Click on Simple Fill to open the style options.

  2. Expand the Fill Colour menu and check the Transparent Fill option. This will make only the boundaries visible, so we will be able to see the layer under this one.

  3. Choose a Stroke Colour, and make the Stroke Width 0.66 Millimeters.

  4. Click OK

  5. Repeat the same process for the Adm1 layer, using the same colour as for Adm0 (it will be in “Recent colors) and leave the stroke width at 0.26.

  6. Now we can see the boundaries of the country and its states, and behind that we can see the districts (Adm2).

  7. Lets make the district layer’s style consistent with the others.



  1. Choose a Fill Color

  2. Use the same Stroke Colour` as for Adm0 and Adm1, but make the width 0.1 Millimeters and the Stroke Style a Dash Line

  3. Click OK and look at your map: hopefully it’s starting to look nicer!

../../_images/en_30.30.2_changing_layer_style_3.png

Fig. 22 The styling of a vector data consists of the colour and the outline#

Use different styles in a single layer

We can use symbology to show the difference between features in the same layer. For example, it could be different types of buildings, quantities of Covid cases by district, or types of roads. We can choose a specific attribute of a dataset to assign different colors, outlines, or sizes to features:

  1. From your shapefile folder, drag the ACLED security incidents shapefile onto your map

  2. Open the Symbology tab for that layer and choose Categorized instead of Single Symbol.

Note

Categorized symbology is used when you have discrete variables.

../../_images/en_30.30.2_categorized_layer_symbology_1.png

Fig. 23 Change the symbology type to “categorized” and choose the Value (variable) you wish to display#

  1. Now we need to choose which attributes we want to display through the symbology. In this case, it could be the number of casualties, or the actor who perpetrated the act. Let’s categorize the features by event_type

  2. Click on Classify to list all the unique values contained in the event_type field (i.e. all the possible types of security incidents recorded in our table)

  3. Now we can change the style of each single value

  4. Double click on the value Explosions

  5. At the bottom of the Symbol selector window, choose a symbol to make Explosion points stand out.

  6. Click on OK, then Apply to preview what the layer will look like.

  7. Click OK again.

../../_images/en_30.30.2_categorized_layer_symbology_2.png

Fig. 24 By double clicking on the unique values in the classified list, you can change the symbol for each value#

Now we have a map of Nigeria where you can locate the areas that are affected by explosions more than others. On the map below, we also added text labels, which will be explained below.

../../_images/en_exercise_map_design_example_Nigeria.png

Fig. 25 Regions affected by explosions in Nigeria#

Style data based on variable ranges (”Graduated” styling)

If a layer contains numeric values that are continuous, they can be organized in intervals. These intervals can be displayed in graduated colours. In this exercise, we assign colours to Adm1 polygons based on the total population of each State.

  1. Download the NGA_Adm1_Pop shapefile [link!!] and save it in your shapefile folder

  2. In QGIS, turn off the Adm1 and Adm2 layer, leaving only Adm0

  3. Drag the shapefile NGA_Adm1_Pop into your map

  4. Open its Symbology options and choose Graduated

  5. Select the value you want to use to assign colours, in this case, it will be Population

../../_images/en_30.30.2_symbology_variable_ranges.png

Fig. 26 With variable ranges, select Graduated symbology and choose the attribute with continuous values#

  1. Click on Classify to list all values divided in classes

  2. Choose how many classes you want the data to be divided into ‒ let’s say 4

  3. By default, the colour ramp will be red. However, red is not the right colour to use for population count, as it is generally used to communicate negative elements, such as food insecurity or cholera cases

  4. Click on the arrow next to the colour ramp to choose another combination of colours - let’s say a color ramp from white to blue

  5. Click Apply to preview the look of your layer, then OK

../../_images/en_30.30.2_symbology_variable_ranges_2.png

Fig. 27 You can categorize the continuous values into classes and assign a colour ramp#

The following map shows the most populated States of Nigeria using a graduated colour categorization. These types of maps are called choropleth maps.

../../_images/en_map_design_example_variable_ranges.png

Fig. 28 A map showing the population of Nigerian states#

Labels#

Labels are text that display information or values of the data. In QGIS, you can either select Single Labels or Rule-based Labelling. For each option, an attribute (value) will be displayed on the map. Additionally, you can change the font, font size, colour and some other styling options for the label text.

  • Creates a single label style for every feature in the layer. You can select an attribute (value) which will be displayed. For example, the name of a settlement. You need to know which attribute displays the information you want to display. Look at the attribute table of the dataset to find it out.

  • Create rules using expressions to select accurately which features are to be labeled.

  • Each rule can have a different text formatting

Note

Label rendering

Sometimes the labels can obstruct other symbols. In that case, you can either adjust the placement of the labels in the Label tab, or use the Move a Label, Diagram, or Callout-tool in Label toolbar

By default, QGIS renders the labels so that they don’t overlap with other labels. This means that not all the labels will be visible if the data is dense or rendered close to each other. You can optimize the rendering under the rendering option.

Adding labels to a layer

  1. In the styling panel, click on the Labels tab underneath the Symbology tab.

  2. Select Single labels.

  3. "Value" is where you choose the attribute that will be displayed as a label. For example *ADM1_EN* will display the English names of Nigerian states for each feature in the data set.

  4. Let’s change the font: Open the Font dropdown menu and select Arial. Make the text Bold in the Style dropdown menu. Change the colour by clicking on Colour, and change the Size to 8 pt

  5. Let’s add a white buffer around the label. In the Labels tab, you will find a list with different options to style the labels. Right now, we are in the Text menu. Select Buffer and check the Draw text buffer option. This will make the labels stand out more on dark or crowded maps.

  6. Click Apply and OK.

../../_images/en_30.30.2_setting_up_labels.png

Fig. 29 Setting up labels in QGIS 30.30.2#

Adding different label styles to the same layer

Sometimes you will need to create two different label styles for different features of a single layer. In this example, we will create one label style for the Country Capital, and another one for the State Capitals

  1. Open the styling panel for the "NGA_settlements_nga" layer and click on the Labels tab

  2. Select Rule-based Labelling

  3. Click on the Add Rule button at the bottom (the “+”-sign) and create the first rule

  4. For Value, select "NAME" (so that the labels will show the name of each city), then click on the "ε"-button next to the Filter bar.

../../_images/en_30.30.2_adding_rule-based_labels.png

Fig. 30 To add rule-based labels, you need to enter an expression#

  1. In the central column, expand Fields and Values to display a list of all the fields in your layer and double-click on "CLASS" to add it to the expression frame on the left.

  2. In the right column, click on All unique to list all unique values contained in the Class field. In this dataset, "CLASS"=1 designates the capital city, whereas "CLASS"=2 designates other major cities. Make sure to familiarise yourself with the dataset at your disposal, so you know what the different attributes represent.

  3. Click on the "=" operator, then double-click on the value 1 (which represents the Country capital in this case). Click OK.

  4. Scroll down to change the label style. Make it Arial, bold, black, 12pt and add a white buffer.

  5. Repeat steps 4 to 9, but select Value 2 (State capitals) and make the label black, bold, 10pt, no buffer.

  6. Click Apply, the OK.

../../_images/en_30.30.2_adding_rule-based_labels_expression_builder.png

Fig. 31 The expression builder: Expression (left); building blocks, operators, fields and values(center); unique values (right)#

Add underlined labels

  1. Set up the labels by following the same steps as before.

  2. TO underline labels, click on the underline-button

Move labels independently

Sometimes the placement of labels is not ideal and can obstruct the readability of the map. In this case, you can move labels independently.

  1. On the label toolbar, there is an option to move labels independently. Click on it to activate the tool. (Note: In some cases, the label toolbar might not be visible. In this case, turn it on by navigating to View>Toolbars>activate the Label toolbar)

  2. Click on the label you want to move.

  3. You will be prompted to select the primary key for joining with internal data storage. You do not need to change it (you can select the ID field of the dataset) and click OK.

  4. Click on the label again, now you can move it freely.

Add labels to roads

When working with line features, the labels will align themselves parallel to the line representing the feature.

Visualising raster data#

Assigning a colour gradient to raster data#

To assign a colour gradient for raster data, you need to:

  1. Open the styling panel for the raster layer

  2. Navigate to the Symbology tab

  3. By default, the colour scheme is set to Singleband gray (if you only have one colour band in the data set). Click on Singleband Gray and switch to Singleband pseudocolour

  4. Click on the arrow to the right of the colour ramp. Here you can choose a pre-made colour ramp

  5. You can modify the colour ramp by clicking on the colour ramp.

../../_images/en_30.30.2_raster_data_colour_gradient.png

Fig. 32 Colour Ramp Selector#

In the colour ramp selector, you can adjust each colour step. On the bottom, you can see a plot for the Hue, Saturation, Lightness and Opacity. The latter three are especially useful to see how your colour ramp will translate. Gradients from light to dark are easier to read: Check if the plot for the Lightness has a more or less linear plot.

Styling a digital elevation model

Elevation data sets are frequently used to communicate the terrain on a map. By default, an elevation model will be displayed with a gray colour ramp. However, if you don’t need to know the elevation at certain points, you can choose to display the hillshade of the terrain. Hillshading will simulate the shadow of the terrain as if it would be exposed to a lightsource. In this example, we will use the elevation raster data (.tiff) of Algeria from the Humanitarian Data Exchange platform (humdata.org) To achieve this,

  1. Open the symbology tab

  2. Click on Render type and select Hillshade. You will have an option to select the direction of the light. Conventionally, the light-source is positioned in the North-West, so we can keep the default settings. In some cases with rough terrain, it can be useful to make the Hillshade Multidirectional.

  3. The Hillshade will be very dark and cover most of the map. We need to make it lighter…

Inverting the colour ramp#

In some cases, the colour ramp should be inverted to make it easier to read the map:

  1. Click on the arrow next to the Colour ramp to open the dropdown menu.

  2. Click on Invert Colour Ramp.

Exporting and Importing styles#

  1. Open the styling panel and click on styles. A dropdown menu will open with the option to export the layer styling.

  2. Since in this case, the styling is for exactly that dataset, you can leave all the boxes checked.

  3. Select a location and name for the styling. The styling will be saved as a .qml file. Make sure it is saved in the same folder as the dataset and give it the same name as the corresponding dataset. This way, when loading the data into QGIS, the styling will automatically be applied.

  1. Open the style manager: Settings > Style manager

  2. Click on import/export and select import items

  3. Navigate to the folder where the style is saved and click import.

  4. The style should now be available as a preset in the styling panel.

Note

You can also import styles directly in the styling panel of a layer. But it will not be added to your style library unless you save it into your library.

With the plugin “Plugin Resource Sharing”, you can install symbol and icon libraries used by the Red Cross and UN, as well as other useful symbols.

  1. Install the “Plugin Resource Sharing” by opening the plugin installation window and searching for the plugin.

  2. Once installed, open the plugin interface by clicking on plugin > Plugin Resource Sharing

  3. Search for packages by the Red Cross and UN

  4. Install the packages.

Now the symbols should be available in the styling manager in the SVG folder.

Tip

Make sure to check out the other resources available in the resource sharing plugin and see if they are useful to you.

  1. Open the styling panel and open the single marker options.

  2. Under Symbol layer type, select “SVG Marker”

  3. Scroll down to the SVG-Browser. Here you will find all the folder of your installed SVG-libraries.

If you have a library of SVG-symbols as a folder you can add them to your Styling manager.

  1. Open the style manager: setting > style manager

  2. Click on Import / Export and select Import items

  3. Navigate to the location where you have saved the library or style and select the file (in most cases .qml but the file type can also be .xml)

  4. Now you can select which symbols you wish to import. In most cases, you can select all symbols.

  5. Click on Import The new SVG-symbols are in your SVG library.