Styling Vector Data#

The previous chapter went over the fundamentals of graphical symbolisation and the visual variables. In this chapter, we want to apply our understanding of visual representation and learn how to use the styling panel in QGIS to customize

Styling Panel#

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Fig. 93 Styling panel in QGIS 3.30.2#

For each layer in QGIS, there is a styling panel where you can change the symbology, colour and label for the features in that layer. There are two ways to open the layer styling options in QGIS:

  1. Right click on the layer you wish to style and select properties. A new window will open up with a vertical tab section on the left. Navigate to the symbology tab.

  2. Open the layer styling panel by enabling it under View>Panels>Styling Panel. Usually, the panel will appear on the right side of map canvas.

On the left of the styling panel you can choose the different tabs to access different styling options.

In the styling panel you can change the styling for all features of a layer, set up categories for different symbols, create labels, and create colour ramps to differentiate between features with variable values.

Video: Opening the styling panel

Symbology for Vector Data#

You can use graphical variables to style vector data. As we have already learned, vector data can be either points, lines, or polygons. There are different options to symbolize these different types of vector data.

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Fig. 94 Symbolization for vector data; Source: White, T. (2017). Symbolization and the Visual Variables. *The Geographic Information Science & Technology Body of Knowledge (2nd Quarter 2017 Edition), John P. Wilson (ed.). DOI: 10.2222/gistbok/2017.2.3#

Note

Remember that the layer’s symbology is saved within your project file, not within your shapefile! If you share a shapefile with a colleague, it will have a different style when they add it to their own project.

In the symbology tab of the layer styling panel, you can set the

Simple Markers are generally used to create the symbols for most elements on a map. For example, simple markers are used to visualise streets, building outlines, waterbodies, administrative boundaries or other polygons. Most simple markers consist of a fill and an outline. Depending on the type of geometry in the layer, you will use have different symbology options.

  • The fill determines the fill colour of the symbol. You can change the colour and transparency. You are also able to make more complex fills such as a line pattern fill, or an SVG-symbol fill.

  • The outline determines the colour, type, and thickness of the outline. Next to the colour and transparency, the outline is the most critical for distinguishing between different elements. For example, thicker lines for roads usually signify roads of a higher order (such as highways), while thin dashed lines might signify footpaths, inaccessible to road vehicles.

  • You can either style a single symbol for each layer or use different styles based on a categorisation method.

In the Symbology Tab, you can select between various symbolization methods (see symbolisation_methods_m4). The most important ones are Single Symbol, Categorised, Graduated, and Rule-based.

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  • 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 (see Module 3: Geodata Classifification).

  • 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.

Below, we will go over some common styling methods used in cartography.

Styling administrative boundaries (Polygons)#

When creating situational reports, you will frequently use administrative boundaries. In the example below, we want to create an overview map using the administrative boundaries of Nigeria. In order to visualise all the three administrative boundaries at the same time, we need to set the symbology for each layer so the layers below are visible, and the hierarchy of the administrative levels is distinguishable.

Optional: Now it’s your turn

You can follow along by downloading the administrative boundaries of Nigeria by OCHA Nigeria

Only display the outlines of polygons#

Now, we want to change the symbology of a layer so that only the outlines of the polygons are visible. This is necessary to make layers below this one 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

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Video: Making the fill colour transparent

Adjusting the styles of multiple overlaying layers#

Step 1: Ordering the layers

  1. Import the administrative boundaries into your QGIS-project.

  2. We need to order the layers in the Layers panel so that the adm0-layer sits on top, followed by adm1 and adm2. At first, this might look weird because Adm0 will cover everything.

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Fig. 95 Order the layers and navigate to the styling panel of the topmost layer#

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

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

  3. 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.

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

  5. Click OK.

  6. 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.

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

  8. Let’s make the districts layer’s style consistent with the others.

  9. Choose a Fill Color

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

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

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Fig. 96 The styling of a vector data consists of the colour and the outline#

Video: Adjusting the style for multiple layers

Creating a choropleth map#

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 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 total_pop

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Fig. 97 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

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Fig. 98 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. 99 A map showing the population of Nigerian states#

Video: How to create a choropleth map

Creating a graduated symbols map#

Graduated Symbols are useful when you have more information on your map, and creating a choropleth map is not possible, or in situations when you want to communicate two variables on a single map. For example, it is easy to combine choropleth maps with graduated symbols. Creating graduated symbol maps is done in a similar way to creating choropleth maps, but it involves one extra step: Creating Centroids of the administrative boundaries. Centroids are points that are placed at the calculated centre of polygons (see Module 5).
We will be using the same layer as for the choropleth map (see Fig. 241): NGA_Adm1_Pop.

  1. In the processing toolbox, search for the tool centroids. Double-Click on it. A new window will open (see Fig. 100)

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Fig. 100 Creating centroids in QGIS 3.36#

  1. Under Input Layer, select the NGA_Adm1_Pop-layer. Click on Run.

  2. A new point layer called Centroids will appear in your layers panel. Open it’s styling panel and navigate to the symbology tab.

  3. Set the symbolisation method to Graduated.

  4. Under Value, select total_pop.

  5. Change the Method from Colour to Size.

  6. Click on Classify.

  7. Optional: Change the Colour and Transparency of the Circles.

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Fig. 101 A map of Nigeria displaying the same data. Once using graduated colours (choropleth) and graduated symbols (proportional circles).#

Video: How to create a proportional circles map

Use different styles in a single layer#

By categorizing or classifying data in a single layer, we are able to assign different styles to each classification. 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:

Setting different point symbols for different features#

  1. Download the ACLED security incidents geopackage and load it into your QGIS-project. It is a point layer with each point indicating a distinct security incident.

  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.

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Fig. 102 Change the symbology type to “categorised” 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.

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Fig. 103 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.

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Fig. 104 Regions affected by explosions in Nigeria#

Video: Set up different symbols in a single layer

Simple Markers, SVG-Symbols and Raster images

On top of simple markers, QGIS let’s you also use SVG-symbols and raster images as symbols for your vector data.

  • Simple markers are simple shapes such as rectangles, circles, or crosses that can be adjusted in the symbolization layer (colour, size, outline, etc.). Most of your styling in QGIS will be done with these markers.

  • SVG-symbols are scalable vector graphic symbols. As vector files, they can be scaled to any size while keeping the same resolution. In most cases, if you want to use a more complex symbol (e.g. hospital, school, train station), SVG-symbols are the best option as they let you adjust the symbol (colours, outline, size, etc.)

  • If you select raster images, the resolution of the symbol is limited by the amount of pixels in the image. It is not advisable to use high resolution images as symbols on your map because it may overload your PC.

Using SVG-Symbols and IFRC symbols

Using SVG-Symbols

In some cases, you might want to use more complex symbols in your map. For example, you want to use a cross to signify a hospital, a book to signify a library, or a plane to signify an airport. In these cases, you can use SVG-symbols. Keep in mind that, ordinarily, SVG-symbols work only for point data. To use SVG-symbols:

  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.

There is already a default library of SVG-symbols. If you are looking for a specific symbol, try searching for it in the search bar

Adding an external SVG-library

QGIS also offers the option to add your own SVG-libraries, for example if your organisation uses a specific set of icons. If you have a library of SVG-symbols as a folder you can add them to your Styling manager.

  1. Open the style manager: Settings > 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.

Using IFRC-Symbols

IFRC- and UN-Symbols repositories

The IFRC provides icons and symbols that can be used in your maps. You can find them under this link.

There is also a library with humanitarian icons by the United Nations Office for the Coordination of Humanitarian affairs which can be found here. The files are available in different formats you can use in QGIS.

Further Resources#