Labeling

We can activate labeling by going to Layer Properties | Labels, selecting Show labels for this layer, and selecting the attribute field that we want to Label with. This is all we need to do to display labels with default settings. While default labels are great for a quick preview, we will usually want to customize labels if we create visualizations for reports or standalone maps.

Using Expressions (the button that is right beside the attribute drop-down list), we can format the label text to suit our needs. For example, the NAME field in our sample airports.shp file contains text in uppercase. To display the airport names in mixed case instead, we can set the title(NAME) expression, which will reformat the name text in title case. We can also use multiple fields to create a label, for example, combining the name and elevation in brackets using the concatenation operator (||), as follows:

title(NAME) || ' (' || "ELEV" || ')'

Note the use of simple quotation marks around text, such as ' (', and double quotation marks around field names, such as "ELEV". The dialog will look like what is shown in this screenshot:

The big preview area at the top of the dialog, titled Text/Buffer sample, shows a preview of the current settings. The background color can be adjusted to test readability on different backgrounds. Under the preview area, we find the different label settings, which will be described in detail in the following sections.

Customizing label text styles

In the Text section (shown in the previous screenshot), we can configure the text style. Besides changing Font, Style, Size, Color, and Transparency, we can also modify the Spacing between letters and words, as well as Blend mode, which works like the layer blending mode that we covered in Chapter 2, Viewing Spatial Data.

Note the column of buttons on the right-hand side of every setting. Clicking on these buttons allows us to create data-defined overrides, similar to those that we discussed at the beginning of the chapter when we talked about advanced vector styling. These data-defined overrides can be used, for example, to define different label colors or change the label size depending on an inpidual feature's attribute value or an expression.

Controlling label formatting

In the Formatting section, which is shown in the following screenshot, we can enable multiline labels by specifying a Wrap on character. Additionally, we can control Line height and Alignment. Besides the typical alignment options, the QGIS labeling engine also provides a Follow label placement option, which ensures that multiline labels are aligned towards the same side as the symbol the label belongs to:

Finally, the Formatted numbers option offers a shortcut to format numerical values to a certain number of Decimal places.

An alternative to wrapping text on a certain character is the wordwrap function, available in expressions. It wraps the input string to a certain maximum or minimum number of characters. The following screenshot shows an example of wrapping a longer piece of text to a maximum of 22 characters per line:

Configuring label buffers, background, and shadows

In the Buffer section, we can adjust the buffer Size, Color, and Transparency, as well as Pen join style and Blend mode. With transparency and blending, we can improve label readability without blocking out the underlying map too much, as shown in the following screenshot.

In the Background section, we can add a background shape in the form of a rectangle, square, circle, ellipsoid, or SVG. SVG backgrounds are great for creating effects such as highway shields, which we will discuss shortly.

Similarly, in the Shadow section, we can add a shadow to our labels. We can control everything from shadow direction to Color, Blur radius, Scale, and Transparency.

Controlling label placement

In the Placement section, we can configure which rules should be used to determine where the labels are placed. The available automatic label placement options depend on the layer geometry type.

Configuring point labels

For point layers, we can choose from the following:

• The flexible Around point option tries to find the best position for labels by distributing them around the points without overlaps. As you can see in the following screenshot, some labels are put in the top-right corner of their point symbol while others appear at different positions on the left (for example, Anchorage Intl (129)) or right (for example, Big Lake (135)) side.
• The Offset from point option forces all labels to a certain position; for example, all labels can be placed above their point symbol.

The following screenshot shows airport labels with a 50 percent transparent Buffer and Drop Shadow, placed using Around point. The Label distance is 1 mm.

Configuring line labels

For line layers, we can choose from the following placement options:

• Parallel for straight labels that are rotated according to the line orientation
• Curved for labels that follow the shape of the line
• Horizontal for labels that keep a horizontal orientation, regardless of the line orientation

For further fine-tuning, we can define whether the label should be placed Above line, On line, or Below line, and how far above or below it should be placed using Label distance.

Configuring polygon labels

For polygon layers, the placement options are as follows:

• Offset from centroid uses the polygon centroid as an anchor and works like Offset from point for point layers
• Around centroid works in a manner similar to Around point
• Horizontal places a horizontal label somewhere inside the polygon, independent of the centroid
• Free fits a freely rotated label inside the polygon
• Using perimeter places the label on the polygon's outline

The following screenshot shows lake labels (lakes.shp) using the Multiple lines feature wrapping on the empty space character, Center Alignment, a Letter spacing of 2, and positioning using the Free option:

Placing labels manually

Besides automatic label placement, we also have the option to use data-defined placement to position labels exactly where we want them to be. In the labeling toolbar, we find tools for moving and rotating labels by hand. They are active and available only for layers that have set up data-defined placement for at least X and Y coordinates:

1. To start using the tools, we can simply add three new columns, label_x, label_y, and label_rot to, for example, the airports.shp file. We don't have to enter any values in the attribute table right now. The labeling engine will check for values, and if it finds the attribute fields empty, it will simply place the labels automatically.
2. Then, we can specify these columns in the label Placement section. Configure the data-defined overrides by clicking on the buttons beside Coordinate X, Coordinate Y, and Rotation, as shown in the following screenshot:
   
3. By specifying data-defined placement, the labeling toolbar's tools are now available (note that the editing mode has to be turned on), and we can use the Move label and Rotate label tools to manipulate the labels on the map. The changes are written back to the attribute table.
4. Try moving some labels, especially where they are placed closely together, and watch how the automatically placed labels adapt to your changes.

Controlling label rendering

In the Rendering section, we can define Scale-based visibility limits to display labels only at certain scales and Pixel size-based visibility to hide labels for small features. Here, we can also tell the labeling engine to Show all labels for this layer (including colliding labels), which are normally hidden by default.

The following example shows labels with road shields. You can download a blank road shield SVG from http://upload.wikimedia.org/wikipedia/commons/c/c3/Blank_shield.svg. Note how only Interstates are labeled. This can be achieved using the Data defined Show label setting in the Rendering section with the following expression:

"level" = 'Interstate'

The labels are positioned using the Horizontal option (in the Placement section). Additionally, Merge connected lines to avoid duplicate labels and Suppress labeling of features smaller than are activated; for example, 5 mm helps avoid clutter by not labeling pieces of road that are shorter than 5 mm in the current scale.

To set up the road shield, go to the Background section and select the blank shield SVG from the folder you downloaded it in. To make sure that the label fits nicely inside the shield, we additionally specify the Size type field as a buffer with a Size of 1 mm. This makes the shield a little bigger than the label it contains.

If you click on Apply now, you will notice that the labels are not centered perfectly inside the shields. To fix this, we apply a small Offset in the Y direction to the shield position, as shown in the following screenshot. Additionally, it is recommended that you deactivate any label buffers as they tend to block out parts of the shield, and we don't need them anyway.