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Some raster data sources, such as simple scanned maps, lack proper spatial referencing, and we have to georeference them before we can use them in a GIS. In QGIS, we can georeference rasters using the Georeferencer GDAL plugin, which can be accessed by going to Raster | Georeferencer. (Enable it by going to Plugins | Manage and Install Plugins if you cannot find it in the Raster menu).

The Georeferencer plugin covers the following use cases:

After loading a raster into Georeferencer by going to File | Open raster or using the Open raster toolbar button, we are asked to specify the CRS of the ground control points that we are planning to add. Next, we can start adding ground control points by going to Edit | Add point. We can use the pan and zoom tools to navigate, and we can place GCPs by clicking on the map. We are then prompted to insert the coordinates of the new point or pick them from the reference map in the main QGIS window. The placed GCPs are displayed as red circles in both Georeferencer and the QGIS window, as you can see in the following screenshot:

Georeferencer shows a screenshot of the OCM Landscape map © Thunderforest, Data © OpenStreetMap contributors (http://www.opencyclemap.org/?zoom=4&lat=62.50806&lon=-145.01953&layers=0B000)

After placing the GCPs, we can define the transformation algorithm by going to Settings | Transformation Settings. Which algorithm you choose depends on your input data and the level of geometric distortion you want to allow. The most commonly used algorithms are polynomial 1 to 3. A first-order polynomial transformation allows scaling, translation, and rotation only.

A second-order polynomial transformation can handle some curvature, and a third-order polynomial transformation consequently allows for even higher degrees of distortion. The thin-plate spline algorithm can handle local deformations in the map and is therefore very useful while working with very low-quality map scans. Projective transformation offers rotation and translation of coordinates. The linear option, on the other hand, is only used to create world files, and as mentioned earlier, this does not actually transform the raster.

The resampling method depends on your input data and the result you want to achieve. Cubic resampling creates smooth results, but if you don't want to change the raster values, choose the nearest neighbor method.

Before we can start the georeferencing process, we have to specify the output filename and target CRS. Make sure that the Load in QGIS when done option is active and activate the Use 0 for transparency when needed option to avoid black borders around the output image. Then, we can close the Transformation Settings dialog and go to File | Start Georeferencing. The georeferenced raster will automatically be loaded into the main map window of QGIS. In the following screenshot, you can see the result of applying projective transformation using the five specified GCPs:

Let's get started with a point layer! Load airport.shp from your sample data. In the top-left corner of the Style dialog, below the drop-down list, we find the symbol preview. Below this, there is a list of symbol layers that shows us the different layers the symbol consists of. On the right-hand side, we find options for the symbol size and size units, color and transparency, as well as rotation. Finally, the bottom-right area contains a preview area with saved symbols.

Point layers are, by default, displayed using a simple circle symbol. We want to use a symbol of an airplane instead. To change the symbol, select the Simple marker entry in the symbol layers list on the left-hand side of the dialog. Notice how the right-hand side of the dialog changes. We can now see the options available for simple markers: Colors, Size, Rotation, Form, and so on. However, we are not looking for circles, stars, or square symbols—we want an airplane. That's why we need to change the Symbol layer type option from Simple marker to SVG marker. Many of the options are still similar, but at the bottom, we now find a selection of SVG images that we can choose from. Scroll through the list and pick the airplane symbol, as shown in the following screenshot:

Before we move on to styling lines, let's take a look at the other symbol layer types for points, which include the following:

Simple marker layers can have different geometric forms, sizes, outlines, and angles (orientation), as shown in the following screenshot, where we create a red square without an outline (using the No Pen option):

Font marker layers are useful for adding letters or other symbols from fonts that are installed on your computer. This screenshot, for example, shows how to add the yin-and-yang character from the Wingdings font:

Ellipse marker layers make it possible to draw different ellipses, rectangles, crosses, and triangles, where both the width and height can be controlled separately. This symbol layer type is especially useful when combined with data-defined overrides, which we will discuss later. The following screenshot shows how to create an ellipse that is 5 millimeters long, 2 millimeters high, and rotated by 45 degrees:

In this exercise, we create a river style for the majriver.shp file in our sample data. The goal is to create a line style with two colors: a fill color for the center of the line and an outline color. This technique is very useful because it can also be used to create road styles.

To create such a style, we combine two simple lines. The default symbol is one simple line. Click on the green + symbol located below the symbol layers list in the bottom-left corner to add another simple line. The lower line will be our outline and the upper one will be the fill. Select the upper simple line and change the color to blue and the width to 0.3 millimeters. Next, select the lower simple line and change its color to gray and width to 0.6 millimeters, slightly wider than the other line. Check the preview and click on Apply to test how the style looks when applied to the river layer.

You will notice that the style doesn't look perfect yet. This is because each line feature is drawn separately, one after the other, and this leads to a rather disconnected appearance. Luckily, this is easy to fix; we only need to enable the so-called symbol levels. To do this, select the Line entry in the symbol layers list and tick the checkbox in the Symbol Levels dialog of the Advanced section (the button in the bottom-right corner of the style dialog), as shown in the following screenshot. Click on Apply to test the results.

Before we move on to styling polygons, let's take a look at the other symbol layer types for lines, which include the following:

A common use case for Marker line symbol layers are train track symbols; they often feature repeating perpendicular lines, which are abstract representations of railway sleepers. The following screenshot shows how we can create a style like this by adding a marker line on top of two simple lines:

Another common use case for Marker line symbol layers is arrow symbols. The following screenshot shows how we can create a simple arrow by combining Simple line and Marker line. The key to creating an arrow symbol is to specify that Marker placement should be last vertex only. Then we only need to pick a suitable arrow head marker and the arrow symbol is ready.

In this exercise, we will create a style for the alaska.shp file. The goal is to create a simple fill with a blue halo. As in the previous river style example, we will combine two symbol layers to create this style: a Simple fill layer that defines the main fill color (white) with a thin border (in gray), and an additional Simple line outline layer for the (light blue) halo. The halo should have nice rounded corners. To achieve these, change the Join style option of the Simple line symbol layer to Round. Similar to the previous example, we again enable symbol levels; to prevent this landmass style from blocking out the background map, we select the Multiply blending mode, as shown in the following screenshot:

Before we move on, let's take a look at the other symbol layer types for polygons, which include the following:

A common use case for Point pattern fill symbol layers is topographic symbols for different vegetation types, which typically consist of a Simple fill layer and Point pattern fill, as shown in this screenshot:

When we design point pattern fills, we are, of course, not restricted to simple markers. We can use any other marker type. For example, the following screenshot shows how to create a polygon fill style with a Font marker pattern that shows repeating alien faces from the Webdings font:

As an alternative to simple fills with only one color, we can create Gradient fill symbol layers. Gradients can be defined by Two colors, as shown in the following screenshot, or by a Color ramp that can consist of many different colors. Usually, gradients run from the top to the bottom, but we can change this to, for example, make the gradient run from right to left by setting Angle to 270 degrees, as shown here:

The Shapeburst fill symbol layer type, also known as a "buffered" gradient fill, is often used to style water areas with a smooth gradient that flows from the polygon border inwards. The following screenshot shows a fixed-distance shading using the Shade to a set distance option. If we select Shade whole shape instead, the gradient will be drawn all the way from the polygon border to the center.

The first group of tools in the Attributes toolbar allows us to select features on the map using the mouse. The following screenshot shows the Select Feature(s) tool. We can select a single feature by clicking on it, or select multiple features by drawing a rectangle. The other tools can be used to select features by drawing different shapes (polygons, freehand areas, or circles) around the features. All features that intersect with the drawn shape are selected.

The second type of select tool is called Select by Expression, and it is also available in the Attribute toolbar. It selects features based on expressions that can contain references and functions that use feature attributes and/or geometry. The list of available functions in the center of the dialog is pretty long, but we can use the search box at the top of the list to filter it by name and find the function we are looking for faster. On the right-hand side of the window, we find the function help, which explains the functionality and how to use the function in an expression. The function list also shows the layer attribute fields, and by clicking on all unique or 10 samples, we can easily access their content. We can choose between creating a new selection or adding to or deleting from an existing selection. Additionally, we can choose to only select features from within an existing selection. Let's take a look at some example expressions that you can build on and use in your own work:

The third type of tool is called Spatial Query and allows us to select features in one layer based on their location relative to features in a second layer. These tools can be accessed by going to Vector | Research Tools | Select by location and Vector | Spatial Query | Spatial Query. Enable it in Plugin Manager if you cannot find it in the Vector menu. In general, we want to use the Spatial Query plugin as it supports a variety of spatial operations such as Crosses, Equals, Intersects, Is disjoint, Overlaps, Touches, and Contains, depending on the layer geometry type.

Let's test the Spatial Query plugin using railroads.shp and pipelines.shp from the sample data. For example, we might want to find all railroad features that cross a pipeline; therefore, we select the railroads layer, the Crosses operation, and the pipelines layer. After we've clicked on Apply, the plugin presents us with the query results. There is a list of IDs of the result features on the right-hand side of the window, as you can see in the next screenshot. Below this list, we can check the Zoom to item box, and QGIS will zoom into the feature that belongs to the selected ID. Additionally, the plugin offers buttons for direct saving of all the resulting features to a new layer:

This is the basic Digitizing toolbar:

The Digitizing toolbar contains tools that we can use to create and move features and nodes as well as delete, copy, cut, and paste features, as follows:

The Advanced Digitizing toolbar offers very useful Undo and Redo functionalities as well as additional tools for more involved geometry editing, as shown in the following screenshot:

The Advanced Digitizing tools include the following:

One of the challenges of digitizing features by hand is avoiding undesired gaps or overlapping features. To make it easier to avoid these issues, QGIS offers a snapping functionality. To configure snapping, we go to Settings | Snapping options. The following screenshot shows how to enable snapping for the Current layer. Similarly, you can choose snapping modes for All layers or the Advanced mode, where you can control the settings for each layer separately. In the example shown in the following screenshot, we enable snapping To vertex. This means that digitizing tools will automatically snap to vertices/nodes of existing features in the current layer. Similarly, you can enable snapping To segment or To vertex and segment. When snapping is enabled during digitizing, you will notice bold cross-shaped markers appearing whenever you go close to a vertex or segment that can be snapped to:

All three use cases are covered by the functionality available through the attribute table. We can access it by going to Layer | Open Attribute Table, using the Open Attribute Table button present in the Attributes toolbar, or in the layer name context menu.

Besides the classic attribute table view, QGIS also supports a form view, which you can see in the lower dialog of the previous image. You can switch between these two views using the buttons in the bottom-right corner of the attribute table dialog.

Another option to edit the attributes of one feature is to open the attribute form directly by clicking on the feature on the map using the Identify tool. By default, the Identify tool displays the attribute values in read mode, but we can enable the Auto open form option in the Identify Results panel, as shown here:

What you can see in the previous screenshot is the default feature attributes form that QGIS creates automatically, but we are not limited to this basic form. By going to Layer Properties | Fields section, we can configure the look and feel of the form in greater detail. The Attribute editor layout options are (in an increasing level of complexity) autogenerate, Drag and drop designer, and providing a .ui file. These options are described in detail as follows.

If we want to change the attributes of multiple or all features in a layer, editing them manually usually isn't an option. This is what the Field calculator is good for. We can access it using the Open field calculator button in the attribute table, or by pressing Ctrl + I. In the Field calculator, we can choose to update only the selected features or update all the features in the layer. Besides updating an existing field, we can also create a new field. The function list is the same one that we explored when we selected features by expression. We can use any of the functions and variables in this list to populate a new field or update an existing one. Here are some example expressions that are often used:

An alternative to the Field calculator—especially if you already know the formula you want to use—is the field calculator bar, which you can find directly in the Attribute table dialog right below the toolbar. In the next screenshot, you can see an example that calculates the area of all census areas (use the New Field button to add a Decimal number field called CENSUSAREA first). This example uses a CASE WHEN – THEN – END expression to check whether the value of TYPE_2 is Census Area:

CASE WHEN TYPE_2 = 'Census Area' THEN $area / 27878400 END

if(TYPE_2 = 'Census Area', $area / 27878400, CENSUSAREA)

if(TYPE_2 = 'Census Area', $area / 27878400, NULL)

Enter the formula and click on the Update All button to execute it:

Since it is not possible to directly change a field data type in a Shapefile or SpatiaLite attribute table, the field calculator and calculator bar are also used to create new fields with the desired properties and then populate them with the values from the original column.

After loading the tabular data from either the spreadsheet or text file, we can continue to join this non-spatial data to a vector layer (for instance, our airports.shp dataset, as shown in the following example). To do this, we go to the vector's Layer Properties | Joins section. Here, we can add a new join by clicking on the green plus button. All we have to do is select the tabular Join layer and Join field (of the tabular layer), which will contain values that match those in the Target field (of the vector layer). Additionally, we can—if we want to—select a subset of the fields to be joined by enabling the Choose which fields are joined option. For example, the settings shown in the following screenshot will add only the some value field. Additionally, we use a Custom field name prefix instead of using the entire join layer name, which would be the default option.

Once the join is added, we can see the extended attribute table and use the new appended attributes (as shown in the following screenshot) for styling and labeling. The way joins work in QGIS is as follows: the join layer's attributes are appended to the original layer's attribute table. The number of features in the original layer is not changed. Whenever there is a match between the join and the target field, the attribute value is filled in; otherwise, you see NULL entries.

You can save the joined layer permanently using Save as… to create the new file.

In QGIS, we can use the Topology Checker plugin; it is installed by default and is accessible via the Vector menu Topology Checker entry (if you cannot find the menu entry, you might have to enable the plugin in Plugin Manager). When the plugin is activated, it adds a Topology Checker Panel to the QGIS window. This panel can be used to configure and run different topology checks and will list the detected errors.

To see the Topology Checker in action, we create a temporary scratch layer with polygon geometries and digitize some polygons, as shown in the following screenshot. Make sure you use snapping to create polygons that touch but don't overlap. These could, for example, represent a group of row houses. When the polygons are ready, we can set up the topology rules we want to check for. Click on the Configure button in Topology Checker Panel to open the Topology Rule Settings dialog. Here, we can manage all the topology rules for our project data. For example, in the following screenshot, you can see the rules we might want to configure for our polygon layer, including these:

Once the rules are set up, we can close the settings dialog and click on the Validate All button in Topology Checker Panel to start running the topology rule checks. If you have been careful while creating the polygons, the checker will not find any errors and the status at the bottom of Topology Checker Panel will display this message: 0 errors were found. Let's change that by introducing some topology errors.

For example, if we move one vertex so that two polygons end up overlapping each other and then click on Validate All, we get the error shown in the next screenshot. Note that the error type and the affected layer and feature are displayed in Topology Checker Panel. Additionally, since the Show errors option is enabled, the problematic geometry part is highlighted in red on the map:

Of course, it is also possible to create rules that describe the relationship between features in different layers. For example, the following screenshot shows a point and a polygon layer where the rules state that each point should be inside a polygon and each polygon should contain a point:

Selecting an error from the list of errors in the panel centres the map on the problematic location so that we can start fixing it, for example, by moving the lone point into the empty polygon.

Sometimes, fixing all errors manually can be a lot of work. Luckily, certain errors can be addressed automatically. For example, the common error of self-intersecting polygons, which cause invalid geometry errors (as illustrated in the following screenshot), is often the result of intersecting polygon nodes or edges. These issues can often be resolved using a buffer tool (for example, Fixed distance buffer in the Processing Toolbox, which we will discuss in more detail in Chapter 4, Spatial Analysis) with the buffer Distance set to 0. Buffering will, for example, fix the self-intersecting polygon on the left-hand side of the following screenshot by removing the self-intersecting nodes and constructing a valid polygon with a hole (as depicted on the right-hand side):

Another common issue that can be fixed automatically is so-called sliver polygons. These are small, and often quite thin, polygons that can be the result of spatial processes such as intersection operations. To get rid of these sliver polygons, we can use the v.clean tool with the Cleaning tool option set to rmarea (meaning "remove area"), which is also available through the Processing Toolbox. In the example shown in this screenshot, the Threshold value of 10000 tells the tool to remove all polygons with an area less than 10,000 square meters by merging them with the neighboring polygon with the longest common boundary:

Tools for converting between raster and vector formats can be accessed by going to Raster | Conversion. These tools are called Rasterize (Vector to raster) and Polygonize (Raster to vector). Like the raster clipper tool that we used before, these tools are also based on GDAL and display the command at the bottom of the dialog.

Polygonize converts a raster into a polygon layer. Depending on the size of the raster, the conversion can take some time. When the process is finished, QGIS will notify us with a popup. For a quick test, we can, for example, convert the reclassified landcover raster to polygons. The resulting vector polygon layer contains multiple polygonal features with a single attribute, which we name lc; it depends on the original raster value, as shown in the following screenshot:

Using the Rasterize tool is very similar to using the Polygonize tool. The only difference is that we get to specify the size of the resulting raster in pixels/cells. We can also specify the attribute field, which will provide input for the raster cell value, as shown in the next screenshot. In this case, the cat attribute of our alaska.shp dataset is rather meaningless, but you get the idea of how the tool works:

Whenever we get a new dataset, it is useful to examine the layer statistics to get an idea of the data it contains, such as the minimum and maximum values, number of features, and much more. QGIS offers a variety of tools to explore these values.

Raster layer statistics are readily available in the Layer Properties dialog, specifically in the following tabs:

For vector layers, we can get summary statistics using two tools in Vector | Analysis Tools:

In both tools, we can easily copy the results using Ctrl + C and paste them in a text file or spreadsheet. The following image shows examples of exploring the contents of our airports sample dataset:

An alternative to the Basics statistics tool is the Statistics Panel, which you can activate by going to View | Panels | Statistics Panel. As shown in the following screenshot, this panel can be customized to show exactly those statistics that you are interested in:

Instead of computing raster statistics for the entire layer, it is sometimes necessary to compute statistics for selected regions. This is what the Zonal statistics plugin is good for. This plugin is installed by default and can be enabled in the Plugin Manager.

For example, we can compute elevation statistics for areas around each airport using srtm_05_01.tif and airports.shp from our sample data:

Heatmaps are great for visualizing a distribution of points. To create them, QGIS provides a simple-to-use Heatmap Plugin, which we have to activate in the Plugin Manager, and then we can access it by going to Raster | Heatmap | Heatmap. The plugin offers different Kernel shapes to choose from. The kernel is a moving window of a specific size and shape that moves over an area of points to calculate their local density. Additionally, the plugin allows us to control the output heatmap raster size in cells (using the Rows and Columns settings) as well as the cell size.

The following screenshot shows us how to create a heatmap of our airports.shp sample with a kernel radius of 300,000 layer units, which in the case of our airport data is in feet:

By default, the heatmap output will be rendered using the Singleband gray render type (with low raster values in black and high values in white). To change the style to something similar to what you saw in the previous screenshot, you can do the following:

Finding nearest neighbors, for example, the airport nearest to a populated place, is a common task in geoprocessing. To find the nearest neighbor and create connections between input features and their nearest neighbor in another layer, we can use the Distance to nearest hub tool.

As shown in the next screenshot, we use the populated places as Source points layer and the airports as the Destination hubs layer. The Hub layer name attribute will be added to the result's attribute table to identify the nearest feature. Therefore, we select NAME to add the airport name to the populated places. There are two options for Output shape type:

It is recommended that you use Layer units as Measurement unit to avoid potential issues with wrong measurements:

It is often necessary to be able to convert between points, lines, and polygons, for example, to create lines from a series of points, or to extract the nodes of polygons and create a new point layer out of them. There are many tools that cover these different use cases. The following table provides an overview of the tools that are available in the Processing toolbox for conversion between points, lines, and polygons:

In general, it is easier to convert more complex representations to simpler ones (polygons to lines, polygons to points, or lines to points) than conversion in the other direction (points to lines, points to polygons, or lines to polygons). Here is a short overview of these tools:

One common spatial analysis task is to identify features in the proximity of certain other features. One example would be to find all airports near rivers. Using airports.shp and majrivers.shp from our sample data, we can find airports within 5,000 feet of a river by using a combination of the Fixed distance buffer and Select by location tools. Use the search box to find the tools in the Processing Toolbox. The tool configurations for this example are shown in the following screenshot:

After buffering the airport point locations, the Select by location tool selects all the airport buffers that intersect a river. As a result, 14 out of the 76 airports are selected. This information is displayed in the information area at the bottom of the QGIS main window, as shown in this screenshot:

If you ever forget which settings you used or need to check whether you have used the correct input layer, you can go to Processing | History. The ALGORITHM section lists all the algorithms that we have been running as well as the used settings, as shown in the following screenshot:

The commands listed under ALGORITHM can also be used to call Processing tools from the QGIS Python console, which can be activated by going to Plugins | Python Console. The Python commands shown in the following screenshot run the buffer algorithm (processing.runalg) and load the result into the map (processing.load):

Another common task is to sample a raster at specific point locations. Using Processing, we can solve this problem with a GRASS tool called v.sample. To use GRASS tools, make sure that GRASS is installed and Processing is configured correctly under Processing | Options and configuration. On an OSGeo4W default system, the configuration will look like what is shown here:

For this exercise, let's imagine we want to sample the landcover layer at the airport locations of our sample data. All we have to do is specify the vector layer containing the sample points and the raster layer that should be sampled. For this example, we can leave all other settings at their default values, as shown in the following screenshot. The tool not only samples the raster but also compares point attributes with the sampled raster value. However, we don't need this comparison in our current example:

Mapping the density of points using a hexagonal grid has become quite a popular alternative to creating heatmaps. Processing offers us a fast way to create such an analysis. There is already a pre-made script called Hex grid from layer bounds, which is available through the Processing scripts collection and can be downloaded using the Get scripts from on-line scripts collection tool. As you can see in the following screenshot, you just need to enable the script by ticking the checkbox and clicking OK:

Then, we can use this script to create a hexagonal grid that covers all points in the input layer. The dataset of populated places (popp.shp), is a good sample dataset for this exercise. Once the grid is ready, we can run Count points in polygon to calculate the statistics. The number of points will be stored in the NUMPOINTS column if you use the settings shown in the following screenshot:

Another spatial analysis task we often encounter is calculating area shares within a certain region, for example, landcover shares along one specific river. Using majrivers.shp and trees.shp, we can calculate the share of wooded area in a 10,000-foot-wide strip of land along the Susitna River:

Sometimes, we want to run the same tool repeatedly but with slightly different settings. For this use case, Processing offers the Batch Processing functionality. Let's use this tool to extract some samples from our airports layer using the Random extract tool:

Using the graphical modeler, we can turn entire geoprocessing and analysis workflows into automated models. We can then use these models to run complex geoprocessing tasks that involve multiple different tools in one go. To create a model, we go to Processing | Graphical modeler to open the modeler, where we can select from different Inputs and Algorithms for our model.

Let's create a model that automates the creation of hexagonal heatmaps!

Another useful feature is that we can specify a layer style that needs to be applied to the processing results automatically. This default style can be set using Edit rendering styles for outputs in the context menu of the created model in the toolbox, as shown in the following screenshot:

Models can easily be copied from one QGIS installation to another and shared with other users. To ensure the usability of the model, it is a good idea to write a short documentation. Processing provides a convenient Help editor; it can be accessed by clicking on the Edit model help button in the Processing modeler, as shown in this screenshot:

By default, the .model files are stored in your user directory. On Windows, it is C:\Users\<your_user_name>\.qgis2\processing\models, and on Linux and OS X, it is ~/.qgis2/processing/models.

You can copy these files and share them with others. To load a model from a file, use the loading tool by going to Models | Tools | Add model from file in the Processing Toolbox.

As an example, we will use SpatiaLite's spatial functions to get all highways that are within 1 km distance from the city of Firenze:

Another thing that databases are really good at is aggregating data. For example, the following SQL query will count the number of towns per region:

SELECT Regions.Name, Regions.Geometry, count(*) as Count
FROM Regions
JOIN Towns
  ON Within(Towns.Geometry,Regions.Geometry)
GROUP BY Regions.Name

This can be used to create a new layer of regions that includes a Count attribute. This tells the number of towns in the region, as shown in this screenshot:

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 individual feature's attribute value or an expression.

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:

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.

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 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:

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:

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.

In this example, we will create a basic map with a scalebar, a north arrow, some explanatory text, and a legend.

When we start the print composer, we first see the Composition panel on the right-hand side. This panel gives us access to paper options such as size, orientation, and number of pages. It is also the place to configure snapping behavior and output resolution.

First, we add a map item to the paper using the Add new map button, or by going to Layout | Add Map and drawing the map rectangle on the paper. Click on the paper, keep the mouse button pressed down, and drag the rectangle open. We can move and resize the map using the mouse and the Select/Move item tools. Alternatively, it is possible to configure all the map settings in the Item properties panel.

The Item properties panel's content depends on the currently selected composition item. If a map item is selected, we can adjust the map's Scale and Extents as well as the Position and size tool of the map item itself. At a Scale of 10,000,000 (with the CRS set to EPSG:2964), we can more or less fit a map of Alaska on an A4-size paper, as shown in the following screenshot. To move the area that is displayed within the map item and change the map scale, we can use the Move item content tool.

Adding a scalebar

After the map looks like what we want it to, we can add a scalebar using the Add new scalebar button or by going to Layout | Add Scalebar and clicking on the map. The Item properties panel now displays the scalebar's properties, which are similar to what you can see in the next screenshot. Since we can add multiple map items to one composition, it is important to specify which map the scale belongs to. The second main property is the scalebar style, which allows us to choose between different scalebar types, or a Numeric type for a simple textual representation, such as 1:10,000,000. Using the Units properties, we can convert the map units in feet or meters to something more manageable, such as miles or kilometers. The Segments properties control the number of segments and the size of a single segment in the scalebar. Further, the properties control the scalebar's color, font, background, and so on.

Adding a North arrow image

North arrows can be added to a composition using the Add Image button or by going to Layout | Add image and clicking on the paper. To use one of the SVGs that are part of the QGIS installation, open the Search directories section in the Item properties panel. It might take a while for QGIS to load the previews of the images in the SVG folder. You can pick a North arrow from the list of images or select your own image by clicking on the button next to the Image source input. More map decorations, such as arrows or rectangle, triangle, and ellipse shapes can be added using the appropriate toolbar buttons: Add Arrow, Add Rectangle, and so on.

Legends are another vital map element. We can use the Add new legend button or go to Layout | Add legend to add a default legend with entries for all currently visible map layers. Legend entries can be reorganized (sorted or added to groups), edited, and removed from the legend items' properties. Using the Wrap text on option, we can split long labels on multiple rows. The following screenshot shows the context menu that allows us to change the style (Hidden, Group, or Subgroup) of an entry. The corresponding font, size, and color are configurable in the Fonts section.

Additionally, the legend in this example is divided into three Columns, as you can see in the bottom-right section of the following screenshot. By default, QGIS tries to keep all entries of one layer in a single column, but we can override this behavior by enabling Split layers.

To add text to the map, we can use the Add new label button or go to Layout | Add label. Simple labels display all text using the same font. By enabling Render as HTML, we can create more elaborate labels with headers, lists, different colors, and highlights in bold or italics using normal HTML notation. Here is an example:

<h1>Alaska</h1>
<p>The name <i>"Alaska"</i> means "the mainland".</p>
<ul><li>one list entry</li><li>another entry</li></ul>
<p style="font-size:70%;">[% format_date( $now ,'yyyy-mm-dd')%]</p>

Labels can also contain expressions such as these:

Other common features of maps are grids and frames. Every map item can have one or more grids. Click on the + button in the Grids section to add a grid. The Interval and Offset values have to be specified in map units. We can choose between the following Grid types:

For Grid frame, we can select from the following Frame styles:

Using Draw coordinates, we can label the grid with the corresponding coordinates. The labels can be aligned horizontally or vertically and placed inside or outside the frame, as shown here:

Maps that show an area close up are often accompanied by a second map that tells the reader where the area is located in a larger context. To create such an overview map, we add a second map item and an overview by clicking on the + button in the Overviews section. By setting the Map frame, we can define which detail map's extent should be highlighted. By clicking on the + button again, we can add more map frames to the overview map. The following screenshot shows an example with two detail maps both of which are added to an overview map. To distinguish between the two maps, the overview highlights are color-coded (by changing the overview Frame style) to match the colors of the frames of the detail maps.

To insert additional details into the map, the composer also offers the possibility of adding an attribute table to the composition using the Add attribute table button or by going to Layout | Add attribute table. By enabling Show only features visible within a map, we can filter the table and display only the relevant results. Additional filter expressions can be set using the Filter with option. Sorting (by name for example, as shown in the following screenshot) and renaming of columns is possible via the Attributes button. To customize the header row with bold and centered text, go to the Fonts and text styling section and change the Table heading settings.

Even more advanced content can be added using the Add html frame button. We can point the item's URL reference to any HTML page on our local machines or online, and the content (text and images as displayed in a web browser) will be displayed on the composer page.

With the print composer's Atlas feature, we can create a series of maps using one print composition. The tool will create one output (which can be image files, PDFs, or multiple pages in one PDF) for every feature in the so-called Coverage layer.

Atlas can control and update multiple map items within one composition. To enable Atlas for a map item, we have to enable the Controlled by atlas option in the Item properties of the map item. When we use the Fixed scale option in the Controlled by atlas section, all maps will be rendered using the same scale. If we need a more flexible output, we can switch to the Margin around feature option instead, which zooms to every Coverage layer feature and renders it in addition to the specified margin surrounding area.

To finish the configuration, we switch to the Atlas generation panel. As mentioned before, Atlas will create one map for every feature in the layer configured in the Coverage layer dropdown. Features in the coverage layer can be displayed like regular features or hidden by enabling Hidden coverage layer. Adding an expression to the Feature filtering option or enabling the Sort by option makes it possible to further fine-tune the results. The Output field can be one image or PDF for each coverage layer feature, or you can create a multipage PDF by enabling Single file export when possible before going to Composer | Export as PDF.

Once these configurations are finished, we can preview the map series by enabling the Preview Atlas button, which you can see in the top-left corner of the following screenshot. The arrow buttons next to the preview button are used to navigate between the Atlas maps.

To create web maps from within QGIS, we can use the qgis2web plugin, which we have to install using the Plugin Manager. Once it is installed, go to Web | qgis2web | Create web map to start it. qgis2web supports the two most popular open source web mapping libraries: OpenLayers 3, and Leaflet.

The following screenshot shows an example of our airports dataset. In this example, we are using the Leaflet library (as configured in the bottom-left corner of the following screenshot) because at the time of writing this book, only Leaflet supports SVG markers:

When you are happy with the configuration, click on the Export button. This will save the web map at the location specified as the Export folder and open the resulting web map in your web browser. You can copy the contents in the Export folder to a web server to publish the map.

Another popular way to share maps on the Web is map tiles. These are basically just collections of images. These image tiles are typically 256 × 256 pixels and are placed side by side in order to create an illusion of a very large, seamless map image. Each tile has a z coordinate that describes its zoom level and x and y coordinates that describe its position within a square grid for that zoom level. On zoom level 0 (z0), the whole world fits in one tile. From there on, each consecutive zoom level is related to the previous one by a power of 4. This means z0 contains 1 tile, z1 contains 4 tiles, and z2 contains 16 tiles, and so on.

In QGIS, we can use the QTiles plugin, which has to be installed using the Plugin Manager, to create map tiles for our project. Once it is installed, you can go to Plugins | QTiles to start it. The following screenshot shows the plugin dialog where we can configure the Output location, the Extent of the map that we want to export as tiles, as well as the Zoom levels we want to create tiles for.

When you click on OK, the plugin will create a .zip file containing all tiles. Using map tiles in web mapping libraries is out of the scope of this book. Please refer to the documentation of your web mapping library for instructions on how to embed the tiles. If you are using Leaflet, for example, you can refer to https://switch2osm.org/using-tiles/getting-started-with-leaflet for detailed instructions.

To create stunning 3D web maps, we need the Qgis2threejs plugin, which we can install using the Plugin Manager.

For example, we can use our srtm_05_01.tif elevation dataset to create a 3D view of that part of Alaska. The following screenshot shows the configuration of DEM Layer in the Qgis2threejs dialog. By selecting Display type as Map canvas image, we furthermore define that the current map image (which is shown on the right-hand side of the dialog) will be draped over the 3D surface:

Besides creating a 3D surface, this plugin can also label features. For example, we can add our airports and label them with their names, as shown in the next screenshot. By setting Label height to Height from point, we let the plugin determine automatically where to place the label, but of course, you can manually override this by changing to Fixed value or one of the feature attributes.

If you click on Run now, the plugin will create the export and open the 3D map in your web browser. On the first try, it is quite likely that the surface looks too flat. Luckily, this can be changed easily by adjusting the Vertical exaggeration setting in the World section of the plugin configuration. The following example was created with a Vertical exaggeration of 10:

Qgis2threejs exports all files to the location specified in the Output HTML file path. You can copy the contents in that folder on a web server to publish the map.