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:

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.

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.

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.

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.