QGIS Georeferencing

Introduction

Georeferencing is used when raster data is not referenced to a CRS - for example a scanned map, or a photograph. The process adds coordinate references to a raster image so that it can be used in QGIS (and other GIS systems) alongside other layers.

Open the project and load the Georeferencer

1. Open project .../Advanced/Georeferencing.qgz.The map shows an area of Exeter, including an Ordnance Survey MasterMap layer, and the OS open data WMS.

2. Go to Layer > Georeferencer.

You are going to use the Georeferencer tool to open a non spatially-referenced image of a map, and reference it to the QGIS layers based on coordinates and common features (for example railway stations, bridges) across the maps. Note that although we are using it for raster images, the tool can also be used for vector data (for example a layer with no CRS).

Load the source data

1. Click the Open raster button in the Georeferencer. 

2. Navigate to and open ...\Data\Raster\Georeferencing\barthol-exeter-1906.jpg

3. Go to Settings > Transformation Settings, and add settings as in the screenshot below:
   

Look at the image alongside the QGIS map to understand how it relates to the other layers. You will need to identify features which are common in the image and on the map, such as road junctions, bridges, or railway stations - you will need at least three and if possible more of these, well spread out across the map. These are called Ground Control Points (GCPs).

Create GCPs

1. Create your first control point by selecting the Add point tool 

2. Click on the map image on a feature you will be able to identify on the QGIS map - for example the main railway station.

3. On the dialogue that appears, click From Map Canvas 
   

4. Identify and click on the same feature in the QGIS map window

5. Click OK - a point will appear on the map, with a corresponding reference in the table below.

6. Complete this task at least a further three times at different locations on the map where features can be identified, ensuring they are well distributed. This might take some time - use zoom controls to be as accurate as possible.

7. Use the Move GCP point tool if necessary to correct point locations, or to delete points and recreate.

Run the Georeferencer

1. When you are happy with the GCPs you have collected, click the Start Georeferencing button

2. You should be able to see how far accurate your georeferencing has been by looking at the length of the red lines next to the red dots in the image representing the GCPs
   
   

3. If they are too long, you can delete and recreate the GCP, and rerun the Georefererencer

4. When you are ready, minimise the Georeferencer to see the results.

5. Your map should now be in the main QGIS map window, lined up with the existing data - you can set transparency to compare the layers.

6. If you are not happy with georeferencing, go back to the georeferencer and try again.

If you have time when you have completed this, repeat the exercise with one of the other images in the same folder.