Georeferencing in QGIS, a widely used open-source Geographic Information System (GIS), involves the process of aligning a digital image of a map or aerial photograph with geographical coordinates. This technique is essential for integrating scanned maps, which initially lack spatial reference, into the GIS environment.
By assigning known coordinate points on the scanned map to their corresponding locations on a spatially referenced grid, georeferencing transforms a simple image into a valuable spatial data layer. This allows users to overlay the scanned map with other spatial data, perform analysis, and use it for various GIS applications.
While there is an abundance of geospatial data readily available for use in GIS software, sometimes the information is only available on a historical map. Many institutions have online repositories where users can readily access scanned copies of these maps.
These scanned maps contain a lot of geographic information but the digital files often lack any kind of geographic coordinates that allows GIS software to properly place the file into a GIS software so it can be used with other geospatial data. Georeferencing, the process of matching raster (images) to coordinates in the real world, is especially important when working with maps that don’t have their own spatial information.
This tutorial will walk you through the whole process, from choosing which scanned map to access in Google Earth Pro and how to georeference the map using the Geoferencer tool in QGIS.
Please note: the steps in this tutorial use an example of how to georeference a scanned map when the map has coordinate grid lines (known as graticules) on it. Here’s how to georeference a scanned map using GIS data in QGIS.
Installing Google Earth Pro
- If you don’t already have Google Earth installed on your computer, visit Google’s Earth Versions web page.
- Select Google Earth Pro for desktop from the menu options to retrieve the installer file.
- Find the Google Earth installer file on your computer and double-click to start the installation process.
Selecting a topographic map in Google Earth pro to import into QGIS
- First click this link to select a scanned map from a kmz file from the University of Texas
- Launch Google Earth Pro’s desktop version.
- From the menu tabs, select File.
- Select Open
- Toggle to the location of the downloaded .kmz file and click OK to import into Google Earth. The file automatically shows under the “temporary places” folder.
- Check the kmz file under the “temporary places” folder. The content of the kmz file shows on the Google Earth viewing canvas.
- Use the pan tool to visualize the overlay of the .kmz file over the google earth canvas.
- Under “temporary places” folder, double-click “US_Army_maps_v_17.kmz” to view its underlying input folders.
- Double-click “US_Army_maps_v_17.”
- Double-click the second folder named “World.”
- Double-click “NE” folder.
- Double-click “NE 36.”
- Right-click on the location pointer for NE 36 showing on the viewing canvas and select copy link.
- Paste the link into a browser to view the topographic map that covers NE 36. The selected map covers a portion of the Nile river and there are latitude and longitude grid lines on the map that we will use to enter the coordinate information for georeferencing this map.
- From the image that displays, right-click on the image and select “Save image as.”
Georeferencing the scanned map in QGIS
- Launch QGIS.
- Open and create a new project in QGIS by selecting New from the Project tab.
- From the menu tabs, select Project.
- From the menu tabs, select Layer –> Georeferencer to open the georeferencing window.
- Use the Open Raster icon on the georeferencing window to import the topographic map into the window for georeferencing.
- From the settings tab, select transformation settings.
- Ensure that transformation type is set to linear.
- Set output raster to your desired output name and location.
- Check “Save GCP points.”
- Check “load in QGIS when done.”
- Click OK.
- Use the zoom tools in the georeferencer window to zoom into the scanned map: the coordinates are displayed clearly on the grids after using the zoom in option. The values on the left and right margins of the map are the Y or Northing values, the values on the top and bottom are the X or Easting values.
- Use the hand icon to pan your map to the convergence of latitude 19 degrees and longitude 31 degrees.
- Zoom in to the intersection of 19 degrees and 31 degrees so you can add the first control point.
- Select “Add point” from the menu items in the georeferencer window
- In the “enter map coordinates” window, enter 31 for the X value and 19 for the Y value. If you know the map projection of the scanned map, select it from the dropdown.
- Once you have entered the coordinates, click “OK”. A red dot will appear in the map window of the Georeferencer showing the entered coordinate and the ground point will be added to a table immediately beneath the map window.
- A minimum of three control points are required to properly georeference the map. Repeat the process for two more ground control points, making sure that the points are fairly distributed across the topographic map.
- The following points used in this tutorial as control points are: latitude 17 degrees and longitude 31 degrees, latitude 18 degrees and longitude 35 degrees.
- Once you have three ground control points, click on the button with the green start arrow to “Start Georeferencing.”
- Once the Georeferencing tool has completed processing, a new TIF file with the spatial information attached will be added to the map canvas in QGIS. Minimize the georeferencer window to view the georeferenced imaged in the map canvas.
- Now that the scanned maps has spatial information attached to it, you can add in GIS data on top of the map like the road layer (dashed white line) on top of the map:
If the GIS data doesn’t align as expected on top of the georeferenced map, you may need to add more control points or change the input map projection information.