ANINTRODUCTIONTOGIS


Georeferencing historical aerial images


The French National Geographic Institute (IGN) provides an important database of historical aerial images. These images can be used to follow the evolution of the agricultural fields following successive land consolidations. But these images must first be georeferenced.

This tutorial shows:

  1. how to download the historical aerial images from the IGN website
  2. how to georeference these images with the open source software QGIS from these data.


I. Consult and download historical aerial images

You may find historical aerial images on the “Remonter le temps” website from IGN.

Figure 1 Compare historical and recent images


The above example illustrates the urbanization of the city of Lempdes (Puy-de-Dôme).

This interface is very easy to use, but it only allows you to view one historical date, while many other images are available in the database.

To access these images, back to the home page, click on the “Télécharger” tab at the center of the window.


Figure 2 Select image to download


A large number of (black and white or color) images can be downloaded this way:

  • Center the image on the area of interest, taking enough distance to see the contours of the proposed images (move the mouse over the yellow dots to make the contours appear)

  • Click on the point of the desired image, then click on download

However, these images are not georeferenced. But this can be done with QGIS, as we will see below.


II. Georeferencing rasters with QGIS

For simplicity, you can use the data downloadable here to perform this application. In this folder, you will find two images:

- a raw image from 1954, not georeferenced

- an image from 2009, already georeferenced and to be used as a reference.


First, open QGIS and set the system of coordinates of your project as Lambert 93 (EPSG:2154), which is the reference coordinate system for France:

  • Project > Properties > CRS

Then add the raster layer that you will use as a reference:

  • Layer > Add layer > Add raster layer
Figure 3 Add raster layer


If you work with a different area of interest, to get a reference map, you may load an aerial satellite image (Google or Bing or…) with the OpenLayer plugin (QGIS2) or, for QGIS3, with QuickMapService plugin (Settings>More Service then update for Google maps and others).

Open the georeferencer plugin:

  • Raster > Georeferencer

You may have to install this plugin with: Plugins > Manage and Install Plugins > Georeferencer GDAL

Inside the georeferencer plugin:

  • Load the image to be georeferenced
    • File > Open raster
  • Set the transformation parameters, in particular:
    • Type of transformation: start with linear then change to Thin Plate Spline
    • Name of the output raster
    • Specify the system of coordinates (here Lambert-93)
      Figure 4 Georeferencing parameters

  • Define at least 10 control points evenly distributed over the image
    • Edit > Add point
    • Define a first control point by clicking on the image to be georeferenced
    • On the window that appears, select “From Map Canvas”
    • Click the corresponding point on the reference raster
    • Repeat for each control point
Figure 5 Select control points


- Once you have enough control points, you may start georeferencing - File > Start Georeferencing

The two rasters can now be overlaid. This makes it easier to track land use changes.