How to georeference an image in Qgis?

¡Let’s start the task of georeferencing an image in Qgis!

It is very common that raster data are not properly associated to any Spatial Reference System (SRC), such as scanned maps, aerial photos, etc…. and/or that their geometry differs from the rest of the data with which we want to work together.

The normal thing is that when we have data of these characteristics (or even if they have some associated Spatial Reference System) we want to adapt them to the geometry of other data of greater precision, whose Coordinate System is known.

This operation is know as georeferencing and for this we can use the tools that Qgis provides us with in its Georeferencing module.

In the first step, you must open a new project in QGIS and access the QGIS settings through the menu Project/Properties/General and in the section Measurements mark Meters.

Now, you must activate the GDAL Georeferencing plugin by accessing from the menu Add-ons/Administer and install add-ons.

Note: It may happen that the plug-in is already installed in QGIS, and in that case we will mention later where you will find it.

In the search bar of the Plugins window type “Georeferencer GDAL” and click on the checkbox next to it to activate and install it.

Next, you must load the image to be georeferenced, adding a new raster layer, accessing from the Add new raster layer menu.

Note: According to the version of your QGIS you will also find the icon

on the menu bar.

Click on the three suspension points, select the route where the raster file (.tiff) or the image that will serve as a reference for georeferencing is saved and click on Add.

With the image already loaded, you must then access the GDAL Georeferencing plugin, through the Raster/Georeference menu, by clicking on the latter.

Now with the new window open, you must find in your files the image or .tiff file that you want to georeference and add it to the georeference.

When you load the image, a new Coordinate Reference System Selector window will appear, in which you will have to indicate the reference system, in your case, it will depend on the image and your tutor’s instructions.

Note: The Georeferencing interface is divided in two parts, in the upper part the image to be georeferenced and in the lower part the space for the control points data (this is the name given to the two pairs of X,Y coordinates of the control points taken in the support data and in the data to be corrected that you will enter during the process).

Now, you can start entering the control points, by selecting the button

Add point

located at the top of the menu of the georeferencer window and mark the control points that you consider necessary. As a tip I recommend you to choose several points easily visible and distributed over several parts of the image, avoiding the accumulation of points in a single part of the file.

The following dialog box will appear for each point:

And you must type the coordinates (if you already know them) or select the From map canvas option, which will redirect you to the QGIS interface, where the reference image is loaded and there you must mark again that same point. With this process, you will have two homologous points marked in each image, one in the Georeferencer interface and another in the QGIS interface, that is, you will assign to each point to georeference its respective coordinates already georeferenced, listing the coordinates of all the points in the inner part of the screen.

With the points previously selected, you will have to click on the icon

a window will pop up asking you to enter the type of transformation and click acept.

Now, you must click on the button

The Transformation Settings button, also located at the top of the Georeferencer menu, will open a window where you will enter the information corresponding to the type of transformation, resampling method, target reference system and the saving path of the georeferenced image. The above data will be indicated by your work objectives and/or your tutor.

Finally, click on the Accept button and wait for the process to finish. The georeferenced image will be loaded into the QGIS interface. You can now close the Georeferencer window and decide whether or not to save the control points you entered.

Note: To make sure that the process was successful, check the properties of the georeferenced image and verify that the reference system matches the expected one.