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Workshop: QGIS

Download QGIS for this website: https://qgis.org/en/site/forusers/download.html.

Make sure to download the latest version, and the “standalone installer” (for Windows).

For this workshop, we’ll be using orthographic images processed from our drone photographs (found here: https://drive.google.com/drive/folders/1zNCEGuzKP7nltHH6mtQ2Q5XIRJMh7Gsa?usp=sharing), and historical aerial images (found here: https://drive.google.com/drive/folders/1vIJzvfNtWNWrZdLChZa76lxiFhizIOli?usp=sharing)

Setting Up QGIS

In the main window of QGIS, chose to create a “New Empty Project”. You’ll be presented with a blank screen.

QGIS is a GIS software, which stands for “Geographic Information System”. It is a software dedicated to the processing and presentation of geographical data. You can use it to plot layers of information that you got from the municipality (for example) or create your own layers of information. In this workshop, we will concentrate on using QGIS for aligning aerial images.

Before we introduce our own images, we need an existing base layer for reference. In the left sidebar, called “Browser”, look for “XYZ Tiles”, and under it for “Open Street Map”. You can double-click Open Street Map to get a basic map on your screen. You can zoom in on Be’er Sheva.

In order to use other base maps, like satellite views, the best way will be to install a certain plugin. In the top menu bar, go to Plugins -> Manage and Install Plugins…

In the plugins window, search and install “QuickMapServices”.

After the installation is complete, go to Web -> QuickMapServices -> Search QMS. That will open a sidebar where you can choose to add many layers available online. You can search for Mapbox Satellite.

Georeferncing Images

There are two major data types used in GIS – vector and raster.

Vector is any kind of graphic information described and displayed using geometry such as points, lines, and polygons. You may use vector data describing real-world things like roads, borders, places of interest, etc.

Raster is pixel-based information layers. These could be aerial images or any information that is displayed as a graphical image – like statistical data (displayed using colored pixels) or topographical height maps.

For the sake of QGIS, our aerial photographs are raster data layers. In order to display them on the map, we first need to reference their exact location. Since these images were sometimes taken from an airplane at an angle, they might be distorted when placed flat on a map.

In order to reference the location of the images, go to Raster -> Georeferencer. That would open a whole new window. To import a new image, press the left top button that looks like a pixelated image with a + sign on it. Choose an image you wish to import.

When georeferencing, we need to select a specific point on the image and indicate their location on the map. After importing a new image, your mouse should automatically change to a + sign, for adding new points. You can also press the “Add Point” button at the top menu. Next to the “Add Point” you will also find “Delete Point” and “Move Point”.

Select a point on the image that you’re certain you know its precise location on the map. You might need to investigate for a while before you’re sure what you’re doing. After pressing the image, a window will pop up. Press “From Map Canvas” in order to go back to the map and indicate the same location over there. After you’re done, press OK. Repeat the process for at least 3 more locations. Try to spread them across the image – the more points – the better.

When you’re done, press the yellow setting button at the top menu. Set your options like this:

  • Transformation type: Thin Plate Spline
  • Resampling method: Lanczos
  • Check “Use 0 for transperancy when needed”
  • Check “Load in QGIS when done”

Press OK. You only need to set these settings once, they will be saved for future use.

If you’re ready, press the green “play” button at the top. After the processing is done, you can check the result on the map. If you want to make any changes and improvements, you can turn off the new layer, and go back to the georeferencing window to make changes.

Note: in future use, you might get a message saying: ‘Please set output raster name’. Press OK, and that would open up the settings menu. You don’t have to do anything, just press OK on that window too, and hit the ‘play’ button again. It would work the second time.