 Hello, this is Hans van der Kwas, Senior Lecturer at IHC Daft Institute for Water Education. In this video, I'm going to explain how projections work in GIS. We will look at on-the-fly reprojection, which is basically the projection of your project. The on-the-fly reprojection makes it possible that layers with different projections can be visualized in the map canvas together, if they have the correct projection. We'll also look at assigning projections. That is when a layer doesn't have a projection or has the incorrect projection assigned, then we can replace that projection. It will not change the coordinates in the file, it will just change the projection that is assigned. And the third thing that we're going to look at is the reprojection. That is if the projection of a layer is not the one that we need for our analysis. In that case, we need to reproject the layer from a source to a target coordinate reference system. If we start a new empty project in QGIS, then in the lower right, we find the default projection of the project. That's the on-the-fly reprojection. It says EPSG4326. In another video, I've explained what EPSG codes are. They represent the projection. EPSG4326 means that the projection is in latitude-longitude. Normally, the first layer that I load will change this on-the-fly reprojection of the project. So I'm going to load a layer from the web. Using the Quick Map Services plugin, I can add, for example, OpenStreetMapStandard. There it is. The OpenStreetMapStandard layer is in EPSG3857. It's the first one that I load here in the map canvas, and therefore it determines the projection of the project. Now I'm going to add another layer. If I type here in the coordinate field world, it will give me a map of the world, of the countries of the world. If I hover my mouse over layers, I can see the projection of the layer. So this world map has EPSG code 4326. The OSM has EPSG code 3857. That they can be visualized together in the map canvas is because of the on-the-fly reprojection, which is set to 3857, but I can also set it to 4326. Then it will visually reproject these layers, and they will still fit on top of each other, even if I use a local projection, like the Dutch one in this case. It will ask us which kind of transformation we want, but I keep the default. And then they will still be visualized on top of each other, while the projection of the project is not the same as this one from the world map, which is still 4326. We didn't change that. And the one of OSM is 3857, the Google Mercator. So what you have to remember is that the on-the-fly reprojection does not change the projection of the layers. It makes it possible to visualize layers of different projections in the map canvas together. I can also assign projections. So I go to properties, and I'm going to change the source coordinate reference system to the one that is used by the other layer. So I choose 3857. I click OK, click OK, and now we lose that layer. Because we assigned a wrong projection, the coordinates are still the same in the layer, but we said from now it's the 3857 EPSG, the Google Mercator. If I do zoom to layer, we end up somewhere in the ocean in another universe. So the whole scale and the whole location is incorrect. So to get it back, I have to assign the correct one, which was 4326, and then it gets again on top of the other layer. So that's the difference between assigning a projection and reprojecting, because I can also make the projection of roadmap the same as OSM standard, or any other one. If I want to do analysis, the roadmap projection needs to be the same as the projection of OSM standard. So in that case, I can do reprojection. To reproject such a vector, I go to the Vector menu, Data Management tool, Reproject Layer. I can also reproject the layer by choosing Export, Save Features S. I give it a new file name, and I call it WorldMap2, and I change it to 3857 Pseudo Mercator, and then I do OK. And it's reprojected. We don't see any difference in the location, because the on-the-fly reprojection is still on the Dutch one. But if I hover my mouse over it, I see that this one is 3857, and the one below is 4326, and the OpenStreetMap is 3857. And they're all visible here in the project, which is in 28992. I can demonstrate that it's the same one by making it transparent. So we have this one, we have that one, and we have that one, and they're all on top of each other. So now you've learned the difference between on-the-fly reprojection, which makes it possible to visualize in the map canvas layers with different projections. You've learned about assigning the projection. You only do that when you are sure that the coordinates that are used in the layer are of a certain projection. Then you can assign that projection if it somehow didn't come with the layer. And the third thing that you can do is reproject the layer. That is if you want to do analysis, you need to convert it from one to another projection. So for vector layers, you can do it with export, save features as, and then choose another projection. And for rusters, you can go here to ruster, projections, and warp. Or you also use the export function. It's very similar, like you also have here, project layer. I hope this was useful. Please subscribe to my YouTube channel if you want updates on new videos. And for more free GIS materials, go to GISOpenCourseWare.org, which has the IHG Delft OpenCourseWare on GIS.