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I'm currently working on a project in QGIS 3.10 . So far, I had somewhat ignored matters pertaining to projections, because things seemed to be going fine. However, I am now beginning to experience some issues. I am not entirely sure how to resolve them, because I suspect I don't fully understand projections in QGIS yet.

The project's CRS is set to EPSG 3857. I have a map with six layers. They are as follows:

enter image description here

The CRS of these layers are:

  1. Output: EPSG 4326
  2. Mean coordinates: EPSG 4326
  3. randstadstedeninfo: EPSG 4326
  4. studstedeninfo: EPSG 4326
  5. GADM36_NLD1: EPSG 4326
  6. Esri Gray (light): EPSG 3857

At the moment, the map looks like this: enter image description here

Apart from the “Output” (the green oval), it looks like I want it to look. I'm not really sure why it looks the way it does though.

How can I change the CRS of these layers in such a way that the Output (the vertical oval green polygon) becomes a nice circle?

When I change the CRS of the Output to EPSG 3857, it (again) goes somewhere else entirely. I'd like the depiction of layers 2 through 6 to remain the same, and change layer 1 in such a way that it fits neatly within the rest of them (as a circle).

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    see if my recent talk helps slides or the video – Ian Turton Nov 7 '20 at 11:44
  • @IanTurton I will check them out. Meanwhile, I've edited the question so it has more focus. May it be re-opened? – Max Muller Nov 7 '20 at 11:45
  • @IanTurton I've watched the video of your talk. I tried exporting the Output layer with “save features as...”, in which I set the CRS to 3857, but the new layer puts the oval at the same place as the old one. Do you know what I should be doing differently? – Max Muller Nov 7 '20 at 12:50
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    Just don't create a buffer using degree. – Erik Nov 7 '20 at 13:05
  • @Erik Right. But how do I change the units of Distance to, say, km? I don't know how to change the units in the QGIS buffer window. – Max Muller Nov 7 '20 at 13:12
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Currently the "output" layer is in a coordinate system EPSG 4326 that has degrees as unit. Using degrees in the Netherlands has the complication that a degree east-west is shorter than a degree north-south, that is why you get an ellipse when you define a circle in degree units. If you create a new layer "output2” with coordinate system EPSG 3857, which has meters as unit, then your east-west distance and north-south distance will be the same, and a circle will look like a circle on your map. The (for novices sometimes confusing) benefit of QGIS is that it will reproject on the fly all your layers to project CRS.

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