Setting projections in QGIS

Question

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:

The CRS of these layers are:

Output: EPSG 4326
Mean coordinates: EPSG 4326
randstadstedeninfo: EPSG 4326
studstedeninfo: EPSG 4326
GADM36_NLD1: EPSG 4326
Esri Gray (light): EPSG 3857

At the moment, the map looks like this:

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).

Hans Erren · Accepted Answer

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.

Correct answer by Hans Erren on December 6, 2020

Setting projections in QGIS

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