I want to figure out (qualitatively) how much area of my city has a bus stop within 400 m walking distance. My approach so far is to load all bus stops and draw circles with radius = 400 m around them. Then I can see how much of the city is covered in circles. I use QGIS, OpenStreetMap and the QuickOSM plugin.
I seem to get something wrong with the measurements / projections though. Starting from knowing nothing, I figured out the following:
- I can easily get all the bus stops as a layer with QuickOSM.
- I can then modify the symbol layer. I tried 800 meters in scale (as the input seems to be diameter) but surprisingly the diameter is then not 800 meters - well, it is, but only if I change the measure tool to cartesian.
- But it should not be Cartesian, it should be the default, ellipsoid, as that is what is used to measure "real life" distances - as in, if I leave my house and walk 100 meters, the same distance will show up as 100 meters in QGIS if I measure based on the ellipsoid.
- I figured out that the bus stops layer uses EPSG 4326 WGS 84 as source while OSM Standard, loaded via the Quick Map Services, uses EPSG 3857 WGS 84 as source. Now if I understood it correctly, one is position on the globe and one is position on my map. It makes sense to me that meters have different meanings then, but still the bus stops end up at their actual positions just fine. I just can't draw circles around them.
I tried re-projecting the bus stop layer to EPSG 3857 but that did not help at all (and probably does not make sense). I also noted that for OSM, "meters in scale" and "map units" seem to be identical.
I read something about EPSG 3857 meters not being real meters but I do not know what to do with that. Is that the issue? How do I correct for the "fake" meters?
How can I address my original problem and what is the explanation? It especially makes me wonder why measuring in Cartesian matches exactly what I entered in the "meters in scale" box.
Meta information: QGIS 3.24, QuickOSM 2.0.1, my location is Germany.
