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This question seems naive at first glance, but it's subtle. I was recently pondering... How does QGIS actually represent maps and features when we select the 'WGS84' CRS at the project level?
I surprisingly cannot find the information here; https://docs.qgis.org/3.16/en/docs/user_manual/working_with_projections/working_with_projections.html (neither in the software itself).

In other words, what is QGIS default 'under-the-hood' projection used when WGS84 is selected for a project?

QGIS WGS84 a project CRS
Fig. 1: WGS84 selected as the project CRS to render an OpenStreetMap background map - my screen is definitely not ellipsoidally shaped.

This question can obviously extend to any other GIS piece of software, so instead of asking for each, if in addition you can tell if it exists a standard which is (almost) always used when displaying WGS84 material on a flat screen, that would be even better to know. It seems that it is not 'embedded' in the WGS84 CRS itself (which is a good thing) as per; https://epsg.org/crs_4326/WGS-84.html?sessionkey=3gcvw501ip and:

$ projinfo epsg:4326
PROJ.4 string:
+proj=longlat +datum=WGS84 +no_defs +type=crs

WKT2_2018 string:
GEOGCRS["WGS 84",
    DATUM["World Geodetic System 1984",
        ELLIPSOID["WGS 84",6378137,298.257223563,
            LENGTHUNIT["metre",1]]],
    PRIMEM["Greenwich",0,
        ANGLEUNIT["degree",0.0174532925199433]],
    CS[ellipsoidal,2],
        AXIS["geodetic latitude (Lat)",north,
            ORDER[1],
            ANGLEUNIT["degree",0.0174532925199433]],
        AXIS["geodetic longitude (Lon)",east,
            ORDER[2],
            ANGLEUNIT["degree",0.0174532925199433]],
    USAGE[
        SCOPE["unknown"],
        AREA["World"],
        BBOX[-90,-180,90,180]],
    ID["EPSG",4326]]

This still remains a Geographic space for me, but which is in 2D only, so how does it actually goes to my flat screen 2D Cartesian space?

Note; I intuitively think it's using an equidistant cylindrical (Plate Carrée) (a.k.a. equirectangular projection) projection, but I definitely wish to strike out that 'intuitively' word from the current sentence.

More on this projection: https://proj.org/operations/projections/eqc.html

This is a parenthesis, but I also think it would be a nice feature to see in a user-friendly way whether a CRS is a Geographic one or a Projected one. For the moment one has to watch to the WKT string to figure it out:

QGIS project CRS list

In my researches, I found this useful article on the ESRI blog:

ArcGis rendering projection

Source: https://www.esri.com/arcgis-blog/products/arcgis-pro/mapping/gcs_vs_pcs/

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    Does this answer your question? How can a geographic coordinate system such as WGS84 draw on the screen?
    – J.R
    Jan 20 at 13:58
  • Not satisfying at all; you cannot treat Geographic coords as planar by definition; there are no rigorous explanations on how it actually does the job and what are the QGIS choices. Jan 20 at 14:01
  • But maybe it is, a map like planar coordinates with a bounding box = [-180,-90 : 180,90] and scale is computed from the pixel at the center of the canvas (look at the scale if you just move a little). So, the bounding box is a rectangle and every coordinate is plot where it is. I think it's no more complicated. Jan 20 at 14:16
  • Indeed, and this is obviously, well... "a projection" (you used the word 'computed'), which picks points from a 3D space to bring them to a 2D surface. It must somewhere have a name (I hope) and a (mathematical) definition? And why choosing this one in particular against another one among the plethora of existing projections? Is this a/the standard choice or way of doing in GIS for representing data? Are there any historical or any other reasons? Jan 20 at 14:24
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    My take on this is that as the user didn't choose any projection (or rather choose to work with unprojected geographic coordinate) the software has no "right" way to display the data (as screen are definitely flat 2D Cartesian space) and it should not choose a projection for you (as only you can decide witch one, if any, is suitable for your mean). But as this leave no option to display the data, it seem developer choose to use the simplest [almost not a] projection (ie. Plate Carré also called the geographic projection, lat/lon projection, or plane chart) rather than displaying nothing...
    – J.R
    Jan 20 at 14:38
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Esri uses 'pseudo-Plate Carrée' because treating the angular decimal degree units as if they're already linear units isn't performing a map projection. Plate Carrée itself converts the decimal degrees into radians and multiplies by the radius of the sphere or the semi-major axis of the ellipsoid that the coordinates are referenced to.

So using the decimal degrees values directly is just a scaling (not) of the usual Plate Carrée projection.

Is the right thing to do with decimal degree data? Nah, but it's simple. Modern GIS software like (I assume) QGIS and ArcGIS have added various mechanisms to work around the weird pseudo-Plate Carrée display like calculating distances using geodesic or great circle calculations.

Disclosure: I work for Esri.

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