[PROJ] Why is Web Marcator EPSG:3857 so popular for web maps (vs EPSG:4326)?

[Duncan Agnew]

To zoom in and out seamlessly and have the results "look right" I think
there are only two choices:
Mercator (most Web services) and perspective (Google Earth). The latter
only works because our visual system is
used to this distortion and (almost always) automatically turns it into a
3-d impression.

On Thu, Nov 26, 2020 at 6:29 AM Charles Karney <charles at karney.com> wrote:

> You don't realize the importance of conformality.
>
> The most common use case for online maps is navigating over short
> distances (~100 km or less).  In this case the size distortions of
> Mercator are not apparent.  However the 2 key properties of Mercator are
> crucial: conformality (the angle of road intersections is preserved) and
> that north is "up".  Transverse Mercator gives you conformality but only
> gives you the "north is up" property locally; so a single transverse
> Mercator projection doesn't work globally.
>
> Presumably the use of "web" Mercator, using the spherical Mercator
> formulas instead of the ellipsoidal ones, was done on the basis of
> simplicity.  I regard this as an unfortunate compromise.
>
> You *can* measure angles with Mercator.  On large scale maps (covering
> small areas), distances and areas are accurately given by applying a
> scale factor.  In both cases, there's a little error (approx 1/300)
> because of the use of web Mercator instead of ellipsoid Mercator.
>
>    --Charles
>
> On 11/26/20 8:47 AM, Idan Miara wrote:
> > Hi all,
> >
> > I was wondering if you can enlighten me regarding why Web Marcator
> > (EPSG:3857) is so popular and used by virtually all major online map
> > providers as opposed to WGS84 lat/lon (EPSG:4326)?
> >
> > I've composed some points that I could think about, but it doesn't add
> > up for why 3857 maps (i.e. slippy map) are more popular than 4326:
> >
> > 1. In 3857 almost all the world fits in a rectangular tile (~85 deg
> > north to ~85 deg south), which makes it easier to divide the tile
> > further into sub tiles.
> > In 4326 all the world fits into two rectangular tiles (so not much more
> > complicated, I guess).
> >
> > 2. Users expect coordinates in 4326 and the transformation from 3857 to
> > 4326 is rather fast (in comparison to ellipsoidal mercator).
> > But if you save the coordinates in 4326 then you wouldn't need to
> > transform at all.
> >
> > 3. 3857 is "almost" conformal - I think that the normal use case is not
> > marine navigation so it doesn't seem so important.
> >
> > 4. Both 3857 and 4326 have size distortion.
> >
> > 5. You can't measure distances, areas or angles easier in 3857 as far as
> > I know.
> >
> > What am I missing? Is 3857 faster or more useful in any other way than
> 4326?
> >
> > Kind regards,
> > Idan
> >
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