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

[Charles Karney]

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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