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

[Nicolas Cadieux]

Hi,
You may like this article.

https://www.spatialsource.com.au/surveying/gda2020-and-overcoming-the-web-mercator-dilemma

Nicolas Cadieux
https://gitlab.com/njacadieux

> Le 26 nov. 2020 à 09:30, Charles Karney <charles at karney.com> a écrit :
> 
> 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
>> _______________________________________________
>> PROJ mailing list
>> PROJ at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/proj
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/proj
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/proj/attachments/20201126/5ec71471/attachment.html>