<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto">Hi,<div>You may like this article.</div><div><br></div><div><a href="https://www.spatialsource.com.au/surveying/gda2020-and-overcoming-the-web-mercator-dilemma">https://www.spatialsource.com.au/surveying/gda2020-and-overcoming-the-web-mercator-dilemma</a><br><br><div dir="ltr">Nicolas Cadieux<div><a href="https://gitlab.com/njacadieux">https://gitlab.com/njacadieux</a></div></div><div dir="ltr"><br><blockquote type="cite">Le 26 nov. 2020 à 09:30, Charles Karney <charles@karney.com> a écrit :<br><br></blockquote></div><blockquote type="cite"><div dir="ltr"><span>You don't realize the importance of conformality.</span><br><span></span><br><span>The most common use case for online maps is navigating over short</span><br><span>distances (~100 km or less). In this case the size distortions of</span><br><span>Mercator are not apparent. However the 2 key properties of Mercator are</span><br><span>crucial: conformality (the angle of road intersections is preserved) and</span><br><span>that north is "up". Transverse Mercator gives you conformality but only</span><br><span>gives you the "north is up" property locally; so a single transverse</span><br><span>Mercator projection doesn't work globally.</span><br><span></span><br><span>Presumably the use of "web" Mercator, using the spherical Mercator</span><br><span>formulas instead of the ellipsoidal ones, was done on the basis of</span><br><span>simplicity. I regard this as an unfortunate compromise.</span><br><span></span><br><span>You *can* measure angles with Mercator. On large scale maps (covering</span><br><span>small areas), distances and areas are accurately given by applying a</span><br><span>scale factor. In both cases, there's a little error (approx 1/300)</span><br><span>because of the use of web Mercator instead of ellipsoid Mercator.</span><br><span></span><br><span> --Charles</span><br><span></span><br><span>On 11/26/20 8:47 AM, Idan Miara wrote:</span><br><blockquote type="cite"><span>Hi all,</span><br></blockquote><blockquote type="cite"><span>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)?</span><br></blockquote><blockquote type="cite"><span>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:</span><br></blockquote><blockquote type="cite"><span>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.</span><br></blockquote><blockquote type="cite"><span>In 4326 all the world fits into two rectangular tiles (so not much more complicated, I guess).</span><br></blockquote><blockquote type="cite"><span>2. Users expect coordinates in 4326 and the transformation from 3857 to 4326 is rather fast (in comparison to ellipsoidal mercator).</span><br></blockquote><blockquote type="cite"><span>But if you save the coordinates in 4326 then you wouldn't need to transform at all.</span><br></blockquote><blockquote type="cite"><span>3. 3857 is "almost" conformal - I think that the normal use case is not marine navigation so it doesn't seem so important.</span><br></blockquote><blockquote type="cite"><span>4. Both 3857 and 4326 have size distortion.</span><br></blockquote><blockquote type="cite"><span>5. You can't measure distances, areas or angles easier in 3857 as far as I know.</span><br></blockquote><blockquote type="cite"><span>What am I missing? Is 3857 faster or more useful in any other way than 4326?</span><br></blockquote><blockquote type="cite"><span>Kind regards,</span><br></blockquote><blockquote type="cite"><span>Idan</span><br></blockquote><blockquote type="cite"><span>_______________________________________________</span><br></blockquote><blockquote type="cite"><span>PROJ mailing list</span><br></blockquote><blockquote type="cite"><span>PROJ@lists.osgeo.org</span><br></blockquote><blockquote type="cite"><span>https://lists.osgeo.org/mailman/listinfo/proj</span><br></blockquote><span>_______________________________________________</span><br><span>PROJ mailing list</span><br><span>PROJ@lists.osgeo.org</span><br><span>https://lists.osgeo.org/mailman/listinfo/proj</span><br></div></blockquote></div></body></html>