[Qgis-user] question on scale bar world map

[Gabriel Cotlier]

Thanks everyone for the explanations; they were very helpful and insightful.
Regards,
Gabriel


On Fri, Jun 27, 2025 at 8:35 AM Richard Duivenvoorde <rdmailings at duif.net>
wrote:

> Hi Gabriel,
>
> Disclaimer: a lot of GEO-nerds here, he :-)
>
> But: given the perimeter on the equator of the earths is about 40000 km,
> as you can see in your image, the scale bar is 'sort of valid' on the
> equator, but totally OFF if you look near the poles (where the perimeter of
> the earth is probably a fraction of the 10000 km scale).
>
> So: in general in such (global) maps you should not use a scalebar, but on
> local level (where you use proper crs's and the unit's are meters), you
> SHOULD always use a scalebar :-)
>
> And yes: do blame Matlab for his error from us ;-)
>
> Regards,
>
> Richard Duivenvoorde
>
> On 27-06-2025 12:55, Gabriel Cotlier via QGIS-User wrote:
> > Thanks to all. I currently have installed QGIS 3.34. I do not know the
> validity of this map from MATLAB at :
> https://la.mathworks.com/help/matlab/ref/geoaxes.html <
> https://la.mathworks.com/help/matlab/ref/geoaxes.html>
> > but I was thinking of a map like this one, just without the small map in
> the upper right corner and the division of countries in QGIS. I do not know
> if that woud be possible and if so, if that scale bar in the figure below
> makes any sense?
> > Thanks a lot again.
> >
> >
> > image.png
> >
> > On Fri, Jun 27, 2025 at 2:22 AM Andreas Neumann <andreas at qgis.org
> <mailto:andreas at qgis.org>> wrote:
> >
> >     Hi all,
> >
> >     Chris explained why it is impossible to have a consistent scale for
> a world projection on a 2D flat plane.
> >
> >     The good news is, that in QGIS 3.44 there is a setting to influence
> the scale calculation method: see
> https://www.qgis.org/project/visual-changelogs/visualchangelog344/#application-and-project-options
> <
> https://www.qgis.org/project/visual-changelogs/visualchangelog344/#application-and-project-options
> >
> >
> >     Best regards,
> >     Andreas
> >
> >     On Fri, 27 Jun 2025 at 02:37, chris hermansen via QGIS-User <
> qgis-user at lists.osgeo.org <mailto:qgis-user at lists.osgeo.org>> wrote:
> >
> >         Gabriel and list,
> >
> >
> >
> >         On Thu, Jun 26, 2025, 16:05 Gabriel Cotlier via QGIS-User <
> qgis-user at lists.osgeo.org <mailto:qgis-user at lists.osgeo.org>> wrote:
> >
> >             Dear QGIS Users,
> >             I have been trying to do a world map with a scale bar and
> found the problem that needs a global projection, such as Greenwich for
> instance then the "scale bar" displayed in the map of the layout is for
> instance, 0.2 m in length but visually not consistent at all with the size
> of the continents and distances of the world map displayed.
> >
> >             How could a world map with a scale bar be produced such that
> the scale bar length and size look consistent with the length and size of a
> world map in QGIS?
> >
> >
> >         Gabriel, the usual problem with trying to do a scale bar on a
> global map is that you have projected the spherical (more or less) globe
> onto a flat map.
> >
> >         This entails some serious distortion of the surface of the earth.
> >
> >         You are hoping for a projection that preserves distances at all
> points on the map.
> >
> >         You may wish to read this Wikipedia article on map projections.
> >
> >         https://en.m.wikipedia.org/wiki/Map_projection <
> https://en.m.wikipedia.org/wiki/Map_projection>
> >
> >         Another visual device that one sometimes sees is several
> "stacked" scale bars, one for each 10 degrees or so of latitude, which of
> course isn't feasible when the two points to be measured are at different
> latitudes.
> >
> >
> >         Chris Hermansen · clhermansen "at" gmail "dot" com
> >
> >         C'est ma façon de parler.
> >         _______________________________________________
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> >
> >
> >
> >     --
> >
> >     --
> >     Andreas Neumann
> >     QGIS.ORG <http://QGIS.ORG> board member (treasurer)
> >
> >
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