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 <[email protected]
<mailto:[email protected]>> 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
<[email protected] <mailto:[email protected]>> wrote:
Gabriel and list,
On Thu, Jun 26, 2025, 16:05 Gabriel Cotlier via QGIS-User
<[email protected] <mailto:[email protected]>> 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
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