[QGIS-Developer] QGIS composer export - issue with projection, scale & scalebar ?

[kimaidou]

Le ven. 16 nov. 2018 à 02:21, Nyall Dawson <nyall.dawson at gmail.com> a
écrit :

>
>
> My 2c: I don't actually believe there's an issue here. QGIS scale bars
> *ALWAYS* use ellipsoidal distances, regardless of the projection (and
> have for longer than I've been involved in the project, so since some
> version earlier than 1.7).
> This means you'll see differences between a 1km size grid created in a
> some local projection vs the scale bar's size, because the grid size
> is a cartesian size measurement based on a projected CRS, vs distances
> measured on the ellipsoid. The differences will be more extreme for
> CRSes which are bad for distance/area preservation, especially 3857.
>

Yes, this is the conclusion I got after some more thinking, as described in
my last comment in the hub issue.
I have one last question about the *numerical* scale "bar", I guess it is
calculated always in meter, and use the spheroid too ?



>
> I think the solution here is really just education -- we need to make
> sure users know that 3857 is unsuitable for anything but web maps, and
> that it's THEIR responsibility to make an informed decision what the
> correct CRS is for their project.**
>

I agree with your


> Nyall
>
> ** in reality, we'll never win this battle, and there'll always be
> uninformed, untrained users pushing out junk results out based on 3857
> calculations.
>

We could probably add some warnings in QGIS composer when using meter | map
units such as "Be aware the scale bar length will be calculated "based on
the projection ellipsoid" | "based on the projection planimetric coordinate
system" )
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-developer/attachments/20181116/c7a531a3/attachment.html>