Le ven. 16 nov. 2018 à 02:21, Nyall Dawson <[email protected]> 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" )
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