[Qgis-user] Distance measures on WGS84 (and other not projected CRSs) project

[Alex Mandel]

On 06/26/2014 11:20 AM, G. Allegri wrote:
> I have a doubt about the setting of the Ellipsoid for the measure tools
> when operating on a WGS84 project.
> If I create a blank project (WGS84) and I add a WGS84 layer, the Ellipsoid
> is set to None/Planimetric, and it cannot be changed until OTFR is switched
> on.
> In this initial state measures are done without reprojecting the points
> grabbed with the measure tool, and the returned measure value is incorrect.
> To be able to set the WGS84 Ellipsoid for the measure, I have to switch
> OTFR before.
> 
> I think it would be better to set the default measure Ellipsoid to be the
> same as the project CRS, wouldn't it?
> 
> giovanni
> 

My assumption (without looking the code) is that with OTFR (on the fly
reprojection) off the map is doing everything in cartesian coordinate
space. So all measures would be assumed to be not ellipsoid based at
all, just get out and measure and do trig directly on a flat paper.

So you have to enable OTFR in order for it to know an ellipse is in use,
and which one to use.

So basically with OTFR off, projection information about layers is 100%
ignored. I can see (but haven't tested) that auto applying ellipse to
other projections could lead to incorrect results too.

Perhaps we need a warning:
When OTFR is off
When user clicks measure tool
Check coordinate space
If it falls within WGS84 bounds pop-up warning
Allow users to either enable OTFR and/or silence this warning in the future.

Best to forward this idea on to the developers.

I see the other option is to let users, set the ellipse on their own if
they so choose, perhaps on the project level.

Thanks,
Alex