[QGIS-Developer] Length measurement weirdness (again)

[C Hamilton]

This is one of the reasons why I wrote Geodesic Measure tool that is part
of Shape Tools. It is based on geographiclib's distance measures and is
relative accurate. I never knew whether I had the default QGIS measure tool
set correctly or not to give accurate measurements. More accurate
measurements could be obtained using geographiclib's gravity models such as
EGM2008, but that would require shipping the associated data which is large.

By the way I found out that the latest versions of proj.4 integrated
geographiclib. Is there a way to call these routines from a python plugin
in QGIS. I would assume the C code would be faster than the python library
which is what I use.

All the best.

On Fri, May 4, 2018 at 3:07 AM, Paolo Cavallini <cavallini at faunalia.it>
wrote:

> Il 04/05/2018 01:18, Nyall Dawson ha scritto:
>
> > It's not a bug as much as very confusing 2.x behavior. The obscure
> > link between a "rendering" setting (OTF projection) and ellipsoidal
> > calculations (area/distance) is extremely confusing for users. We
> > couldn't change this in 2.x because of the API freeze and how embedded
> > this logic was in many areas of QGIS code and API.
>
> I understand, thanks for the explanation. Then why not disabling
> measurements when OTF is off? giving a totally false measurement does
> not seem a good idea.
> I believe 2.18 will be around for a good while, and I hate sending
> around wrong results of which the user has no clue whatsoever.
> Still unclear to me why the lack of ellipsoid would lead to 40% error,
> but that's my ignorance.
> All the best.
> --
> Paolo Cavallini - www.faunalia.eu
> QGIS & PostGIS courses: http://www.faunalia.eu/training.html
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