[QGIS-Developer] Is the measurement tool using proj library?

[Nyall Dawson]

On Fri, 19 Jul 2019 at 23:15, Andreas Neumann <a.neumann at carto.net> wrote:

> For myself, I decided to never make fun again about other people not understanding projections, given that I make such mistakes myself ...

I had this moment of realisation too while reading through

https://lists.osgeo.org/pipermail/proj/2019-July/008703.html
and
https://www.openstreetmap.org/user/StephaneP/diary/390290

(all those "spatial isn't special" proponents trying to do data
science on raw x/y alone are about to have a ten ton weight dropped on
their heads)

Nyall




>
> I wonder if it would make sense to issue a warning if a user creates data outside of the valid bounds of a projection? It would definitely make sense for stupid people like myself ...
>
> Thanks and greetings,
>
> Andreas
>
> On 2019-07-19 01:18, Nyall Dawson wrote:
>
> On Fri, 19 Jul 2019 at 00:38, Andreas Neumann <a.neumann at carto.net> wrote:
>
>
> Hi,
>
> We are having issues on the QGIS 3.4 release that the difference between and ellipsoidical and planimetric distance (and area) is huge. Too huge.
>
> We have this issue on Windows 3.4.9, but on Linux (a bit newer commit) I don't have the same huge difference, but rather a very reasonable one.
>
> I noticed that while the Windows version was built with proj 520 and the Linux version with 493, so a bit older.
>
> But I don't know if the measurement tool in QGIS is using proj at all or not? If yes, could it be that newer proj releases made things worse regarding ellipsoidical measurements?
>
>
> It's possible. The actual measurement calculation uses code ported
> from GRASS (and not the proj implementation of these algorithms), but
> proj IS used in transforming map/measure coordinates prior to
> calculating the distance. Possibly something has changed there which
> is impacting this.
>
> Could you please test:
>
> 1. Do you see the same distance values when measuring feature lengths?
> I.e. setup a layer with a feature with the same line as you are
> measuring, and check the identify tool derived attributes or $length
> value for it.
>
> 2. Can you confirm that the project ellipsoid settings are identical
> between the windows & linux builds?
>
> 3. Can you share an example I can test with?
>
> Nyall
>
>