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

[Nyall Dawson]

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