[Qgis-developer] Why is ellipsoidal calculation tied to project OTF setting?

[Blumentrath, Stefan]

Hi Nyall,

Just as a note: The ellipsoidal measure is quite uncommon in other GIS packages.
PostGIS, SAGA, GRASS and the mayor proprietary GIS are all using a Cartesian measure. Deep down in the history of the several related tickets, you can see that users are often surprised if measures in QGIS differ from measures they got from other sources (including official statistics).

I am no geodesist, so I do not have the expertise to judge which type of measure should be favored / the default. But I would expect the other packages have their reasons for using Cartesian measurements... Given that the code for the ellipsoidal measurement is actually copied from GRASS, which mostly does not use it, I – as a lay person in this matter – would be a bit concerned to have an ellipsoidal area/distance measure as the default...

But that probably does not answer your question...

Cheers
Stefan


From: Qgis-developer [mailto:qgis-developer-bounces at lists.osgeo.org] On Behalf Of Nyall Dawson
Sent: 5. februar 2016 23:02
To: qgis-developer <qgis-developer at lists.osgeo.org>
Subject: [Qgis-developer] Why is ellipsoidal calculation tied to project OTF setting?


Hi all,

While looking into the outstanding area/distance calculations issues there's something I'm struggling to understand.

In QGIS ellipsoidal distance/area calculations are ONLY enabled when the project has OTF crs transformation enabled. It could be because I've spent too long staring at this code and lost sight of the bigger picture, but why is this a requirement??

I can't see any reason why an ellipsoid can't be used even when layers aren't being reprojected on the fly. To me, it's a totally different setting without any relation to distance/area measurement.

If you don't want ellipsoidal calculation you could still set the ellipsoid to none. But you *could* potentially want ellipsoidal calculation even when OTF is off.

Am I missing something here?

Nyall
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