[Qgis-user] Question on Project Properties - Ellipsoid

[Nyall Dawson]

On Tue, 14 Jul 2020 at 09:41, Greg Troxel <gdt at lexort.com> wrote:
>
> Nyall Dawson <nyall.dawson at gmail.com> writes:
>
> > On Tue, 14 Jul 2020 at 05:59, Randal Hale
> > <rjhale at northrivergeographic.com> wrote:
> >>
> >> I hope everyone is staying safe out there.
> >>
> >> So I've had some questions regarding setting up a QGIS Project and now
> >> I've convinced myself that I'm doing this wrong. When I get a new
> >> project in and I start setting up QGIS generally my project will be in a
> >> Stateplane projection - in this case EPSG:2274 (TN Stateplane NAD83).
> >> When I set up the project I change the ellipsoid to be
> >> "none/planimetric" (In the project properties). I don't set up the
> >> ellipsoid to be GRS 1980 and haven't for some time.
> >
> > Can I ask why you do this?
>
> I too am boggled by this.  EPSG:2274 is a Lambert Conformal Conic
> projection of NAD83, and I don't understand what it means to do that
> projection to/from geodetic coordinates without an ellipsoid definition.
> Using the GRS80 ellipsoid is part of the definition of this CRS, so if
> you change that you are using a different CRS that is not actually 2274,
> and that 2274prime CRS should throw an exception if you try to convert
> to/from NAD83 geodetic.

It's actually unrelated to projection. You CAN'T change the ellipsoid
used by a projection, at least without creating your own custom
projection.

The ellipsoid settings in project properties are only utilised for
area/length calculations, not transformations.

Nyall