[Qgis-user] Question on Project Properties - Ellipsoid

[Randal Hale]

On 7/13/20 8:35 PM, Nyall Dawson wrote:
> 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.
So the next question would be - way back in a past life I worked with 
mapping at Fossil/Nuclear Plants and some had custom grids - no 
projection just a 0,0 and then everything was on a grid. In that case 
would it be acceptable to do "none/planimetric" when working on a 
project like that? No projection - just a grid.
> Nyall

-- 
-----------
Randal Hale
rjhale at northrivergeographic.com
https://www.northrivergeographic.com
(423)653-3611