On 7/13/20 8:35 PM, Nyall Dawson wrote:
On Tue, 14 Jul 2020 at 09:41, Greg Troxel <[email protected]> wrote:
Nyall Dawson <[email protected]> writes:
On Tue, 14 Jul 2020 at 05:59, Randal Hale
<[email protected]> 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
[email protected]
https://www.northrivergeographic.com
(423)653-3611
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