[Qgis-user] Question on Project Properties - Ellipsoid

[Nyall Dawson]

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?

> Now I'm getting a
> ballpark transformation message if I move data from 2274 to 4326 (for
> instance I'm moving points to a GPX Format).....BUT - if I set the
> Ellipsoid to be GRS 1980 (which it is for that projection) - no warning.

Can you open a ticket on this and attach your data? It's quite
possible that the warning is correct, but it's also possible it's a
spurious warning and I can hide it in this circumstance.

> Which now has me thinking MAYBE I should set the ellipsoid..... EXCEPT
> my area measurements always come out a bit larger than expected.

Which **IS** expected! :D An ellipsoidal area will almost always
result in a larger area vs a cartesian area calculation, because it's
calculating the area over a curved surface rather than a flat surface.
(same applies to length measurements).

Generally my advice is to use ellipsoidal area/length calculations,
unless the specific constraints and specifications of a particular
task or discipline dictate otherwise. If in doubt, I'd:

1. compare the two area calculation results. The identify tool
"derived attributes" section handily shows BOTH cartesian and
ellipsoidal areas/lengths, if you've setup your project to use an
ellipsoid. If the results are relatively similar (e.g. say within 0.1%
of each other), then ask yourself whether it makes any difference
practically which of them you use. Chances are the accuracy of your
input datasets will be a bigger factor here then the area calculation
method used, and you may be chasing false accuracy by reporting areas
to that many significant figures anyway! Or, in other cases, the
actual purpose of the calculations you're making aren't even required
to a degree of accuracy to warrant being affected by the choice!
(e.g. if you're calculating species density, then it's an inherent
estimate only and the practical outcome of choosing between
cartesian/ellipsoidal calculations is nill).

2. if you've checked and the input datasets ARE sufficiently accurate
that the choice matters, then push the choice back to your customer.
Ask them which method is suitable for the results they're after. Get
the answer in writing and your method is justified!

3. keep in mind that even ellipsoidal measurements are an estimation.
The earth isn't an ellipsoid! Local variations in elevation are likely
to have a much more significant impact on large scale calculations vs
the cartesian/ellipsoidal choice.

Hope that helps!
Nyall







> So I go
> back to "none/planimetric".....BUT - is that causing issue with
> reprojecting data........
>
> Any thoughts? Thanks for any support or pointing me in a direction.
>
> Randy
>
> --
> Randal Hale
> rjhale at northrivergeographic.com
> (p) 423.653.3611
> http://www.northrivergeographic.com
>
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