On 7/13/20 7:22 PM, Nyall Dawson wrote:
On Tue, 14 Jul 2020 at 05:59, Randal Hale
<rjh...@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?
That's a great question - somehow I became convinced this was right.
Mostly it resulted from a job I was doing for a forester - the area
calculations from ellipsoid were coming out larger than it should have
been. Planimetric put me close to what was "right". I started doubting
this was right a few weeks ago wrestling with the "ballpark transform"
error message.
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.
I can!
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).
I'm really starting to wonder if this started back at 3 (in my head) and
I've continued down the wrong path since then. Maybe it was a bug or
something in the beginning and I never adjusted based on everything
getting fixed. Maybe. I really don't have a good answer.
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!
Yes - it helps. I'm getting a lot of questions from people getting
"ballpark transform" messages which led to the last little bit of
testing, head scratching, and "Hey I can be completely wrong on what I'm
doing".
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
rjh...@northrivergeographic.com
(p) 423.653.3611
http://www.northrivergeographic.com
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--
-----------
Randal Hale
rjh...@northrivergeographic.com
https://www.northrivergeographic.com
(423)653-3611
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