<div dir="ltr"><div>It looks as though these conventions are for "where do I put the (new) geographic pole?", which is "what aspect will this projection have?'' But this is different from "what is the pole of rotation (about which I rotate everything)?" which is the usual question for finite spherical rotations. This</div><div>could be added to the General Oblique Transformation by adding parameters; say lat_r, long_r, alpha_r.</div><div><br></div><div>How does Proj actually do the G.O.T? I don't mean what are the parameters, I mean what is the actual code that does the arithmetic?</div><div>(Github is not something I've used much--but a link would be good).</div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Nov 9, 2022 at 7:39 AM Even Rouault <<a href="mailto:even.rouault@spatialys.com">even.rouault@spatialys.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<p>Daven,</p>
<p>finding the right parameters is always a bit tricky. For example,
for meteorological data, there are at least 2 different
conventions to express angles.</p>
<p>See PROJ_WKT2_NAME_METHOD_POLE_ROTATION_GRIB_CONVENTION and
PROJ_WKT2_NAME_METHOD_POLE_ROTATION_NETCDF_CF_CONVENTION at
<a href="https://urldefense.com/v3/__https://github.com/OSGeo/PROJ/blob/ae3180303d974653b126bc65dcfdad3cecb5a313/src/iso19111/operation/conversion.cpp*L3783__;Iw!!Mih3wA!CLqRz9MqXF-PSnAEuYeY5ZvHpRdIzdPBOk7sZ-arOkP9TgEEjtjlBw7cQbwsGqV4lhKPrdhoZgWyMSyVDt-lZ_c2$" target="_blank">https://github.com/OSGeo/PROJ/blob/ae3180303d974653b126bc65dcfdad3cecb5a313/src/iso19111/operation/conversion.cpp#L3783</a>
where there's a mapping from south pole / north pole conventions
to +proj=ob_tran. Setting lon_0 is often necessary.</p>
<p>Even<br>
</p>
<div>Le 09/11/2022 à 13:00, DAVEN P QUINN
via PROJ a écrit :<br>
</div>
<blockquote type="cite">
<div name="messageBodySection">
<div dir="auto">Hello all,<br>
<br>
I’m a geologist who is working with paleogeographic
reconstructions (see example <a href="https://urldefense.com/v3/__https://davenquinn.com/viz/corelle-demo-pbdb/?time=295__;!!Mih3wA!CLqRz9MqXF-PSnAEuYeY5ZvHpRdIzdPBOk7sZ-arOkP9TgEEjtjlBw7cQbwsGqV4lhKPrdhoZgWyMSyVDvT4V4JT$" target="_blank">here</a>). These
require composite reproductions where different parts of a
feature dataset (continents) are rotated with different
axis-angle transformations. I’ve had good luck doing these
rotations with quaternion math in the browser/Python
environments, but I am now trying to use Proj transformations
in order to apply rotations directly within PostGIS queries.<br>
<br>
<span>This </span><a href="https://urldefense.com/v3/__https://pbs.twimg.com/media/Fgpx4UGXEAQmPOt?format=jpg&name=4096x4096__;!!Mih3wA!CLqRz9MqXF-PSnAEuYeY5ZvHpRdIzdPBOk7sZ-arOkP9TgEEjtjlBw7cQbwsGqV4lhKPrdhoZgWyMSyVDiL82wyr$" target="_blank">linked image</a> <span>shows the desired result, a
plate reconstruction to 250 Ma with a different rotation
applied to each plate. This was produced by applying the
desired quaternions through pl/pgsql math. Moving this math
to Proj internals would result in a ~50-100x speedup.</span><br>
<br>
The tool that seems most fitting is the `ob_trans` family of
projections, but I have been unable to define a rotation that
can handle my preferred representation (a pole defined in
Lon-lat coordinates and an associated angle of rotation around
it). From my reading of the docs, I believe the `o_lon_c`,
`o_lat_c` and `o_alpha` parameters should do this, but I
cannot get them to work reliably. In fact, I can’t even
reliably define a ’no-op’ transformation that leaves
coordinates unchanged. Perhaps I have the angular coordinate
system wrong, or the rotation is being done in Cartesian space
even though `proj=lonlat` is used.<br>
<br>
Is it possible to define an arbitrary spherical spatial
rotation in Proj transformations? Maybe I need to use
pipelines instead? I’d appreciate any guidance! More details
below the fold...<br>
<br>
Regards,<br>
<br>
<strong>Daven P. Quinn</strong><br>
Research scientist II · <em>U of Wisconsin Madison</em><br>
PhD · structural geology · <em>Caltech</em> ‘18<br>
<a href="https://urldefense.com/v3/__https://davenquinn.com__;!!Mih3wA!CLqRz9MqXF-PSnAEuYeY5ZvHpRdIzdPBOk7sZ-arOkP9TgEEjtjlBw7cQbwsGqV4lhKPrdhoZgWyMSyVDtXQU_j5$" target="_blank">https://davenquinn.com</a><br>
+1 704 920 8487<br>
<br>
-------------<br>
<br>
Here is an example of the pl/pgsql I am currently using to
assemble a projection (I have gone through many iterations
testing different offsets and the different ways to specify
the transformation):<br>
```<br>
RETURN '+proj=ob_tran +o_proj=longlat +o_alpha=' ||
pi()/2+angle || 'r +o_lon_c=' || pi()/2+lon || 'r +o_lat_c='
|| lat || 'r' proj<br>
```<br>
where (lat, lon, angle) defines a rotation pole.<br>
<br>
This results in some geometries (with a fortuitous set of
poles, I guess) attaining ballpark-correct transformations
while other features are shifted far outside of their origin
tiles.</div>
</div>
<br>
<fieldset></fieldset>
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