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<p>Mircea,</p>
<p>could you file an issue at <a class="moz-txt-link-freetext" href="https://github.com/OSGeo/PROJ/">https://github.com/OSGeo/PROJ/</a> about
that ? It would help if you could provide evidence that a change
is needed: like wrongly transformed coordinates (possibly with a
screenshot of a reprojected map before and after your fix) or that
the current forward formula doesn't roundtrip with the inverse
formula before your fix, but does after, etc.</p>
<p>I see in
<a class="moz-txt-link-freetext" href="https://github.com/OSGeo/PROJ/commit/5cad2f68f2a68c8c6854d0bfc62700299598dfa1">https://github.com/OSGeo/PROJ/commit/5cad2f68f2a68c8c6854d0bfc62700299598dfa1</a>
that I already did a atan() -> atan2() change in another
equation (9-8) of ocea, so maybe equation (10-4) also needs it.</p>
<p>Even<br>
</p>
<div class="moz-cite-prefix">Le 14/03/2023 à 19:18, Mircea Neacsu a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:186e158b2ef.128053f992568495.2791380176809508392@neacsu.net">
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<div style="font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 10pt;">
<div>Hello,<br>
</div>
<div><br>
</div>
<div>I think I may have found a small error in the above
mentioned projection (file ocea.cpp). The error can be
directly traced to Snyder and the numerical example he gives
for the spherical form of this projection.<br>
</div>
<div><br>
</div>
<div>In his numerical example (page 279, formula for x) Synder
has the following derivations:<br>
</div>
<div>0.98X arctan [-0.6223338/(-0.9720102)l<br>
</div>
<div>= 0.98X(0.5694937 + rr), adding .rr since denominator is
negative.<br>
</div>
<div><br>
</div>
<div>In ocea.cpp (line 29) this translates into the following
code:<br>
</div>
<div>if (t < 0.)<br>
</div>
<div> xy.x += M_PI;<br>
</div>
<div><br>
</div>
<div>I believe that Synder should have subtracted M_PI as the
numerator of the atan fraction is negative (see Snyder page ix
discussion about ATAN2 function) and consequently ocea
projection should also use atan2.<br>
</div>
<div><br>
</div>
<div>I have to admit I'm writing this with a certain trepidation
as reporting an error in Snyder is almost like finding faults
in the Bible, but I'd like to have this clarified.<br>
</div>
<div><br>
</div>
<div>The error is probably insignificant and I ran into it only
because I'm working on an online version of the Snyder book.
You can check it at <a target="_blank"
href="https://neacsu.net/docs/geodesy/snyder/"
moz-do-not-send="true" class="moz-txt-link-freetext">https://neacsu.net/docs/geodesy/snyder/</a>
Currently I'm transcribing the numerical examples to JS and
MathJax and that's how I ran into this issue.<br>
</div>
<div><br>
</div>
<div>Thank you for looking into it.<br>
</div>
<div class="zmail_signature_below">
<div id="" data-zbluepencil-ignore="true"
data-sigid="5934168000004827002">
<div>-- <br>
</div>
<div><br>
</div>
<div>Mircea Neacsu <br>
</div>
</div>
</div>
<div><br>
</div>
</div>
<br>
<br>
<fieldset class="moz-mime-attachment-header"></fieldset>
<pre class="moz-quote-pre" wrap="">_______________________________________________
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</pre>
</blockquote>
<pre class="moz-signature" cols="72">--
<a class="moz-txt-link-freetext" href="http://www.spatialys.com">http://www.spatialys.com</a>
My software is free, but my time generally not.</pre>
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