<div dir="ltr"><div>Using "Orthographic" instead of "Local" seems to work. At least returns some values that make sense.<br></div><div>However the parameter "Azimuth" is not used. Is there any reason for that? I was expecting just a rotation of the input before continuing with the transformation.<br></div><div><br></div><div>Thanks<br></div><div><div><div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">.___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__<br>Entre dos pensamientos racionales <br>hay infinitos pensamientos irracionales.<br><br></div></div><br></div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, 24 Sep 2020 at 11:01, 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"><u></u>
<div style="font-family:"monospace";font-size:9pt;font-weight:400;font-style:normal">
<p style="margin:0px;text-indent:0px">On jeudi 24 septembre 2020 05:42:22 CEST Kristian Evers wrote:</p>
<p style="margin:0px;text-indent:0px">> Nyall,</p>
<p style="margin:0px;text-indent:0px">> </p>
<p style="margin:0px;text-indent:0px">> </p>
<p style="margin:0px;text-indent:0px">> > </p>
<p style="margin:0px;text-indent:0px">> > Based on this, would it be appropriate to directly map this projection</p>
<p style="margin:0px;text-indent:0px">> > method across to the "Orthographic" method? Or am I missing something</p>
<p style="margin:0px;text-indent:0px">> > important?…</p>
<p style="margin:0px;text-indent:0px">> </p>
<p style="margin:0px;text-indent:0px">> </p>
<p style="margin:0px;text-indent:0px">> Yes, it would seem so. Be aware though, the current Orthographic projection</p>
<p style="margin:0px;text-indent:0px">> implementation in PROJ is only valid in the spherical case. You would have</p>
<p style="margin:0px;text-indent:0px">> to extend that to the ellipsoidal case for the mapping described by Esri. </p>
<p style="margin:0px;text-indent:0px">> We’ve got an old issue [0] which contains some hints on how to go about</p>
<p style="margin:0px;text-indent:0px">> that.</p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px">"Recent" EPSG guidance note 7.2</p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px"><a href="https://drive.tiny.cloud/1/4m326iu12oa8re9cjiadxonharclteqb4mumfxj71zsttwkx/62018e48-9da4-43e7-b598-1202cd96ec9f" target="_blank">https://drive.tiny.cloud/1/4m326iu12oa8re9cjiadxonharclteqb4mumfxj71zsttwkx/62018e48-9da4-43e7-b598-1202cd96ec9f</a></p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px">(link got from <a href="http://epsg.org" target="_blank">epsg.org</a>)</p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px">has the maths for the ellipsoidal formulation of the orthographic projection in paragraph 3.3.5, page 89</p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px">I cannot comment on the full equivalence of the "Local" projection, but hopefully in the Scale_Factor=1 + Azimuth=0 case, they would match.</p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px">One way to check that the theory local = ortho on the sphere is valid would be to use ArcGIS with a sphere as the ellipsoid and compare results with +proj=ortho</p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px">Other point:</p>
<p style="margin:0px;text-indent:0px">Looking at</p>
<p style="margin:0px;text-indent:0px"><a href="https://pro.arcgis.com/en/pro-app/help/mapping/properties/orthographic.htm" target="_blank">https://pro.arcgis.com/en/pro-app/help/mapping/properties/orthographic.htm</a> ,</p>
<p style="margin:0px;text-indent:0px">there's the Auxiliary Sphere Type parameter that can be used to change how orthographic is computed on an ellipsoid, but I'm not sure any of the listed method is equivalent to the ellipsoidal formulation from EPSG, as the ellipsoidal formulation from EPSG doesn't seem to use authalic latitude.</p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px">Even</p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px">-- </p>
<p style="margin:0px;text-indent:0px">Spatialys - Geospatial professional services</p>
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</blockquote></div>