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<p class="MsoNormal">Hello,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I am trying to implement the <u>inverse ISEA projection</u> documented in:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><i><span style="color:#222222">An Equal-Area Map Projection For Polyhedral Globes, John Snyder,
</span></i><i><span style="color:#555555">October 1992</span></i><i><span style="color:#222222"><o:p></o:p></span></i></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><i><span style="color:#555555">Cartographica The International Journal for Geographic Information and Geovisualization 29(1):10-21<o:p></o:p></span></i></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><i><span style="color:#222222"><o:p> </o:p></span></i></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><span style="color:#222222">For the purposes of my current project, I extended the code for the forward Snyder ISEA projection found at
<a href="https://github.com/OSGeo/proj.4/blob/master/src/PJ_isea.c">https://github.com/OSGeo/proj.4/blob/master/src/PJ_isea.c</a> to work with a rhombus-based Discrete Global Grid System (DGGS). I converted this from C to Python.
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><span style="color:#222222">Now, I am trying to implement the inverse projection according to the description provided on page 14 of the above paper.
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><span style="color:#222222">However, the problem is that the value for the
<i>spherical distance, z</i>, is not getting properly calculated (it is about 0.5-2 units “off” from the value that results after using the forward ISEA formula for the same lat,long point). The
<i>azimuth Az</i>, however, is getting properly calculated, and the only problem it seems is with the value of z. Therefore, the latitude and longitude of a point gets incorrectly calculated from the inverse, and is off by 50-100 kms from the true location.
I am attaching the main part of the Python code to this email, and it uses the same constants defined in the forward projection code.
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><span style="color:#222222">If anyone can provide some guidance as to where I am going wrong, or what the problem could be, I would greatly appreciate it. If any additional information is
needed, please let me know. <o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><span style="color:#222222">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><span style="color:#222222">Neeraj Sirdeshmukh<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><span style="color:#222222">Student, Delft University Of Technology<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:11.25pt;background:white"><span style="color:#222222"><o:p> </o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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