<div>The radius of 6370997 is the integer part of the authalic radius of the Clarke 1866 ellipsoid.</div><div>The calculated value would be around 6370997.24063266 m.</div><div> </div><div> </div><div> </div><div>Greetings,</div><div> </div><div>Oscar van Vlijmen</div><div><br> </div><blockquote style="margin-right: 0px; margin-left: 15px;">----Origineel Bericht----<br>Van : ndzinn@comcast.net<br>Datum : 07/07/2016 19:59<br>Aan : proj@lists.maptools.org<br>Onderwerp : Re: [Proj] Difference in Orthographic projection between Proj4 and Global Mapper<br><br><div dir="ltr">
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Hi Huw,
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Given ellipsoidal parameters there are many spherical radii from which to choose. Wikipedia gives a good summary:
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<a title="https://en.wikipedia.org/wiki/Earth_radius" href="https://en.wikipedia.org/wiki/Earth_radius">https://en.wikipedia.org/wiki/Earth_radius</a>
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But where does GCTP’s 6370997 come from? It’s close to some radii for WGS84, but no cigar. Is it just a convention? Anyone know?
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Of course, if you’re mapping on a sphere “closeness” shouldn’t really matter very much. Consistency should matter more, and, in that sense, a conventional radius would be useful.
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Noel
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Noel Zinn, Principal, Hydrometronics LLC
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<br>noel.zinn@hydrometronics.com (email)
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<a href="http://www.hydrometronics.com">http://www.hydrometronics.com</a> (website)
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