[Proj] 6370997 origin

[Melita Kennedy]

-----Original Message-----
>Date: Thu, 7 Jul 2016 12:59:30 -0500
>From: "Noel Zinn \(cc\)" <ndzinn at comcast.net>
>Subject: Re: [Proj] Difference in Orthographic projection between
>	Proj4 and	Global Mapper
>To: "PROJ.4 and general Projections Discussions"
>	<proj at lists.maptools.org>
>
>Hi Huw,
>
>Given ellipsoidal parameters there are many spherical radii from which to choose.  Wikipedia gives a good summary:
>
>https://en.wikipedia.org/wiki/Earth_radius
>
>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?
>
>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.  
>
>Noel
>
>Noel Zinn, Principal, Hydrometronics LLC
>+1-832-539-1472 (office), +1-281-221-0051 (cell)
>noel.zinn at hydrometronics.com (email)
>http://www.hydrometronics.com (website)
>---------------------------------------------------

It's the authalic radius (rounded) of Clarke 1866. A more precise value is 6370997.24 m.

Melita

Melita Kennedy
Sr. Product Engineer
Esri