[Proj] Difference in Orthographic projection between Proj4 and Global Mapper

[vanadovv at hetnet.nl]

The radius of 6370997 is the integer part of the authalic radius of the Clarke 1866 ellipsoid.
The calculated value would be around 6370997.24063266 m.
Greetings,
Oscar van Vlijmen
 ----Origineel Bericht----
Van : ndzinn at comcast.net
Datum : 07/07/2016 19:59
Aan : proj at lists.maptools.org
Onderwerp : Re: [Proj] Difference in Orthographic projection between Proj4 and	Global Mapper
 
  
   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)
   
  
 
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