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

[Noel Zinn (cc)]

Frederick Pearson, II?  I didn't know that, Cliff, but, after going down the rat holes of innumerable typos in his book "Map Projections: Theory and Applications", I sort of gave up on Pearson.  CRC Press didn't serve him well.  -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)


From: Clifford J Mugnier 
Sent: Thursday, July 07, 2016 2:28 PM
To: ndzinn at comcast.net ; vanadovv at hetnet.nl ; PROJ.4 and general Projections Discussions 
Subject: Re: [Proj] Difference in Orthographic projection between Proj4 and Global Mapper

Noel,

Which then begs the question, why not use the Authalic Latitude function for your spherical equal area projection …

That’s what Pearson tried to do when proposed the idea for the old Bonne projection applications in reproducing 19th century graticules.
Not mathematically equivalent, but that’s what he proposed.

Cliff 

Clifford J. Mugnier, CP,CMS,FASPRS
Chief of Geodesy,
Center for GeoInformatics (C4G)
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Dept. of Civil & Environmental Engineering
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA  70803
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From: <proj-bounces at lists.maptools.org> on behalf of "vanadovv at hetnet.nl"
Reply-To: "vanadovv at hetnet.nl", "PROJ.4 and general Projections Discussions"
Date: Thursday, July 7, 2016 at 2:22 PM
To: "PROJ.4 and general Projections Discussions"
Subject: Re: [Proj] Difference in Orthographic projection between Proj4 and Global Mapper


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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