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

Clifford J Mugnier cjmce at lsu.edu
Thu Jul 7 14:28:00 EST 2016


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.


Clifford J. Mugnier, CP,CMS,FASPRS
Chief of Geodesy,
Center for GeoInformatics (C4G)
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Dept. of Civil & Environmental Engineering
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Cell          225-328-8975

From: <proj-bounces at lists.maptools.org<mailto:proj-bounces at lists.maptools.org>> on behalf of "vanadovv at hetnet.nl<mailto:vanadovv at hetnet.nl>"
Reply-To: "vanadovv at hetnet.nl<mailto: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.


Oscar van Vlijmen

----Origineel Bericht----
Van : ndzinn at comcast.net<mailto:ndzinn at comcast.net>
Datum : 07/07/2016 19:59
Aan : proj at lists.maptools.org<mailto: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:


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 Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com<mailto:noel.zinn at hydrometronics.com> (email)
http://www.hydrometronics.com (website)

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