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

[Clifford J Mugnier]

Noel,

By the way, … GCTP was originally written in FORTRAN77 by Dr. Atef Elassal (a Photogrammetrist) who passed away earlier this year.  His obituary was published in PE&RS.  Everything in GCTP was kosher John P. Snyder, since both worked at the time for USGS Reston.

Clifford J. Mugnier, CP,CMS,FASPRS
Chief of Geodesy,
Center for GeoInformatics (C4G)
266 ERAD (Research)
3335 PFT  (Academic)
Dept. of Civil & Environmental Engineering
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA  70803
Research  225-578-4578
Academic   225-578-8536
Cell          225-328-8975


From: Cliff Mugnier
Date: Thursday, July 7, 2016 at 2:28 PM
To: "ndzinn at comcast.net<mailto:ndzinn at comcast.net>", "vanadovv at hetnet.nl<mailto: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)
266 ERAD (Research)
3335 PFT  (Academic)
Dept. of Civil & Environmental Engineering
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA  70803
Research  225-578-4578
Academic   225-578-8536
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.



Greetings,

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:

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<mailto:noel.zinn at hydrometronics.com> (email)
http://www.hydrometronics.com (website)


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