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

[Noel Zinn (cc)]

6367440 is approximately the average (3 meters off) of the semi-major and semi-minor axes of the WGS84 ellipsoid, which is of absolutely no significance geodetically.  It's not the radius of the sphere with the same volume or the same area as WGS84.  It's just an uniformed guess of an orthographic radius.  Maybe Blue Marble haven't yet gotten around to cleaning up Global Mapper's cartography, so I'd hesitate forcing proj4 to match GM.  Anyway, it's time to use an ellipsoidal orthographic instead, which you can read about here:



http://www.hydrometronics.com/downloads/Ellipsoidal%20Orthographic%20Projection.pdf



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: Kristian Evers 
Sent: Thursday, July 07, 2016 4:48 AM
To: PROJ.4 and general Projections Discussions 
Subject: Re: [Proj] Difference in Orthographic projection between Proj4 and Global Mapper

No, it is the same as using +ellps=sphere – as was already tried. By doing a bit of manual bisection on the radius I came up with the following that more or less matches the results from GM:

 

> echo -3.1 52.1 | proj +proj=ortho +lat_0=52 +lon_0=-3 +a=6367440 +units=m +no_defs

-6826.72        11117.97

 

I have no idea if that works for other coordinates though! Probably not… 

 

Kristian

 

Fra: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] På vegne af Thomas Knudsen
Sendt: 7. juli 2016 11:32
Til: PROJ.4 and general Projections Discussions
Emne: Re: [Proj] Difference in Orthographic projection between Proj4 and Global Mapper

 

Does this solve the problem? (taking the radius from the remotesensing page you referred to):

 

$ echo -3.1 52.1 | proj +proj=ortho +lat_0=52 +lon_0=-3 +x_0=0 +y_0=0 +a=6370997 +f=0 +units=m +no_defs

 

Output:

 

-6830.53        11124.18

 

/Thomas



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