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

Ills mangahair at gmail.com
Thu Jul 7 03:51:16 EST 2016

Hello List, 

I have encountered an issue where I am getting a mismatch between Global
Mapper and Proj4 results for orthographic projection.

I have contacted GM and dug around online so I know that they use GCTP for
their orthographic implementation and that there are issues listed here
about the difference in the results -

Our test lat and lon origins were lat=52, lon=-3

This gives us the following definitions from each system, 

Here is the proj4 string: 

+proj=ortho +lat_0=52 +lon_0=-3 +x_0=0 +y_0=0 +ellps=WGS84 +units=m +no_defs 

Here is the WKT representation exported from Global Mapper: 


Next we used a test point of lat=52.1, lon=-3.1

Our results were:
                     GM                            Proj4 
                     11,117.97, -6826.7   11136.65 -6838.1881 

I also tested proj4 by swapping +ellps=WGS84 for +ellps=sphere the results
were closer but not exact, 

sphere                             11124.1789, -6830.5331

Can anyone give me any insight into what is happening here and if I can
force proj4 to match Global Mapper (GCTP)? (I know there is reference to
that on the remotesensing.org random issues page, but how do I 'override'
the radius in proj to get the same answer?) 

Thanks in advance, 


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