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

[Ills]

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 -
http://www.remotesensing.org/geotiff/proj_list/random_issues.html#orthographic




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: 

PROJCS["Orthographic",GEOGCS["GCS_WGS_1984",DATUM["D_WGS84",SPHEROID["WGS84",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Orthographic"],PARAMETER["longitude_of_center",-3],PARAMETER["latitude_of_center",52],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["Meter",1]]

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, 

Ills 





--
View this message in context: http://osgeo-org.1560.x6.nabble.com/Difference-in-Orthographic-projection-between-Proj4-and-Global-Mapper-tp5275097.html
Sent from the PROJ.4 mailing list archive at Nabble.com.