Maciej,

thank you for your contribution of the General Oblique Transform to 
GeoTools.
https://github.com/geotools/geotools/blob/master/modules/library/referencing/src/main/java/org/geotools/referencing/operation/projection/GeneralOblique.java
https://osgeo-org.atlassian.net/browse/GEOT-5190

While attempting use the General Oblique transform, I have encountered a 
discrepancy between the latitude of the origin and the projected latitude.

In the test script:

https://github.com/geotools/geotools/blob/master/modules/library/referencing/src/test/resources/org/geotools/referencing/test-data/scripts/GeneralOblique.txt

The test General Oblique coordinate system is specified with:

PARAMETER["central_meridian", 19.3],
PARAMETER["latitude_of_origin", 37.5]

The following assertion is made:

source pt = (0, 0)
target pt = (19.300000,52.500000)

This looks wrong to me because I expected the origin of the rotated 
coordinate system to have the latitude specified in the parameter 
latitude_of_origin, that is 37.5.

If I specify my own General_Oblique CRS in GeoServer with 
PARAMETER["central_meridian", -106] and PARAMETER["latitude_of_origin", 
54] and use the reprojection console to reproject (0, 0) to WGS84, I get 
the (lon, lat) = (-106.0, 36.0) which is incorrect. Likewise (-106.0, 
54.0) projected to this same General Oblique CRTS is incorrectly 
projected to (0.0, 18.0) (should be (0,0)).

In each case, it appears to me that the transformed latitude is 90 
degrees minus the expected latitude. This is consistent with the 
latitude being measured from the North Pole not the equator. As this is 
a common mathematical convention, I wonder if there is a problem with 
the implementation of the General Oblique transformation. Can you see 
anything wrong? Or do I misunderstand?

If this is changed, we will need your help to generate and validate 
changes to the test script.

Kind regards,

-- 
Ben Caradoc-Davies <[email protected]>
Director
Transient Software Limited <http://transient.nz/>
New Zealand

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