[geotk] "Custom" location local projection

Martin Desruisseaux martin.desruisseaux at geomatys.fr
Wed Sep 16 04:18:31 EDT 2009

Hello Sam

As you guessed, the most tricky part is likely to be axis orientation. I don't 
think you will need to bother about the kind of Stereographic projection. When 
asking for a "Stereographic" projection, Geotk will select automatically one of 
the "Oblique", "Equatorial" and "Polar" flavor, but actually the "Equatorial" 
and "Polar" cases are simplifications of the "Oblique" case in the special case 
where some internal coefficients are set to 0 or 1 because of the latitude at 0 
or 90°. Consequently the transition from the "Polar" to the "Oblique" case when 
you move from 90° to 89.999° should be continuous (I think so; I have not verified).

However the axis directions may need more verifications. In the "Oblique" case, 
you should get the classical (East,North) directions (assuming you are using the 
DefaultCartesianCS.PROJECTED constant or something equivalent). In the South 
Pole case, I would expect the axis directions (North along the 90° meridian, 
North along the 0° meridian) to be continuous with the (East, North) case, but 
some trial may be needed in order to verify the actual behavior. If you spot an 
anomaly, I would be very interrested in a small test case reproducing the 
problem so I could fix it.

Is the above coherent with your question?


Sam B a écrit :
> Hi all !
> Sorry to ask again, but I still have one last question regarding this post :
> I spoke about the orientation of the projected map (where does the Y 
> axis points to). I just need to know if it is possible that, for a given 
> projection point, my grid is oriented in one direction, and for a second 
> point very close to the second one, the grid is oriented totally 
> differently.
> I'll use an example to make my question clearer : Let's consider a 
> projection point being exactly on the North pole. I think Geotoolkit 
> will automatically select a stereographic polar projection (latitude of 
> origin being 90°). I guess that the Y axis of my grid will be oriented 
> towards Greenwhich (but maybe I am wrong). Now I consider another case, 
> where the projection point is extremely close from the previous one, 
> let's consider longitude = 0° and latitude = 89.999999°. The projection 
> will probably be different. Is that possible now that the Y axis of my 
> grid is oriented towards the North pole ?
> Because in my project, I have to do such "custom projections" and I just 
> would like to know if I have to take special precautions for handling 
> such situations.
> I hope I was clear enough... :-)
> Many thanks in advance and best regards,
> Sam

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