[Proj] Re: Newbie Polar stereographic question

[Gerald I. Evenden]

On Wednesday 12 July 2006 2:22 pm, starjohn wrote:
> Hello Gerald!
>
> Thanks a lot for the reply!
>
> As far as I know, the projection plane is on latitude 60 and parallel to
> the equator (this is the meaning of true latitude).
>
> There is a ray from the center of earth to this plane. The center of the
> projection
> is the point on this plane where the ray from the center of earth goes
> through the center
> point and "meets" the plane.. this is the center of the projection. Makes a
> sense ?

Basically.  The source point of all the rays is the "antipod" of the point of 
tangency of the projection plane.  That is at a point -lat_0,-lon_0.  Thus 
the ray from the ray source to lat_0, lon_0 passes through the center of the 
earth.  And the plane of the projection is orthogonal to this ray.

> There is a drawing on mathworld to see this in better detail on
> http://mathworld.wolfram.com/StereographicProjection.html
>
> Thanks again and all the best
>    John
>
> > On Tuesday 11 July 2006 2:29 pm, starjohn wrote:
> >> Hello!
> >>
> >> I got image from a korean customer which is polar stereographic with
> >> center
> >> Longitude=127, and center latitude = 35
> >> The true latitude is 60. How do I use polar stereographic in this case ?
> >> Is
> >> it possible to specify true lat other than
> >> 90 or -90 ?
> >
> > "Polar" Stereographic may be a bit of a misnomer.  I suspect simple
> > Steregraphic is  what is meant with the point of tangency at 127E? and
> > 35N?.
> > In proj4 terms: +lat_0=127E +lat_0=35N.
> >
> > I cannot imagine anyone using truly polar stereographic at a latitude of
> > 35
> > degrees.
> >
> > --
> > Jerry and the low-riders: Daisy Mae and Joshua
> > "Cogito cogito ergo cogito sum"
> >   Ambrose Bierce, The Devil's Dictionary
>
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-- 
Jerry and the low-riders: Daisy Mae and Joshua
"Cogito cogito ergo cogito sum"
   Ambrose Bierce, The Devil's Dictionary