# [Proj] pj_transform function

Sandeep Kumar Jakkaraju sandeepkumar.jakkaraju at gmail.com
Fri Oct 6 04:57:04 EDT 2006

```Hi Hugo

So what should be the  projection definition then .. to make it general..???

I am trying to convert the geographical co-ordinates to screen co-ordinates
..
so i require to convert the lat/lon to some projection and the convert that
X,Y to pixels...
hope this is correct ...

or is there some function in proj4/or anywhere else which does this ....??

Thanx again

sandeep

On 10/6/06, Hugo Tyson <Hugo.Tyson at cpslocates.com> wrote:
>
> proj-bounces at lists.maptools.org <> wrote:
> > I want to transform ... this lat/lon 39.739/-104.985...
> > to world mercator project ..with the defintition
> >
> > World Mercator <54004> +proj=merc +latts=0 +lon0=0
> > +k=1.000000 +x0=0 +y0=0 +ellps=WGS84 +datum=WGS84 +units=m no_defs <>
> >
> > but the method gives an "error"....
> >
> > this method works fine if i give lat/lon say 42.4182,-74.2643
>
> I've not heard of "world mercator", but looking at the parameters
> it's projecting into a plane tangent to the sphere at lat 0, lon 0.
> So it only works for longitudes from -90 to +90 (though these map
> to infinities in the projected plane).
>
> Your example of -104 longitude is around the back of the earth,
> on the "far side" from the projection plane.   Therefore a
> line from the origin through the point never intersects the
> projection plane, so no result can be calculated.  Hence your
> error.  Lon -74 is in range, so it's OK.
>
> HTH,
>        - Huge
>
>
> --
> Hugo Tyson, Cambridge Positioning Systems Ltd
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--
Sandeep Kumar Jakkaraju
WeBlog:
http://jakkarajus.blogspot.com
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