[Proj] pj_transform function

Hugo Tyson Hugo.Tyson at cpslocates.com
Fri Oct 6 04:38:01 EDT 2006

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.

	- Huge

Hugo Tyson, Cambridge Positioning Systems Ltd
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