[Proj] cs2cs and PDC Mercator

[Mikael Rittri]

José Gonçalves wrote:
 
> I think that is the expected result.

I agree. The interrupted meridian (where the Mercator cylinder is cut)
should be antipodal to the central meridian.  For PDC Mercator, this 
means that projected coordinates should be continuous in the entire 
Pacific Ocean. (PDC = Pacific Disaster Center, Hawaii.)

By the way, note that EPSG:3349 was deprecated in 2008 and replaced 
by EPSG:3832.  The corrected CRS has central meridian at 150 E,
instead of 150 W. 

Regards,
--
Mikael Rittri
Carmenta AB
SWEDEN
www.carmenta.com

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Jose Gonçalves
Sent: den 6 januari 2010 01:22
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] cs2cs and PDC Mercator

Hello

I think that is the expected result. The formula for the X coordinate is:
X=a*(lon-lon_0)*PI/180
where a is the semi-major axis and longitudes are in degrees. If the result of the longitude difference is out of the interval (-180,+180) it is reduced to this interval by adding or subtracting 360 degrees.

 -179.999999 + 150 =  -29.999999   ===>  -29.999999

+179.999999 + 150 = +329.999999   ===>  -30.000001

So the result is approximately the same (2 microdegree difference, which corresponds to approx. 22 cm).
Probably you get the result you want by changing +lon_0 to 0 and including the appropriate x_0 value.

Regards

José Gonçalves




2010/1/5 John Cartwright <John.C.Cartwright at noaa.gov>:
> Hello All,
>
> I'm trying to convert geographic coords (EPSG 4326) to PDC Mercator 
> (EPSG 3349) and was using the command line listed below.  I was 
> surprised to find that the PDC mercator X values were the same for
> -179.999999 or +179.999999.  Can someone please explain what I'm doing 
> wrong?
>
> Thanks!
>
> --john
>
>
> cs2cs +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs  +to +proj=merc
> +lon_0=-150 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m 
> +no_defs <<EOF
>> -179.999999 -89.999999
>> 179.999999 -89.999999
>> 179.999999 89.999999
>> -179.999999 89.999999
> EOF
>
> -3339584.61   -118315559.69 0.00
> -3339584.84     -118315559.69 0.00
> -3339584.84     118315559.73 0.00
> -3339584.61     118315559.73 0.00
>
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