[OSRS-PROJ] prime meridians

Frank Warmerdam warmerdam at pobox.com
Mon Dec 9 10:59:55 EST 2002


I have introduced a form of prime meridian support to PROJ.4.  The
+pm=<prime_meridian> option can be used to set a prime meridian.  The
argument can be a longitude (relative to Greenwich) or the name of
a well known prime meridian.  These can be listed with the -lm flag:

warmerda at gdal[317]% cs2cs -lm
    greenwich 0dE
       lisbon 9d07'54.862"W
        paris 2d20'14.025"E
       bogota 74d04'51.3"E
       madrid 3d41'16.48"W
         rome 12d27'8.4"E
         bern 7d26'22.5"E
      jakarta 106d48'27.79"E
        ferro 17d40'W
     brussels 4d22'4.71"E
    stockholm 18d3'29.8"E
       athens 23d42'58.815"E
         oslo 10d43'22.5"E

The prime meridians are applied within pj_transform() so that if the
input or output coordinate system is proj=latlong with a prime meridian
set the longitude will be offset accordingly.  The prime meridian does
not effect pj_fwd() and pj_inv() all. The u and v results from these
functions will always be relative to Greenwich.  So (by association)
the prime meridian will have no effect on the proj program, it is
only utilized in cs2cs (based on pj_transform()).

When I generate the new epsg init database I will be including non
greenwich meridians where appropriate.

I also looked at implementing an angular units option but decided
against it in the end.  Currently PROJ.4 input/output angles are all
radians, and it is up to the client programs (including proj and cs2cs)
to convert to degrees or other units if they wish.  Therefore I don't
feel it appropriate for the proj.4 core to "known" about coordinate
system angular units.  I have implemented support for angular units in
my OGRCoordinateTransformation wrapper around PROJ.4.

Let me know if there are any questions.

Best regards,

I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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