[OSRS-PROJ] Re: Prime Meridian Support in PROJ.4

Frank Warmerdam warmerdam at pobox.com
Thu Oct 3 14:58:16 EDT 2002

Luis Mota wrote:
> Hi again Frank
>> One concern I have is whether the prime meridian should be considered to
>> have any relationship to longitude parameters for a projection.  For
>> instance, if a TM projection uses a geographic coordinate system with a
>> Lisbon prime meridian, is the central meridian supposed to be 
>> specified as
>> degrees from Lisbon or Greenwich?  What is common practice?
> This is an easy question , I think. Both alternatives are possible, but, 
> as far as I remember, both EPSG and the ISO TC 211 (which deals with GI) 
> speak of the 'greenwich longitude', which, in the case of the lisbon 
> meridian, and as you can check in 
> http://gdal.velocet.ca/cgi-bin/csv2html/TABLE=p_meridian/CODE=8902/, sth 
> like -9.07. This means that this meridian is west of greenwich.


If this is the case, the pj_fwd() and pj_inv() function which map from
projected coordinates to lat/long in the same GCS would need to take
the prime meridian into account as well.  As I think about it, this isn't
really such a terribly hard thing to do and would be worthwhile to avoid
disconnects with EPSG, ISO and OGC standard data models.

> Anyway, in the meantime, I would like to take a look at the code to see 
> if I understand it. Would you kindly point out what is function that 
> addresses GCS to GCS conversion?

I believe you will want to insert the prime meridian support in
pj_transform() in src/pj_transform.c.  Depending on our approach above,
the pj_inv() and pj_fwd() functions may also require prime meridian support.

I would suggest that the prime meridian should be parsed out by pj_init()
and placed as a new field in the PJ structure defined in projects.h.  Perhaps
just under the "int islat_long" flag.

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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