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

Luis Mota luis.mota at iscte.pt
Thu Oct 3 14:57:09 EDT 2002

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.

 >For practical purposes in PROJ.4 I would have preferred that prime 
 >issues remain completely divorced from the projection routines (pj_fwd,
 >pj_inv) and only addressed at the point where transations are done 
from one
 >lat/long coordinate system to another.  But this sort of implies that all
 >longitude parameters to a projection with an alternate prime meridian are
 >implicitly relative to that pm ... that may be a peculiar convetion.

Seems a good idea to me.
 >I would like the feedback of the PROJ.4 community on this issue before we
 >commit to an approach to prime meridian support.

Yes, it's better to do it that way.

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?

Bye, Luís

PROJ.4 Discussion List
See http://www.remotesensing.org/proj for subscription, unsubscription
and other information.

More information about the Proj mailing list