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

Paul Ramsey pramsey at refractions.net
Thu Oct 3 14:39:05 EDT 2002

Frank Warmerdam wrote:

> 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?

Cursed colonial countries! :) Henceforth I am going to declare the 
Ramsey GEOGCS to be a GRS80 ellipsoid with Victoria as a the PRIMEM!


This example appears to use the declared PRIMEM (Lisbon) as the 
parameter in the Transverse_Mercator projection following. Personally, I 
think that makes sense -- the GEOGCS is declared because it 
contextualizes the planar projection which follows, both in terms of 
ellipsoid used and also in terms of origin of parameters.

> For practical purposes in PROJ.4 I would have preferred that prime meridian
> 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.
> I would like the feedback of the PROJ.4 community on this issue before we
> commit to an approach to prime meridian support.
> Best regards,

      | Paul Ramsey
      | Refractions Research
      | Email: pramsey at refractions.net
      | Phone: (250) 885-0632

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