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