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

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

Frank Warmerdam wrote:
 > In short, I don't think you can operate on coordinate systems with
 > non-greenwich prime meridians properly at this time.
 > The following bugzilla enhancement request addresses this issue:
 >  http://bugzilla.remotesensing.org/show_bug.cgi?id=186 Luis Mota wrote:

Luis Mota wrote:
>  From what I see on this link, you are the person this enhancement has 
> been assigned to. Also, I suspect that this enhancement is not a 
> priority to you...
> The thing is that it is quite important to me, since almost all my data 
> is expressed on one of this non-greenwich-prime-meridian-datum. It seems 
> to me that this enhancement is not too complicated: it would be a 
> question of subtracting the pm value to an input datum and adding it to 
> the output, isn't it? Or is it more difficult?
> If this is the case, what about if I tried to solve this question? You 
> would send me the code and I'd try to contribute with this 
> improvement... Is the code written in C? Would you guide me through the 
> process and check that the results are correct?


I have moved this discussion to the PROJ.4 mailing list, since that is
more appropriate.

PROJ.4 is indeed a C library and you are certainly welcome to prepare a
patch yourself.  If so, please attach the completed and tested patch to
the bug report and I will (after a bit of review) apply it to the core.

If you want to do this, please prepare the patch against the current CVS
version of PROJ.4.  Various information about PROJ.4, including anonymous
CVS access to the source is available at:


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?

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,
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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