[Proj] Re: [libproj4] WGS84 to NTF - datum shift issues

Patrick Mézard pmezard at gmail.com
Fri Feb 25 08:32:55 EST 2005

[schizophrenic exchange with my other self]

On Fri, 25 Feb 2005 12:14:24 +0100, Patrick Mézard <pmezard at gmail.com> wrote:
> Hello,
> I am trying to figure how to convert correctly from WGS84 to NTF
> (Lambert II carto étendue) using libproj4. My first attempts returned
> points with offsets of 100 meters around in X and 10 meters in Y (I am
> comparing with an existing points codebase which accuracy I am pretty
> confident in).
> Then I tried every parameter set I could find online, including those
> from the PROJ.4 espg file. Nothing changed. Eventually, I looked on
> the mailing list and stumble on this one :
> <http://article.gmane.org/gmane.comp.gis.proj-4.devel/721/match=ntf>
> If I understand correctly, Gerald says that libproj4 does not support
> datum shifts. I am not as knowledgeable as you are but his arguments
> look rather sensible to me. So (yet, yet another pile of questions for
> you :-) :
> 1- Do you think my offset problems are related to datum shifts ? Or do
> you know someone who managed to convert from WGS84 to NTF with good
> accuracy ?

Yes, my problem si related to datum shifting (or whatever you call the
towgs84 option).

> 2- Will PROJ.4 solve my problems ?

Yes, it just solved my problem 5 minutes ago, thank you. And the
pj_transform function is great, exactly what I needed.

Patrick Mézard

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