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

[Patrick Mézard]

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 ?

2- Will PROJ.4 solve my problems ?

3- If datum shifts are responsible for this, could you add a clear
warning in libproj4 manual for ignorant users like me :-) ?

Best regards.

Patrick Mézard