[Proj] proj-datumgrid-1.4 issued

Richard didier didier.richard at ign.fr
Wed Jul 23 07:48:30 EDT 2008

Le Mardi 22 Juillet 2008 19:28, Paul Kelly a écrit :
> On Tue, 22 Jul 2008, Frank Warmerdam wrote:
> > Richard Greenwood wrote:
> >> On Mon, Jul 21, 2008 at 7:59 PM, Frank Warmerdam <warmerdam at pobox.com>
> >>
> >> wrote:
> >>> Folks,
> >>>
> >>> I have uploaded a new version of the datum grid shift file collection
> >>> that includes the ntf_r93.gsb shift file.  Richard, perhaps you could
> >>> more fully
> >>> explain how this grid shift file works?
> >>
> >> I wish I could, but I'm still struggling with the .lla files!
> >
> > Rich,
> >
> > Sorry, I actually mean Richard Didier who added this .gsb file in PROJ. 
> > :-)

It's me !-)

> I imagine this would be similar to my france.gsb file to convert between
> the French NTF and RGF93 datums, documented in detail here:
> http://www.stjohnspoint.co.uk/gis/france.htm
> so perhaps that page would be of interest?

Well, there is a difference between your grid and our : while your grid is 
correct and our has been built using the same overall process, our grid 
ensure the centimeter precision (they are something like 2 more iterations 
when computing the grid's nodes).

You can keep our page on by just informing users the difference in precision 
between your grid and our.

> Paul
> (BTW I never pressed to have it included in GRASS or PROJ.4 as I wasn't
> sure about the licence/legal status of the tables I derived it from.)

No problem for the licence/legal status : the original text file found on 
IGN's web side is public data. A lot of GIS editors have been using it 
directly to implement the projection usign this grid. So, feel free to use by 
keeping in mind the above point on precisions.

All the best,

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Didier RICHARD, chef du pôle technique Géoportail pour l'IGN
Institut Geographique National           tel: +33/0
2/4 Avenue Pasteur                       fax: +33/0
F-94165 St-Mande Cedex                e-mail: Didier.Richard at ign.fr

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