[GRASS5] error in French projection definition

[Paul Kelly]

Hello Moritz

On Tue, 22 Nov 2005, Moritz Lennert wrote:

> Hello,
>
> On the french-speaking list a discussion has been taking place concerning the 
> definition of the French NTF datum. Apparently it is not correct as 
> datum.table defines it as using clark80, but the definition of clark80 in 
> ellipse.table does not correspond exactly to the one used by the French 
> geographical institute.
>
> I am quite unknowledgeable both about projections and about the way they 
> function in GRASS, especially the interplay between the GRASS definitions and 
> the proj definitions. So before I do anything wrong or unnecessary:
>
> - Should I change the two files to reflect the correct defintions. Mathieu 
> Basille proposes the following changes:
>
> In datum.table :
> # Nouvelle Triangulation Française
> ntf      "Nouvelle Triangulation Française"      clark80IGN    dx=-168.0 
> dy=-60.0    dz=320.0

First of all that should be "Nouvelle_Triangulation_Francaise" even if it 
is not quite grammatically correct. It should correspond to the official 
EPSG name which only uses ASCII characters. I have now added a comment to 
datum.table to indicate that.

> In ellipse.table :
> # Clarke 1880 IGN
> clark80IGN      "Clarke 1880 IGN"               a=6378249.200 
> f=1/293.466021294

Two things:
1) The differences are so small from the version already in GRASS that 
it's not clear to me how big a difference it would make (although I could 
be wrong). What is the problem that resulted in the proposal for these
changes?
2) I would rather fix the existing clark80 definition to get it right than 
introduce an alternative different version.

For better datum transformation you can also use
nadgrids: france.gdb
in the PROJ_INFO file (instead of dx,dy,dz)
Get the france.gsb file at http://www.stjohnspoint.co.uk/gis/france.htm
I should maybe see about integrating this file into the GRASS distribution 
as it is hard to find good datum transformation parameters for France.

One other thing to look out for is the way to specify the Lambert 
Conformal Conic projection with GRASS/PROJ. I suspect g.setproj has some 
bugs with regard to this as there is more than one way of specifying the 
parameters for this projection. But I'm not too sure about it all, just 
have an uneasy feeling!

> - Or is this handled by proj ? What is the relation between the two ?

The ellipse.table file in GRASS dates from the mid-late 1980s and was 
originally only used by some d.* commands that converted UTM co-ordinates 
to lat/long. It pre-dates the introduction of PROJ into GRASS in the early 
1990s as far as I know.

Best regards,

Paul