[GRASS5] error in French projection definition
Paul Kelly
paul-grass at stjohnspoint.co.uk
Mon Nov 28 11:22:57 EST 2005
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
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