[MetaCRS] CS-Map RFC 1 - Creation of a GDC file to handle French grid files

[Frank Warmerdam]

Hugues Wisniewski wrote:
> Hi,
> 
>  
> 
> I would like to submit "CS-Map RFC 1 - Creation of a GDC file to handle 
> French grid files" for review and comment:
> 
> http://trac.osgeo.org/csmap/wiki/CsMapRfc1

Hugues,

I have reviewed this RFC and I see no obvious problem with it.

As I understand it the .gdc "catalog" mechanism is already used in CS-Map.
It seems the only thing being done is to create a catalog file for the NTF
datum.  Is that right?

In this regard I might suggest an official RFC may not be required.

However, since the topic is raised I do have a few questions, mostly oriented
towards increasing integration of grid shift datasets between different
packages.

1) My understanding is that NTv2 files already provide a heirarchical
set of grids.  Would it be sensible to convert the gr3df97a.txt
and the metropolitan NTv2 grid shift files into a single NTv2 for all of
France?

2) As a followon, does CS-Map have mechanism for creating NTv2 files? PROJ.4
can read them but I haven't tried producing them.

3) Could you provide an example of the proposed GDC file, possible with
references to a .GDC specification or notes?

4) Have you considered having a syntax in the .GDC file that indicates
'use this grid shift file if available, otherwise pass on to the next
option'?  In PROJ.4 we use a syntax like:

   +nadgrids=@ntv2_0.gsb,ntv1_can.dat

to indicate use ntv2_0.gsb if it is available, but if not try the next
option.  But since the ntv_1_can.dat is not prefixed with '@' it is
not considered to be optional and an error will occur if it is not
found.  I'm not suggesting the same syntax be used, but perhaps that
this approach would be valuable in CSMap .GDC files so that users don't
need to edit a text file when they add the local grid shift files.

---

So what I'm really hoping to get to is a situation where we have more
sophisticated grid shift file management tools (creation, etc), and
where the same grid shift files can be used with CS-Map, PROJ.4,
and perhaps other packages like GeoTools.  If it is expressive, and
not too complicated perhaps I could add .GDC format support to
PROJ.4 and be able to more easily take advantage of CS-Map datum
shift stuff.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent