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

[Hugues Wisniewski]

Hi Frank,

>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?
Yes, the mechanism is already in place for other places in the world.
Since there was only one grid file for France in the past, there was no need for a gdc file at that time and that single grid file was harcoded inside CS-Map.

>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?
I suppose Norm could answer this in more detail but I see a few problems doing this:
     - we do not have the right to redistribute the NTV2 grid files whereas we have the right to redistribute gr3df97a.txt
     - grid files come from official organisms like the IGN in the current scenario, and we do not change their content. When a new one comes, it can be
       added directly to the grid folder, or it can replace an old version of the same grid file
     - The NTV2 grid files cover specific area, the metropolitan areas, and each one of them overlaps the area covered by gr3df97a.txt by 100%
       With the use of GDC file, the end user can change the order in which the grid files will be used in case of an overlap, comment out a grid file if
       it doesn't have to be used, or add new grid files in the future if they become available
The main advantage of the gdc files is that they are text files editable by the end user to change the behavior of the conversion technique.

>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.
I don't think so, but Norm can confirm that

>3) Could you provide an example of the proposed GDC file, possible with
>references to a .GDC specification or notes?
http://svn.osgeo.org/metacrs/csmap/trunk/CsMapDev/Dictionaries/Nad27ToNad83.gdc
You will see that some grid files are commented out and the order of the Nadcon files can be changed by the end user when the data is in an area that is overlapped by multiple grid files. The first grid file in the list is the one that will be chosen if there is an overlap.

>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'?
That's quite an interesting feature but I don't think that's possible right now since we have to comment out the grid files that we cannot redistribute.
This triggers some testing problems when we forget to uncomment those grid files used in lab to test the library on a test machine that has the grid files available. That would likely be a nice add on to the gdc file management inside the CS-Map code as we would always use the same files for testing and distribution without constantly editing them.

Hugues


-----Original Message-----
From: Frank Warmerdam (External)
Sent: Tuesday, October 28, 2008 12:59 PM
To: Hugues Wisniewski
Cc: metacrs at lists.osgeo.org
Subject: Re: [MetaCRS] CS-Map RFC 1 - Creation of a GDC file to handle French grid files

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,
--
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