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

[Norm Olsen]

Hello All . . .

Frank mentioned that an NTv2 data file can have sub-grids.  This is certainly true.  There are, however, very specific restrictions on sub-grids within the BTv2 format.  That is, a sub-grid must fully completely cover a rectangular set of grid cells in the parent grid.  A sub-grid may also contain sub-grids within itself.  Again, the restriction applies; the sub-grid must completely cover a rectangular set of grid cells in the parent grid.

Using this sub-grid feature, the IGN could indeed produce a single NTv2 data file where the parent grid would represent the current text file, and each municipality be represented as a sub-grid. Code to handle this situation already exists in CS-MAP.

As proposed, the implementation of a .gdc file for the RGF93 <-> NTF transformation would handle either case, as it is proposed that this implementation would handle both file formats.

Norm

-----Original Message-----
From: metacrs-bounces at lists.osgeo.org [mailto:metacrs-bounces at lists.osgeo.org] On Behalf Of RICHARD Didier
Sent: Tuesday, October 28, 2008 3:25 PM
To: Frank Warmerdam (External)
Cc: metacrs at lists.osgeo.org
Subject: Re: [MetaCRS] CS-Map RFC 1 - Creation of a GDC file to handle French grid files

Hi all,

> 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?
>

I did not read yet the RFC, but we have (IGN France) built a NTv2 file
from the gr3df97a.txt file. We are currently working on a more generic
grid format to support the shift with the height (for instance for the
Reunion island) and with different axis and axis orientation.

> 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.
>

We used Perl script for converting the .txt to .gsb. It could worth
sharing it.

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

Yes, it would be interesting to have such a spec at least for comparison
with our undergoing work.

> 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.
>

Agreed.

As a matter of fact such a GDC file cannot be created if we are not
involved in the process and check if it delivers the quality of
transformation we except in the end.

Sincerely,

didier
> ---
>
> 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
>
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--
RICHARD Didier - Chef du pôle technique du Géoportail
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