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

[Norm Olsen]

Hello Frank . . .

The Canadian NTv2 format would be a good choice; it has one major and two trivial draw backs:

1> It's only two dimensional (major).
2> Internally, it uses west positive longitude (minor, but a pain for developers).
3> Half of the disk space it takes up (and data which must parsed when processing the file) is wasted, it includes some error values which are never used.

I've always been partial to the French format as it is 3D, uses a better geodetic algorithm, and would be easier to produce.  But it doesn't handle the sub-grid issue well which is the whole purpose of this discussion.

Norm

-----Original Message-----
From: metacrs-bounces at lists.osgeo.org [mailto:metacrs-bounces at lists.osgeo.org] On Behalf Of Frank Warmerdam (External)
Sent: Wednesday, October 29, 2008 11:50 AM
To: metacrs at lists.osgeo.org
Subject: Re: [MetaCRS] CS-Map RFC 1 - Creation of a GDC file to handle French grid files


Hugues, Norm, Didier,

Thanks for the answers to my questions.  It doesn't sound like there is any
immediate opportunity for sharing anything other than the original datum
shift grids between PROJ.4 and CS-Map but I will try and read up on .gdc
files some more to see if at some point we can take more direct advantage
of them in some cases.

I am still interested in "grid shift file preparation" tools being shared
within MetaCRS (though they might technically live in one of the subprojects).
I'm interested in Didier's mention of a more generic grid shift format.

On the one hand I find NTv2 pretty generally and it has the benefit of having
achieved fairly wide adoption.  But if there are serious issues with it, then
we (as MetaCRS) have the opportunity to assist in bringing a better format
into popular use.

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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