[Proj] How common is it that a zero shift means undefined in an NTv2 file?

[Mikael Rittri]

Oops, I happened to hit Control + Enter by mistake, which means "Send". Sorry.
The end was supposed to be:

I wonder if anyone knows of other NTv2 files where zero shifts are supposed to mean undefined?

As far as I know, most NTv2 files extrapolate the datum shifts, so that they can fill a rectangular grid with
shift values that are continuous and sensible (for some definition of sense).

But maybe there are other countries that use the British style?

Best regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

________________________________
From: Mikael Rittri
Sent: Thursday, February 09, 2012 4:46 PM
To: 'proj at lists.maptools.org'
Subject: How common is it that a zero shift means undefined in an NTv2 file?

Hello,

I have noticed that the grid shift files for Great Britain (that do datum shift from OSGB36 to ETRS89)
have meaningful shift values only up to 10 km offshore.  Beyond that limit, OSGB36 is not defined,
the shift values are zero, and the software is supposed to signal an error.

At least, the original British software does so:

    When transformation software encounters a zero parameter it will return an "outside transformation boundary" type error.
    http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/about/surveying_osnet.html#6

However, when using the NTv2 representation of the grid file, the zero shifts may be accepted as valid:

    The 10km off shore buffer is retained in the NTv2 version of OSTN02 so users must be aware that
    no transformation will take place beyond this limit. Unlike in a dedicated implementation of the
    original OSTN02 grid, a GIS system using the NTv2 version of OSTN02 is unlikely to warn the
    user that coordinates are outside the 10km boundary and will therefore not be transformed.
    http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/furtherinfo/ostn02_ntv2.html
I have noticed that the grid shift files for Great Britain (that do datum shift from OSGB36 to ETRS89)

have meaningful shift I wonder if anyone knows of other NTv2 files where zero shifts are supposed to mean undefined?
As far








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