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

[Mikael Rittri]

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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