[Proj] ICSM (Australia) transformation file licensing

[Greg Troxel]

Sebastiaan Couwenberg <sebastic at xs4all.nl> writes:

> On 01/18/2017 01:01 AM, Alex Leith wrote:
>> I'm working with ICSM to incorporate Australian ntv2 grids into open source
>> software. There are two existing GSB files, and there will be another soon
>> for the new GDA2020 datum. ICSM are able and prepared to license the files
>> in any way.
>> 
>> I'd like to know what license to apply to these files in order to best
>> enable their inclusion into Proj4 and other projects?
>
> The NTv2 grids already included in the proj-datumgrids distribution are
> in the public domain. Creative Commons CC0 is a better choice than PD
> because it covers more jurisdictions.
>
> The license for the Australian grids needs to be compatible with the
> terms of the GPL-2+ to allow other projects (PostGIS specifically) to
> include it too.
>
> Please don't disallow modification of the correction values like EPSG
> and many other national grids following their lead, those terms are
> incompatible with the GPL-2+.

Agreed.  Before I read Sebastian's reply I was going to suggest CC0.  I
really do not think there is much actual danger of trouble.  Everyone in
the community knows that randomly changing projection code files to
return wrong data is not helpful and I really don't think people are
going to do it.  And if they did, everybody else would then shun such a
distribution.  For example, proj is legally free to make a change that
computes UTM by adding 17m to easting and northing, and that would have
a similar effect of users getting wrong data.  But no one is worried
about that happening, and rightly so.

A license that doesn't permit modifications is not compatible with all
Free Software.  It's not just GPL; such a license is also not compatible
with the BSD license.  (It is true that BSD-licensed projects are less
bothered by non-free data licenses, because distributing a tarball that
is mostly Free plus some non-free data, while not under a Free license,
is under a messy license that is compatible with the BSD license.  But
the package is no longer Free Software.)

A middle ground might be CC0 with some extra text:

  To preserve integrity of the projection data, users are strongly
  requested not to make modifications that change numeric results.

Another approach might be to use a 2-clause BSD license that, while
permitting modification, does not allow the modified work to be
distributed under the original name.   But I would suggest that this is
not necessary and just causes needless work from people who are making
changes like fixing == to = to make shell scripts follow POSIX, who then
have to think about whether their change is one that triggers the clause
prohibiting using the name for modified works.

The real way to ensure integrity of geodetic calculation is to publish
test vectors and perhaps code, and encourage those to be added to unit
tests in software that uses the transforms.  Again these can be CC0 --
no one is going to pay attention to modified test vectors.
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