[Proj] Switch utm from tmerc to etmerc

Charles Karney charles.karney at sri.com
Sun Oct 25 18:29:02 EST 2015

I have a different understanding of the process from you.

I made the change to the source code repository in git.  This is not
typically what end users or downstream packagers use.  They will pick up
an official release.  The change I just made will therefore be
incorporated into release 4.9.3.  At that time there will be an
announcement with the changes (including this one) described.  There's
will only be one version 4.9.3 and specifying that you linked with this
version will be sufficient to specify the code you used.

This is how proj.4 and most other open source projects are usually

I hope this addresses your concerns.


On 10/25/2015 07:09 PM, Roger Bivand wrote:
> Charles Karney <charles.karney <at> sri.com> writes:
>> This change has now been made to the master branch of the source.  The
>> "utm projection" now follows the NGA recommendation and uses etmerc
>> instead of tmerc (which is less accurate).  You can expect small
>> (typically < 1mm, except at high latitudes) changes in the results for
>> the utm projection.
> While correcting wrong parameterisations is good, here we are doing what the
> mooted metacrs definitions were trying to avoid, that is altering in-place
> code without user-facing versioning. Different versions (by commit hashes
> now) of the underlying code will now give different answers, so re-running
> an analysis with the same data and apparently the same code in software
> linking to proj will give different answers, without the user necessarily
> being able to re-instate the earlier relationship. It would be great to bear
> in mind the fact that research does need to be reproducible (data from times
> before this correction was known don't need to be obliged to use it). I
> guess we have to assume that users really read commit logs and NEWS files.
> Roger
>>     --Charles
>> On 10/06/2015 11:19 AM, Charles Karney wrote:
>>> Reposted from
>>>     https://github.com/OSGeo/proj.4/issues/316

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