[GRASS-dev] v.transform: what is zrot supposed to do ?

[Moritz Lennert]

On Fri, 19 Jun 2015 08:28:56 -0400, Anna Petrášová <kratochanna at gmail.com>
wrote:
> On Fri, Jun 19, 2015 at 7:48 AM, Moritz Lennert <
> mlennert at club.worldonline.be> wrote:
> 
>> I have the feeling I'm missing something, here, and I prefer asking the
>> question here before posting a possible bug report.
>>
>> What is the exact meaning of zrot in v.transform ? I would have thought
>> that with no scaling and no shift, zrot would create a simple rotation.
>>
>> I create a square as such:
>>
>> v.in.lines in=- out=testline sep=, --o << EOF
>> 610000,200000
>> 680000,200000
>> Nan,Nan
>> 680000,200000
>> 680000,270000
>> Nan,Nan
>> 680000,270000
>> 610000,270000
>> Nan,Nan
>> 610000,270000
>> 610000,200000
>> EOF
>>
>> I then try to rotate that squate by 45 degrees:
>>
>> v.transform input=testline output=testline45 zrotation=45
>>
>> The result is shown the attached image: The rotation works, but I get a
>> shift of about 445km NW. Is this expected ?
> 
> 
> I think the rotation axis is in (0, 0), I would say that is expected
> behavior, although should be better documented.

Yes, that does make sense. So if I want to rotate a specific feature, I
have us translation to center it on (0,0), then rotate it and then
translate it back ? Would probably be nice to have 'rotate' as a tool in
v.edit...

Thanks for the quick response !

Moritz