[postgis-users] How to use ST_Affine

[Frank Durstewitz]

Hi Dylan,

thanks for posting the file on your blog. I'm going to play with R ;-)

Best regards, Frank

Dylan Beaudette schrieb:
> On Thursday 17 January 2008 12:26:12 pm Frank Durstewitz wrote:
>> Hi Dylan,
> 
> Hi Frank. Just a reminder, lets keep the discussion CC-ed to the postgis list 
> for documentation. I am sure that there will be others interested.
> 
> 
>> i installed a binary package of R. Seems to be a _very_ powerfull
>> language, and i for shure will learn it, because i assume we can do much
>> more with our data...
> 
> Oh yeah... R is a real gem!
> 
>> I assume "dylans blog" is yours? If yes, maybe you can help me out:
> 
> I can claim responsibility for that. It is more of a way to keep indexed, 
> searchable notes than anything else....
> 
>> Can you send me an example of a file "stan.c_points" with the "good" and
>> the "bad" points? I can't figure out what format the points should have...
>>
>> Kindly regards, Frank
> 
> Ah yes. I should have posted that file for others to tinker around with!
> 
> See the attachment at the bottom of the post:
> 
> http://casoilresource.lawr.ucdavis.edu/drupal/node/433
> 
> Note that the control points are interleaved within this file!
> 
> Cheers,
> 
> Dylan
> 
> 
> 
>> Dylan Beaudette schrieb:
>>> On Thursday 17 January 2008 08:25:51 am Bruce Rindahl wrote:
>>>> Dylan
>>>> I thought your tutorial was great.  How often do you think something
>>>> like this will be used?  I am thinking it could be straight forward to
>>>> have a "link" table in PostGIS with say the x,y "from" coordinates and
>>>> "to" coordinates populated via some interface.  Then with the PL/R
>>>> package installed use R to not only create the transformation matrix but
>>>> then pass the correct values to ST_Affine and then translate the
>>>> geometry.  Something like
>>>>
>>>> Select transformation(link_table,the_geom) from my_tiger_set
>>>>
>>>> which would return a transformed geometry.
>>>>
>>>> Might be a nice procedure to have.
>>>>
>>>> Bruce Rindahl
>>> Computing an affine transformation matrix seems like a common vector
>>> operation to me. It shouldn't be too hard to use the code in v.transform
>>> to get a working prototype.
>>>
>>> Here is a link to the source:
>>> http://trac.osgeo.org/grass/browser/grass/trunk/lib/vector/transform/tran
>>> sform.c
>>>
>>> It would be a nice function to have within the context of PostGIS!
>>>
>>> Cheers,
>>>
>>> Dylan
>>>
>>>> -----Original Message-----
>>>> From: postgis-users-bounces at postgis.refractions.net
>>>> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
>>>> Dylan Beaudette
>>>> Sent: Thursday, January 17, 2008 8:03 AM
>>>> To: Frank Durstewitz; PostGIS Mailingliste
>>>> Subject: Re: [postgis-users] How to use ST_Affine
>>>>
>>>> On Thursday 17 January 2008 12:20:47 am Frank Durstewitz wrote:
>>>>> Hi Dylan,
>>>> Hi Frank
>>>>
>>>>> thanks for your answer. I have 2 problems:
>>>>> a) i do know nothing about R and GRASS
>>>> time to start going over some tutorials and manuals...
>>>>
>>>>> b) the "transformation" has to run on the server (just php and
>>>>> postgres/postgis)
>>>> ok. it is possible to use R via a web-form or CGI. it is also possible
>>>> to build a custom CGI around R using the shared libraries.
>>>>
>>>>> If i remember correctly, there is a PostGres-package which does R. Can
>>>>> i install this extension and use R in PostGres to get the matrix as
>>>>> mentioned on http://casoilresource.lawr.ucdavis.edu/drupal/node/433?
>>>> If you get the R interpreter for postgres working that should get you
>>>> started.
>>>> I do not have any experience with that, so I cannot offer any comentary.
>>>>
>>>>> Kind regards and sorry for my dumb questions ;-) I still learning.
>>>> No problem. Keep trying and post back to the list with progress.
>>>>
>>>> Dylan
>>>>
>>>>> Frank
>>>>>
>>>>> Dylan Beaudette schrieb:
>>>>>> On Wednesday 16 January 2008 09:09:01 am Frank Durstewitz wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> i wonder how i would use ST_Affine. I found a small note how to use
>>>>>>> (with R), but that's over my mind.
>>>>>>>
>>>>>>> I want to shift the coordinates of some points. On a map i show the
>>>>>>> points, then i select at least 3 of them to move them manually to the
>>>>>>> correct coordinates and use the new coordinates as reference for the
>>>>>>> shift. So i have 3 "old" coordinates and 3 "new" cordinates. But i
>>>> don't
>>>>
>>>>>>> know what goes where...
>>>>>>>
>>>>>>> Can someone please explain what to do?
>>>>>>>
>>>>>>> Regards, Frank
>>>>>> Hi Frank,
>>>>>>
>>>>>> Was it this page
>>>> (http://casoilresource.lawr.ucdavis.edu/drupal/node/433)
>>>>
>>>>>> which caused the confusion?
>>>>>>
>>>>>> I was recently confronted with the same problem: I knew the locations
>>>>>> of 'good' and 'bad' coordinates (control points) but did not know how
>>>>>> to compute the transformation matrix, which ST_Affine() wants.
>>>>>>
>>>>>> There are two approaches:
>>>>>>
>>>>>> 1. use R to 'fit' the transformation
>>>>>> 2. use GRASS and v.transform to 'fit' the transformation
>>>>>>
>>>>>> Both approaches can be used to compute and print the transformation
>>>>>> matrix based on a set of control points.
>>>>>>
>>>>>> If you use R, the trick is interpreting the format of the
>>>>>> transformation matrix:
>>>>>>
>>>>>> use the coef() function and transpose the results:
>>>>>>
>>>>>> t(coef(l))
>>>>>>
>>>>>> ... and the format will be :
>>>>>>
>>>>>>  xoff a b
>>>>>>  yoff d e
>>>>>>
>>>>>>
>>>>>> put the resulting numbers into the order that postgis wants:
>>>>>> ST_Affine(geom, a, b, d, e, xoff, yoff)
>>>>>>
>>>>>>
>>>>>> If you use GRASS and v.transform, be sure to include the '-m' flag,
>>>>>> and the transformation matrix will be printed, along with a note on
>>>>>> the format.
>>>>>>
>>>>>>
>>>>>> As the linked page suggests this approach works well when there is a
>>>>>> systematic shift in coordinates which can be modeled by an affine
>>>>>> transform.
>>>>>>
>>>>>> Cheers,
>>>>>>
>>>>>> Dylan
>