[postgis-users] How to use ST_Affine

[Dylan Beaudette]

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/transform.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





-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341