[postgis-users] use of the affine() function

[Stephen Woodbridge]

Dylan Beaudette wrote:
> I have a problematic TIGER  module - i.e. a single county in Ca is really 
> messed up. I thought that a simple translate() would do the job, but it looks 
> like there is both a translation and rotation involved. I do not really 
> understand the postgis manual entry for the affine function.
> 
> How would one go about converting a series of coordinate pairs ([good], [bad]) 
> into the terms of the affine transform matrix?
> 
> thanks,
> 
> 
> 
Dylan,

You will have a secondary problem if you apply an affine function ot 
otherwise transform the county in that TIGER is topologically correct 
even is the interior segments are out of place.

If you trans form it you might better align the segments on the interior 
but you will disconnect the roads from the road network at the county 
boundaries. If you are only displaying on county and not the adjacent 
counties then this might not matter too much. Otherwise you can expect 
to get discontinuities of segments the cross the county boundaries.

Some GIS programs support transforming a network of segments by 
providing anchor points and vertices to move, and then transform based 
on linear or bicubic transforms built on these control points for the 
other points in the network. Someone with GRASS knowledge might be able 
to tell you if GRASS can do this.

-Steve