[postgis-users] Affine transformations of GIS data.

[Andreas Neumann]

Hi Bruce and others,

I think the functionality to compute the affine transformation
coefficients from a set of control points would be a useful and welcome
addition to the Postgis tool-set.

I agree with you that it would be very nice to have an interactive tool to
edit individual control points in the link table in a Desktop GIS, such as
QGIS or uDig. QGIS already has a tool for georeferencing rasters, so maybe
some of the GUI and functionality could be re-used for the adjustment of
vectors as well. I don't know too much about uDig. I am more a QGIS user.

I would be willing to test both, a non-interactive Postgis-function and an
interactive tool in Desktop GIS.

Andreas


> Based on a previous post on affine transformations of GIS data in PostGIS,
> I
> have put together a function that takes a table of control points and
> computes the affine transformation coefficients using the least squares
> code
> from GRASS.  A nice tutorial by Dylan Beaudette on this is at:
> http://casoilresource.lawr.ucdavis.edu/drupal/node/433
>
> At this point I would like some feed back on how this could be used.  I
> envision a GIS package (say uDIG) would allow a user to select pairs of
> points and populate a table with a similar layout as ESRI's link table
> when
> georeferencing.  A query would give the RMS error and similar query would
> return the table back with the residual errors. If the user were then
> happy
> with the values (after deleting poor matching points), a final query could
> then be made to the ST_Affine function to modify the geometry based on the
> control points.
>
> Comments?
>
> Bruce Rindahl
>
>
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-- 
Andreas Neumann
Böschacherstrasse 6, CH-8624 Grüt/Gossau, Switzerland
Email: a.neumann at carto.net, Web:
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