[Proj] Arbitrary translation and custom projection

[Yves Moisan]

Hi All,

In PostGIS, we have points in a local system that we have made an affine
transformation for to get them in EPSG 900913.  We followed instructions
at http://casoilresource.lawr.ucdavis.edu/drupal/node/433 so we
collected a bunch of GCP's, ran a regression and applied the resulting
coefficients to transform our local coordinate points to 900913 using
affine().  

I guess we could alternatively determine a custom projection string and
use transform.  I would like to know about pointers to tutorials as to
how to do that.

TIA,

Yves Moisan