[GRASS-user] Setting up PROJ file

[Harri Kiiskinen]

Hello all,

I'm trying to set up the projection parameters for a local coordinate
system. The coordinate system is 'attached' by five points of which I
know both lat/long and UTM33 coordinates. It is relatively easy to
perform an affine transformation between my local system and UTM33 using
v.transform, and the transformation parameters can also easily be found
with R (Thanks Dylan for the example at
http://casoilresource.lawr.ucdavis.edu/drupal/node/433.)
	The problems arise when I try to set up the projection with g.setproj
(and these probably reflect the fact I'm no geographer). As I have
understood, I should set the +proj and +towgs84 parameters. The latter
reflects the affine transformation coefficients (rotation, scale,
transformation), but relative to what should they be calculated? And
what should the +proj parameter then be, considering that the
geometrical shape on which the local system is based is actually a flat
surface?
	Since I have the 'real' coordinates in UTM33, I considered the
following:

+proj=UTM
+zone=33
+towgs84=dx,dy,dz,rx,ry,rz,scale

where the +towgs84 coefficients come from the affine transformation
matrix between my coordinates and UTM33 coordinates.

But the results are, well, less than right... And I'm quite at loss how
to go on further.

Harri K.