[Gvsig_english] Raster georeferencing

[ernesto sferlazza]

Hi all (Developers & Users)
If I have correctly understood, when I georeference a raster image using
GVSIG, the program accepts in input a set of point given in two coordinate
system (e.g. file coordinates, in pixel and map coordinates, in meters) and
performs a affine transformation (resolved in change of scale in X, change
of scale in Y, skew of X axis, skew of Y axis, shift of X coord. of origin
and shift of Y coord of origin). Then, the parameters of the above mentioned
transformation are written in the world file (e.g. the "tfw" file associated
to a tiff, or the "jgw" file associated to a jpg image). At the same time,
the same information added to other regarding the projection and the image
size (both in pixel and in meters) are stored in another file contaning xml
code (with extention "tmp").
I have two remarks to the point:
1) the "skew" parameters (rotation of X and Y axes) written in the world
file are ignored (are considered equal to "0") by some GIS sw (e.g. ESRI
ArcView 3.x) and there is a lack of "interoperability";
2) it would be appreciated very much the possibility to use polinomial
transformation of higher order (2nd, 3rd) than the affine (1st order), or,
even more, projective transformation (using the homology theory), or rubber
sheeting, to perform different kinds of transformations. In this cases,
obviously, the transformation informations cannot be stored in the world
file and arises the need of create a new raster, by performing a resampling
based on the transformed original raster. In this way the axes of the new
raster file will be parallel to the map axes and the world file will not
contain rotation axes parameters.

I found that ILWIS (Open Source) performs many transformation, but the
change of raster format consumes too much time.
Best regards
-- 
ing. Ernesto Sferlazza
responsabile nodo provinciale di Agrigento del SITR Sicilia
resp. U.O. Sistema Informativo Territoriale
Provincia regionale di Agrigento
sede istituzionale: piazza Aldo Moro, 1 - 92100 AGRIGENTO
sede operativa: via Acrone, 27 - 92100 AGRIGENTO
cell. 3392718463
tel. uff 0922401935
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