[Gvsig_english] world file and on-the-fly transformation for vector layers

[ernesto sferlazza]

Hi all.
The world file is actually used by ESRI ArcGIS.
When I load a CAD layer into the view the software ask if user wants
to apply a trasformation using a world file, then the user browse to
the file and the transformation is applied on-the fly.
The behaviour in gvSIG could be similar to what happens when user
choose to apply a transformation (NTV2, EPSG, etc.) when loading a
layer to a view with a different coordinate system.

The world file is simply a text file with suffix ".wld". Opening this
file with notepad it shows at least one or two lines like the
following:

766.0866,681.395 392609.74,4128305.49
1754.757,871.1515 393442.64,4127737.41

Othe lines are ignored by the software and can be used for comments
and expanations, like the following:
"
the above rows contains the coordinates of two points in the drawing "naro.dwg"
the start reference frame in in local coordinates not better defined
the target frame is in WGS84 - UTM 33 N
"

For each of the lines in the text file the first couple of coordinates
refers to X,Y in the "from" coordinate system, while the second couple
refers to the coordinates of the same point in the "to" coordinate
system.

If the file contains only one row (id est only one point) the
transformation reduces to a simple translation without rotation, while
with two rows the transformation is a little more complex
(translation, overall scale factor, overall rotation), but is not a
complete affine transformation (which allows also different scale
factors along the axes and different rotation of the axes).

Since the "wld file" method already is adopted by ArcGIS (and
therefore can be considered like a standard de facto), for it's
simplicity no shame nor presuntuosness is to be considered in adopting
the same ability to use it.

Best regards
Ernesto Sferlazza

-- 
ing. Ernesto Sferlazza
responsabile nodo provinciale di Agrigento del SITR Sicilia
resp. gruppo Sistema Informativo Territoriale
Provincia regionale di Agrigento
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