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

[Rahkonen Jukka]

Hi,

While world files are pretty often used for georeferencing rasters I have never heard about using them for georeferencing vector data. World file format gives a possibility to give the anchor point, two rotation parameters and pixel sizes in x- and y-directions. It would be possible to georeference vectors somehow with this information (add this value to each CAD x-coordinate and another value to each y-coordinate and rotate). I am not sure if vectors could be stretched to any direction by the information that can be stored into world file. It would still give an usable result for georeferencing CAD drawings and also total station measuments using local coordinates. However, I feel that instead of trying to re-use world file format to something else than it is designed for it could be better to design a simple text file format just for the this special use. No other software would support world file referencing for vectors anyhow so nothing would be lost by using an own gvSIG format.

Most simple format could take just source and target coordinates for two or more reference points for performing a simple vector warp. More advanced option would be to store all the parameters which are needed for affine transformation. Simple warp option should be good enough for georeferencing most CAD works and total station measurements.  The file extension .wrp feels rather good.

-Jukka Rahkonen-



ernesto sferlazza wrote:

> Hello gvSIG developers.
> I reply a very old issue (15-01-2007), that I wrote to gvSIG team
> ahead of my subcrition to the international list.
> >
> > " it would be very useful to implement "on the fly 
> transformation" of CAD layers, using "wld" files. In this way 
> it is possible , for example, to use a CAD cartography 
> georeferenced in local coordinates in a view frame referenced 
> in UTM-WGS84, without the need of create a new CAD file 
> moving all the features from the first to the second 
> cartographic system.
> > [...]
> > The "wld" file is simply a text file containing the 
> informations that allow the software to calculate the 
> parameters of a particular affine transformation (i know it 
> is called "similitude") composed of a rigid rotation, a shift 
> of the origin of axis  and an uniform change of overall 
> scale. The called informations consist of the coordinates of 
> two points, given in two different frames of reference: the 
> start frame and the target frame [...] .
> > From the informations contained in the "wld" file it is 
> possible, using a worksheet and a CAD software [...], to 
> perform quickly the transformation, calculating the 
> parameters of rotation, overall scale and shifting and 
> applying them to the CAD drawing, using the command line"
> 
> 
> Nowadays the only way I have found to perform such transformation with
> gvSIG + Sextante consists of the following steps:
> 1) export the CAD file to the SHP format;
> 2) given the coordinates of two points,  both in in the start frame
> end in the target frame, I can calculate the parameters of
> translation, rotation, scale factor and anchor point coordinates
> needed to perform a similitude transformation (I can use the
> following spreadsheet:
> http://www.provincia.agrigento.it/flex/cm/pages/ServeAttachmen
> t.php/L/IT/D/D.c460f62201e21373447d/P/BLOB%3AID%3D309
> 
> 3)  use the "transform" algorithm of Sextante, using the above
> mentioned parameters as input.
> 
> This procedure works fine, but is quite farraginous compared to the
> simplest use of "wld" file performed by ArcGIS.
> Implementing this feature, even spread to all vector formats, shp
> included,  would be a great improvement for interoperability too.
> Best regards.
> Ernesto
> 
> 
> --
> ing. Ernesto Sferlazza
> responsabile nodo provinciale di Agrigento del SITR Sicilia
> resp. gruppo Sistema Informativo Territoriale
> Provincia regionale di Agrigento
> sede istituzionale: piazza Aldo Moro, 1 - 92100 AGRIGENTO
> sede operativa: via Acrone, 27 - 92100 AGRIGENTO
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> tel. uff 0922593752
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