[Gvsig_english] world file and on-the fly transformationfor vectorlayers

Thu Sep 30 12:26:18 CEST 2010

> .j2w and so on). However, if there are not images with with gdal .wld
> world files and vector files having the same base name in the same
> directory there will not be any trouble.

That's why I wouldn't want to see .wld files used with GIS vector
files: it's sure to cause confusion in some way.

> 
> Warping with more than two control points can make sense if someone is
> digitising features from scanned maps. They can have bad local
> distortions, especially if they are scanned from books.

Such warping can be done (using many different methods) with the
Transform tool of the Topology Extension. Just as with images,
it's important to do this manually, with great control over the
process and some sort of error measure.

The .wld is only intended to compute an affine transformation
for CAD drawings in non-geodetic systems. This makes a few
assumptions:

- the drawing is not distorted
- X and Y axes have the same scaling

Those assumptions are pretty safe for CAD projects, so the
two points in the .wld file are good enough to serve the
purpose. Let's not make it overly complicated. All that needs
to be done is to read the .wld file on opening the CAD file,
computing the affine transformation parameters from the
point coordinates and then transforming the data accordingly.

Cheers,

Ben

> 
> -Jukka Rahkonen-
> 
> > -----Alkuperäinen viesti-----
> > Lähettäjä: gvsig_internacional-bounces at listserv.gva.es
> > [mailto:gvsig_internacional-bounces at listserv.gva.es] Puolesta
> > Benjamin Ducke
> > Lähetetty: 30. syyskuuta 2010 12:02
> > Vastaanottaja: Users and Developers mailing list
> > Aihe: Re: [Gvsig_english] world file and on-the fly
> > transformationfor vectorlayers
> >
> > http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?TopicName=
> World_files
> >
> > Apparently, the Esri GIS world uses these for georeferencing CAD
> > files, which have objects whose coordinates (just like raw
> > image files)
> > do not represent locations in defined geodetic systems
> > (so called "local systems" in CAD).
> >
> > So the original request was to support these transformation files
> > just like ArcMap does.
> >
> > Like I argued in my last email, I do not think it should be used
> > for anything else than CAD files.
> >
> > Ben
> >
> > ----- Original Message -----
> > > 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
> > > > cell. 3392718463 (personale)
> > > >        3316874296 (aziendale)
> > > > tel. uff 0922593752
> > > > _______________________________________________
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