Hi again; > Can you give an example of what you mean by above? What is an example > of a data format and how does a data format specify its CRS. Sure - a shapefile is a common data format that is made up of the following files: - file.shp - holds the geometry (we parse these into a JTS Geometry and you can get it out as the "default" geometry) - file.shpx - some kind of index - file.dbf - the attributes stored as a dbase3 dbf file - file.prj - a little text file with the projection information stored in WKT; this is turned into a CRS
For PostGIS the information comes from: - any old table - holds the attributes; including geometry attributes - geometry_columns table - holds a list of all the geometry columns used; and records the SRID - spatial_ref_sys table - holds a table of SRID numbers and the WKT definitions of the CoordinateReferneceSystem for each SRID Does that help? Each spatial data format has a different way of holding this kind of information. > This (udig) was a great visual example to make your point that > features from different sources using different CRS can all be used in > the same map. How does udig determine what CRS to use for each feature > set loaded from a shape file? Where in the code is that determination > done? uDig does not really know what kind of data it is working with; it trusts the FeatureType.getDefaultGeometry().getCoordinateReferenceSystem(). Behind the scenes shapefile and postgis handle things differently. All the best, Jody ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Geotools-gt2-users mailing list Geotools-gt2-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/geotools-gt2-users
