Farrukh Najmi wrote: > Jody Garnett wrote: >> Farrukh Najmi wrote: >>> Thanks Jody for the very helpful response. >> No worries; now if you can tell me where to put that information in >> the user guide for the next person we will be ahead of the game :-) >> Where would you look ... > > It should probably be under Chapter 03 JTS Topology Suite perhaps > under a page titled "Relationship Between JTS Coordinate System and > Geotools CoordinateReferenceSystem" or some such. > > In http://docs.codehaus.org/display/GEOTDOC/01+How+to+Create+a+Geometry > it should be clarified that SRID has no signficance when creating a > GeometryFactory. > Otherwise it seems as if the GeometryFactory uses specified SRID and > corresponding CRS > for its create operations. > >>> I am unclear about one important conceptual understanding though. >>> You said that "JTS does not care (or use) the SRID value". So what >>> does JTS use for its internal CoordinateReferenceSystem? Does that >>> mean that before geometry objects are stored using JTS they are >>> normalized to that JTS internal CoordinateReferenceSystem? How else >>> could JTS do spatial operations on geometry objects that use >>> different SRID values? >> Just so; there is a JTS utility class here to help you reproject your >> JTS Geometry into the CoordinateReferneceSystem of your choice. >> - http://docs.codehaus.org/display/GEOTDOC/02+JTS+Utility+Class > What CRS does geotools use internally to normalize all geometry > instances before storing? It does not; that is up to you the developer. It blindly stores the data in JTS Geometry objects (where the only concern is the "shape"); the Feature holding the data keeps track of the CoordinateReferneceSystem (ie what the data means). This is just the same as the difference between the number 1 and the meaning being held by a Unit (ie 1 meter vs 1 foot).
GeoTools uses both pieces of information to figure out "where" a feature is, only relying on a transform to "reproject" the geometry when needed (say if you are going to reproject some data for display in a different projection). So there is no need to go to a common format; leave the data alone and manipulate it into the shape you need *just* when you need it. >>> Said another way, geotools allows apps to use a large number of >>> CoordinateReferenceSystems among different geometry objects. I had >>> assumed JTS did the same and the only difference between the two was >>> use of SRID vs. CoordinateReferenceSystem with some mapping between >>> the two. Your statement "JTS does not care (or use) the SRID value" >>> puts in doubt my understanding of how CoordinateReferenceSystems are >>> handled in the two libraries. >> JTS does the mathematical "topology part"; see this page: >> http://docs.codehaus.org/display/GEOTDOC/Point+Set+Theory+and+the+DE-9IM+Matrix >> >> >> GeoTools does the mathematical "referencing part", and the features, >> and the rendering and the xml and the database access and the .... >> well we try and get the job done. > Sounds like a good division of labor :-) > > Thanks for bearing with me my many perhaps basic questions. No it is good; I need dialog like this for the user guide - it all fuels the fire. 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 [email protected] https://lists.sourceforge.net/lists/listinfo/geotools-gt2-users
