[Geotools-devel] Pointers on CoordinateOperations

[Adrian C. <ac...@gm...>]

Hey all,

I've been working on the 'Georeferencing' chapter for the user manual.
I've gotten a long way towards an outline, something like:

        Background:
                Overview: CRS's and Operations
                Warnings: 2d only/ axis order issue/ Spatial error size
                Resources: Standards / Literature / Web
                Code: Layout / Packaging / FactorySystem+GeoAPI /
                premade
        
        Essential Geodesy:
                DirectPositions are...
                Datums: benchmarks, geoid, ITerrestrialRefFrame,
                ICelestialRF
                
        The Class Hierarchy:
                Units: JSR-108 vs. JSR-275 (JScience)
                Parameter:
                Identified Object:
                Datum: anchor point? / ellipsoid / prime meridian
                Coord Sys: axis / CS
                CRS:
                DirectPosition:
                
                CoordOperation: ...
        
        Tutorials:
                CRS Tutorial (a formalized version of CRSTutorial.java)
                CoordOp Tutorial ( the second half of CRSTutorial.java)
        
        Test Apps:
                Concoqt - Create a DirectPosition
                Reproject - Display a dataset (shapefile), then
                reproject

which seems like it's a good basis for the docs to help users understand
what's going on. 



However, I need some help on the issues below.


Math Notation:
-------------
EPSG uses the notation of JPSnyder's "Map Projections - A working
manual". Is this what Geotools uses as well? 

Is there any place where the equations which are used are presented? I'm
especially interested in how the dimension reduction happens going from
a geocentered XYZ coordinate to the geographic 2D. Does this happen by
defining special matrices to augment/diminish the dimension?



Stuck on CoordinateOperations:
-----------------------------
I'm really stuck on CoordinateOperations since I can't get traction to
get started and learn from there. Unfortunately, I don't understand
enough to ask my question coherently so, when i say nonsensical things,
you'll have to guess between my lines.

Which operations are supported?

EPSG tech note 7 part 1 page 11 has a diagram of the general case of
different CRS's and the steps to convert between them. Since GT2.2.x
only has 2D DirectPositions, persumably GT has operations only for a
subset of that diagram. 
        (1) What subset of that diagram does Gt perform? Only the
        coordinate operations which are actually 2D -> 2D?
        
        (2) Transforms seem to go through earth centered cartesian
        coordinates but is this, for GT2.2.x, actually 
          geographic2D -> geocentric (point on ellipsoid surface) ->
        geographic2D? Does this happen only for Bursa-Wolf transforms?


How to create / obtain coordinate operations?

>From the CRSTutorial and javadocs, it's easy to create a 'transform'
that goes from CRS to CRS. 
        (1) is this possible for any CRS in the EPSG database and any
        user defined CRS that has a complete toWGS84 information?
        
        
How do we get operations from Geotools?

        (1) The CRSTutorial.java has the following:
        MathTransformFactory mtFactory = 
        	FactoryFinder.getMathTransformFactory(null);
        ParameterValueGroup params =
        	mtFactory.getDefaultParameters("Hotine_Oblique_Mercator");
        where do we get a list of valid string parameters to get
        pre-named parameter groups? Are these pulled from the EPSG
        database or from some Geotools class?
        
        (2) Are there static, pre-made CoordinateOperations?
        
        
        
How do I create coordinate operations by hand? 

        I'm interested in creating a conversion, a conversion which is a
        projection, and a transformation. 
        
        (1) The CRSTutorial.java has the following:
        MathTransformFactory mtFactory = 
        	FactoryFinder.getMathTransformFactory(null);
        ParameterValueGroup params =
        	mtFactory.getDefaultParameters("Hotine_Oblique_Mercator");
        Where does the magic string value come from, and where are
        others defined?
        
        (2) For each operation (perhaps specifically projection) like
        that above, where does Gt2 define the parameters that are in the
        group?
        
        (3) Can I get operations from the EPSG database, as in the
        currently not working:
                CoordinateOperationAuthorityFactory coaf =
        FactoryFinder.getCoordinateOperationAuthorityFactory("EPSG",null);
                CoordinateOperation co =
        coaf.createCoordinateOperation("");
        //TODO find an operation code

Thanks for any answers or pointers to get me unstuck,

--adrian