Firstly, many thanks for the detailed and helpful reply.
[...snip..]
> I suspect that ... the difference lies in the datum shift step. More than one
> datum shifts may exists for a given source and target CRS. For example there
> is 40 different transformations from 4230 to 4326 declared in the EPSG
> database! Geotools try to pickup the most accurate one.
> My understanding is that the epsg-wkt factory is derived from the PostGIS
> database, and we known that epsg-wkt factory doesn't handle datum shift.
> Maybe PostGIS doesn't handle datum shift neither? You may test this
> hypothesis by trying to run yours program again with the epsg-wkt factory
> instead of epsg-hsql (don't forget to swap longitude and latitude order!). If
> you get the same result with epsg-wkt than with PostGIS, then it would means
> that PostGIS doesn't handle datum shift, and Geotools computation with
> epsg-hsql would be more accurate.
> If epsg-wkt produces yet an other result, then my hypothesis is wrong. I
> would be interrested to know yours result.
I tried using epsg-wkt, but it simply throws an OperationNotFoundException
("Bursa wolf parameters required") because of the lack of a datum shift within
DefaultCoordinateOperationFactory.createOperationStep(final GeocentricCRS
sourceCRS, final GeocentricCRS targetCRS). So I can't compare the
values with PostGIS. Also, I know that PostGIS uses PROJ.4 for transformations,
so doesn't that guarantee that it can handle datum shifts? Perhaps PROJ.4
simply picks a different algorithm?
Given that there are potentially many algorithms, I have since compared the
GeoTools results with two other sets of values - one set from www.multimap.com
and another given by "Grid InQuest" software from www.qgsl.com. For these
values the GeoTools results are much closer - for the latter, not more than 12
metres difference, and usually only 1 or 2).
Simon
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