[geotk] Tranforms created in jboss and eclipse are different

[Bertrik Sikken]

Hi Martin, 

-----Original Message-----
Le 09/11/10 10:34, Bertrik Sikken a écrit :
> I noticed that when converting coordinates between ETRS89 (UTM31N) and 
> ED50
> (UTM31N) I get slightly different results depending on the environment 
> I’m running in. Coordinates converted when running in jboss are a few 
> meters off compared to coordinates converted when running in Eclipse.
>
> I’m importing the coordinate systems using method CRS.parseWKT and 
> creating the transform using CRS.findMathTransform(crsSource, 
> crsTarget, false), so with lenient mode explicitly turned off.
>
> On closer inspection, I found that the transforms created to do the 
> conversion are different.
>
> The transform created when running in Eclipse contains a “PARAM_MT["Affine

”
> step, while the one created
> in jboss contains a PARAM_MT["Position Vector transformation (geog2D
domain)
"
> step, but are identical otherwise.
>
> Can anyone explain why I’m seeing two different transforms?

The EPSG database is not only a collection of CRS. For some coordinate
operations, 
the EPSG database also specifies explicitly which operation methods shall be
applied.

In the JBoss case, the referencing module has been able to etablish a
connection 
to the EPSG database. The "Position Vector transformation (geog2D domain)" 
operation method come from the EPSG database. This information overwrite any

"TOWGS84" elements in WKT.

In the Eclipse case, no such connection to the EPSG database has been
etablished. 
Consequently the referencing module has build its own transformation chain
using 
the Bursa-Wolf parameters provided by the "TOWGS84" elements that appear in
the 
WKT of source and target CRS.

Many different values are possible for the "TOWGS84" elements for the same
CRS. 
Geotk tries to guess a set of parameters on a "best effort" basis, but the
choice
 is not always optimal (see http://jira.geotoolkit.org/browse/GEOTK-80). 
It depends on the pair of (source, target) CRS, on the geographic area where

transformation are going to be applied, etc.

In any case, when the method is explicitly specified by the EPSG database, 
this is the authoritative method for that pair of (source, target) CRS.

-----------------------

Your analysis was correct, this seems to be exactly what was going on in my
case.
Thank you very much for the clarification!

With kind regards,
Bertrik Sikken