Frank Warmerdam a écrit :
> The slightly different TOWGS84 value in PROJ.4 is because it is a "user
> supplied override". I'm surprised you only found one transformation from
> EPSG:4289 to WGS84 in the EPSG database. In fact, this scared me enough
> about my own logic, that I thought I should dig into it. When I search
> I find these records in the coordinate_operation table:
>
> 1112,Amersfoort to WGS 84 (1),transformation,4289,4326,NCG-Nld
> 93,1,1275,?,1,9606,,,Superseded by Amersfoort to WGS 84 (2) (code
> 1672).,Nederlandse Commissie voor Geodesie publication 30;
> 1993.,EPSG,2002-12-09,97.07 2002.90,1,0
>
> 1672,Amersfoort to WGS 84
> (2),transformation,4289,4326,EPSG-Nld,2,1275,Approximation at the +/- 1m
> level.,1,9607,,,Parameter values from Amersfoort to ETRS89 (1) (code
> 1751) assuming that ETRS89 is equivalent to WGS 84 within the accuracy
> of the transformation. Supersedes Amersfoort to WGS 84 (1) (code
> 1112).,EPSG,EPSG,2002-12-09,2002.90,1,0
You are right, there is 4 set of parameters declared in the EPSG database, 2 for
"WGS 84" and 2 for "European [snip]". Geotools retains only one set of
parameters per distinct target datum, which is why the previous table displayed
only two of them.
> While the comments of the second claim it superceeds the first, it does
> not seem that the deprecated flag of the first is set. Do you just
> arbitrarily a transformation when there are multiple paths available?
For a given target datum, we pickup the first set of parameters returned by a
SQL statement that contains the following clause:
ORDER BY ABS(DEPRECATED), COORD_OP_ACCURACY, COORD_OP_CODE DESC
So deprecated parameters are last. If there is more than one non-deprecated set
of parameters, the most accurate one is selected. If they all have the same
accuracy, we choose the one with the highest primary key value, assuming that it
is the most recently added. I admit that this last rule is not a very reliable
one, but in the particular case of EPSG:4289, it seems to work.
Martin
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