[Proj4j] ESPG:28992

[Gertjan Idema]

Hi Martin,

Thank for the quick fix. I works just fine.
I compared the results from the two +towgs string version to the results
of Postgis.
As you already noticed, the difference are small, but the +towgs version
below is much closer to the Postgis results:
+towgs84=565.237,50.0087,465.658,-0.406857,0.350733,-1.87035,4.0812
However, this ticket: http://trac.osgeo.org/gdal/ticket/1987 advocates
the +towgs string currently in proj4j.
That might indicate that I have the wrong +towgs string in my postgis
version. 

Here are my proj4j results with the +towgs string above:
5.387638889,52.156160556 (EPSG:4326) ->
155029.78919920223,463109.9541111593 (EPSG:28992)
155000.0,463000.0 (EPSG:28992) -> 5.3872035804217715,52.15517230193107
(EPSG:4326)
50000.0,350000.0 (EPSG:28992) -> 3.8871491439988644,51.12978774865556
(EPSG:4326)
50000.0,600000.0 (EPSG:28992) -> 3.8094540580375993,53.37600149352827
(EPSG:4326)
250000.0,350000.0 (EPSG:28992) -> 6.7444286180777056,51.13154120698355
(EPSG:4326)
250000.0,600000.0 (EPSG:28992) -> 6.81474749295472,53.37787337579902
(EPSG:4326)
250000.0,600000.0 (EPSG:28992) -> 6.814747494636736,53.377873382361884
(EPSG:4326)

One more very strange thing I can't explain can be seen in the last two
lines.
When I run the conversion twice (or more times) the results don't agree.
Even when I create new Objects for the CRSFactory, the
CoordinateReferenceSystems and the BasicCoordinateTransforms.
The difference are maybe to small to worry about, but to me it's very
strange.
Can you confirm this behavior?

Gertjan Idema


On Wed, 2012-01-11 at 21:47 -0800, Martin Davis wrote:

> As is usually the case with projection issues, this is getting
> complicated...
> 
> Gertjan, in the test you give below you are using a slightly different
> +towgs string than the one in the PROJ4 definition.  However, the
> effect of the difference seems to be fairly minor. I just tested both
> towgs84 definitions in Proj4J and the results are within a few mm of
> each other.  (This is *with* the towgs84 parameter conversion
> implemented, of course!)
> 
> So, it would be great if you can confirm that the value below is
> correct within mm (using some independent program?).  In that case,
> Proj4J seems to be working correctly.  
> 
> Not sure why the PROJ4 value that Jeff reported was off by 30 or so
> metres, though.
> 
> 
> 
> On 1/11/2012 3:03 PM, Gertjan Idema wrote: 
> 
> > Hi Fitz,
> > 
> > I just wrote a test script and can confirm your result.
> > Then I remembered that there was a difference between proj
> > (c-version) and proj4j in handling the +towgs parameters.
> > The c version has some conversion code for parameters 4-7.
> > Parameters 4-6 get converted from arc seconds to radians.
> > (param=param*pi/180/3600)
> > Parameter 7 gets converted from ppm to scaling factor (param=1
> > +param/1000000)
> > 
> > Here's the code from pj_datum_set.c:
> >   /* transform from arc seconds to radians */
> >   projdef->datum_params[3] *= SEC_TO_RAD;
> >   projdef->datum_params[4] *= SEC_TO_RAD;
> >   projdef->datum_params[5] *= SEC_TO_RAD;
> >   /* transform from parts per million to scaling factor */
> >   projdef->datum_params[6] =  (projdef->datum_params[6]/1000000.0) +
> > 1;
> > 
> > This code seems to be missing in proj4j.
> > 
> > Apart from that, as far as I know, the +towgs should be :
> > +towgs84=565.237,50.0087,465.658,-0.406857,0.350733,-1.87035,4.0812
> > Applying the above calculation to the last 4 parameters as a work
> > around gives:
> > 
> > +towgs84=565.237,50.0087,465.658,-1.972e-6,1.7004e-6,-9.0677e-6,1.0000040812
> > 
> > When I put this into the nad/epsg file for proj4j I thought I would
> > get the same results you got from proj, but I didn't.
> > I get 155029.79163595638 463109.9538034333 for your reference point
> > instead.
> > 
> > However, the result seems to agree with some other data I have. I'll
> > do some more research tomorrow.
> > 
> > Gertjan Idema
> > 
> > 
> 
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