WGS84 to krovak

[schneidermichal]

Hi, I have a problem with projection from WGS84 to krovak.

Example code:

var point = [ -747963.64999999851, -1041493.3299999982 ];
proj4.defs( "EPSG:5514", "+title=Krovak +proj=krovak +lat_0=49.5 +lon_0=24.83333333333333 +alpha=30.28813972222222 +k=0.9999 +x_0=0 +y_0=0 +ellps=bessel +units=m +towgs84=570.8,85.7,462.8,4.998,1.587,5.261,3.56 +no_defs" );
var coord_wgs = proj4( "EPSG:5514" ).inverse( point );
console.log( coord_wgs ); // [14.346996204306555, 50.09465901453681] - OK
var coord_jtsk = proj4( "EPSG:5514" ).forward( coord_wgs );
console.log( coord_jtsk ); // [-747983.7727820469, -1041499.9068752637] - wrong

Inverse projection is absolutely OK, but forward projection gives the resulting point on about 20 meters wrong position :-(

When I tried to use cs2cs utility with same input ([14.346996204306555, 50.09465901453681]):

cs2cs +proj=latlong +datum=WGS84 +to +proj=krovak +ellps=bessel +towgs84=570.8,85.7,462.8,4.998,1.587,5.261,3.56

then the resulting point is accurate: [-747963.65, -1041493.33]

Thank you for any advice!

[schneidermichal]

adding a link to fiddle: http://jsfiddle.net/79QHW/

[xiceph]

Hi,
I noticed this problem too. I found that when the proj definition is used directly the result is correct. I update your fiddle to show this behaviour:
http://jsfiddle.net/xiceph/79QHW/1/
Unfortunately I need correct transformation with EPSG code for using together with ol3.

[calvinmetcalf]

so it looks like we are duplicating something when we store it

On Tue, Jul 29, 2014 at 8:02 AM, xiceph notifications@github.com wrote:

Hi,
I noticed this problem too. I found that when the proj definition is used
directly the result is correct. I update your fiddle to show this behaviour:
http://jsfiddle.net/xiceph/79QHW/1/
Unfortunately I need correct transformation with EPSG code for using
together with ol3.

—
Reply to this email directly or view it on GitHub
#115 (comment).

-Calvin W. Metcalf

[schneidermichal]

I think it is connected with 932ca62. I tested our example again against master and both forward tranformations are correct.

Source JTSK coordinates: -747963.6499999985,-1041493.3299999982
WGS 84 transformation: 14.346996204306555,50.09465901453681
Transformation back to JTSK: -747963.649753845,-1041493.3287851872
Difference between source and transformated JTSK: 0.00024615356232970953,0.0012148109963163733
Transformation back to JTSK (proj def): -747963.649753845,-1041493.3287851872
Difference between source and transformated JTSK (proj def): 0.00024615356232970953,0.0012148109963163733

So thanks Jachym!