[geotk] coordinate transformation from 3857 to 4326

[juergen sorg]

thanks a lot - that solved my problem


On 19/03/2021 13:21, Alexis Manin wrote:
> Hello,
> 
> Geotk referencing engine is strict about CRS definition, and as stated 
> by the official EPSG database[1], The 4326 CRS axes order is latitude 
> first then longitude.
> If you want a longitude first system, the easiest way is to use "CRS:84" 
> code instead of "EPSG:4326", which indicates official OGC CRS for WGS84, 
> whose axes order is longitude then latitude.
> 
> The jts transform does not update crs definition. You'll have to do it 
> yourself by calling JTS.setCRS(geometry, crs)[2];
> 
> Regards,
> 
> [1] https://epsg.org/crs_4326/WGS-84.html
> [2] 
> https://github.com/Geomatys/geotoolkit/blob/97cfa40dca66617069afc23cbb8a21a667e1b5c1/modules/feature/geotk-geometry/src/main/java/org/geotoolkit/geometry/jts/JTS.java#L761
> 
> 
> Le ven. 19 mars 2021 à 12:48, juergen sorg <j.sorg at fz-juelich.de 
> <mailto:j.sorg at fz-juelich.de>> a écrit :
> 
>     hi,
> 
>     i have a question about transforming coordinates from epsg:3857
>     (google)
>     to epsg:4326 (lon lat). (i am not sure if this is the right place to
>     ask
>     for - but i am using geotk with jts).
>     in [0] you can find a code snippet to transform a point and in [1] the
>     results.
>     my questions are:
> 
>     1. why are the coordinates not swapped, because after the
>     transformation
>     x-coord and the y-coord are at the wrong place.
>     2. do i have to swap it by my own and if yes then how can i do it with
>     e.g. polygons
>     3. why the crs is still 3857 after transformation (it should 4326) and
>     if i have to change it by my own how this can be accomplished?
> 
>     thanks in advance
> 
>     regards,
>     juergen
> 
> 
> 
>     [0]
> 
>     WKTReader WKT_READER = new WKTReader(
>                                      new GeometryFactory(new
>     PrecisionModel(PrecisionModel.FLOATING), 3857));
>                      String pt_google="POINT(704377.78268943
>     6533181.663612)";
>                      Geometry pt_google_geom=WKT_READER.read(pt_google);
> 
>                      Geometry pt_wgs84_geom=JTS.transform(pt_google_geom,
>                                     
>     CRS.findOperation(CRS.forCode("EPSG:3857"),
>     CRS.forCode("EPSG:4326"), null).getMathTransform());
>                      Geometry
>     pt_google_geom_transformed=JTS.transform(pt_wgs84_geom,
>                                     
>     CRS.findOperation(CRS.forCode("EPSG:4326"),
>     CRS.forCode("EPSG:3857"), null).getMathTransform());
>     System.out.println(pt_google_geom);
>     System.out.println(pt_google_geom.getSRID());
>     System.out.println(pt_wgs84_geom);
>     System.out.println(pt_wgs84_geom.getSRID());
>     System.out.println(pt_google_geom_transformed);
>     System.out.println(pt_google_geom_transformed.getSRID());
> 
> 
>     [1]
> 
>     POINT (704377.78268943 6533181.663612)
>     3857
>     POINT (50.499200582184564 6.327533279841338)
>     3857
>     POINT (704377.7826894299 6533181.663611999)
>     3857
> 
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> 
> 
> 
> -- 
> Alexis Manin,
> Développeur JAVA/JEE.
> Geomatys.
> 
> 


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