[postgis-users] Strange EPSG:3857 transformation results.

[Wolfgang Glas]

Hello,

  Sorry for the false alarm, my testcases used SRID 4236 "Hu Tzu Shan
1950" instead of SRID 4326 "WGS 84" ;-)

  Best regards, Wolfgang

Am 18.06.16 um 21:44 schrieb Wolfgang Glas:
> Hi all,
> 
>   I desperately tried to recalculate WGS84/EPSG:3857 (pseudo-mercator)
> transformations and have really weird results using postgis 2.2.2 with
> postgres 9.5.3
> 
> First example is the origin 0/0 of WGS84:
> 
> #  select ST_AsText(ST_Transform(
>     ST_GeomFromText('POINT(0.0 0.0)',4236),3857));
>                   st_astext                  
> --------------------------------------------
>   POINT(-549.033225716555 -204.380569358876)
>  (1 row)
> 
> To my understanding the origin of EPSG 3857 is also the origin of EPSG
> 4326/WGS84, so the result seem to be plain wrong to me :-/
> 
> Another unit test was:
> 
> # cs2cs +init=epsg:3857 +to +init=epsg:4326 -f %.17lg
>  1400000 6000000
>  12.576413977673299 47.353704702433106 0
> 
> For this test, postgis gave different values as in:
> 
> # select ST_AsText(ST_Transform(
>    ST_GeomFromText('POINT(1400000 6000000)',3857),4236));
>                  st_astext                  
> ------------------------------------------
>   POINT(12.5816693645354 47.3508570080732)
> 
> According to my research, the results of cs2cs are exact and e.g. in
> line with http://wiki.openstreetmap.org/wiki/Mercator
> 
> My postgres/postgis versions are:
> 
> PostgreSQL 9.5.3 on x86_64-pc-linux-gnu, compiled by gcc (Debian
> 4.9.2-10) 4.9.2, 64-bit POSTGIS="2.2.2 r14797" GEOS="3.4.2-CAPI-1.8.2
> r3921" PROJ="Rel. 4.8.0, 6 March 2012" GDAL="GDAL 1.10.1, released
> 2013/08/26" LIBXML="2.9.1" LIBJSON="0.11.99" RASTER
> 
>   Dose anybody have experience with mercator transformations and "false"
> offsets like in my origin example,
> 
>    TIA, Wolfgang