2009/2/26 Ric S <[email protected]>: > Hi Folks, > > this is a little bit offtopic, but I think someone here might know the > solution. > I'm trying to transform osm data from my postgis db to lat lon values. > > SELECT name, place, asText(transform(way,4326)) from planet_osm_point > WHERE name = 'Ingolstadt' AND place = 'city' ORDER BY name; > > The result is: > > 11.4333 48.9570163662354 > > If I use the values for example with google maps the resulting point is > off by about 20km. The expected result would be rather around 11.41 48.76. > > Any clue what's wrong?
I know what is wrong, but not why in Your case. 20km offset at ~48 degree is from additional Spheroid -> Elipsoid reprojection (changing reference elipsoid besides datum conversion). I had similar problem when converting SRTM -> GeoTIFF and needed to tell gdal, that srtm was latlon projection with spheroid not WGS84 elipsoid. It means that in some point, probably at import time, besides latlon->merc conversion there is also WGS84 elipsoid(6378137,298.257223563) -> spheroid(+a=6378137 +b=6378137). Second reason could be bad srids definition in postgis. I have data in 900913, and transform works fine, I use osm2pgsql application i.e.: > select getsrid(way),place, name, AsText(way), AsText(transform(way,4326)) > from planet_osm_point where place is not NULL and name = 'Płock'; > getsrid | place | name | astext | > astext > ---------+-------+-------+------------------------------------------+--------------------------------- > 900913 | city | Płock | POINT(2192192.46829378 6900082.03008444) | > POINT(19.6928 52.5494099990897) and in spatial_ref_sys srid definitions I have: > select * from spatial_ref_sys where srid = '900913' or srid = '4326'; > srid | auth_name | auth_srid | > > > srtext > > > | > proj4text > --------+----------------------+-----------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------- > 4326 | EPSG | 4326 | GEOGCS["WGS > 84",DATUM["WGS_1984",SPHEROID["WGS > 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]] > > > | +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs > 900913 | spatialreference.org | 6 | PROJCS["unnamed",GEOGCS["unnamed > ellipse",DATUM["unknown",SPHEROID["unnamed",6378137,0]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433]],PROJECTION["Mercator_2SP"],PARAMETER["standard_parallel_1",0],PARAMETER["central_meridian",0],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["Meter",1],EXTENSION["PROJ4","+proj=merc > +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m > +nadgri...@null +wktext +no_defs"]] | +proj=merc +a=6378137 > +b=6378137+lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m > +nadgri...@null +wktext +no_defs > (2 rows) I based my 900913 definition on: http://genuchten.blogspot.com/2008/04/srid-stuff-in-postgis.html PS. I'm not GIS expert, only what I needed at given moment and I could confuse some definitions or terms, for what I apologize in advance -- Marcin Rudowski _______________________________________________ Mapnik-users mailing list [email protected] https://lists.berlios.de/mailman/listinfo/mapnik-users
