very nice explanation Mike On Thu, Aug 12, 2010 at 11:39 AM, Mike Toews <mwto...@gmail.com> wrote:
> Hi, > > Your coordinates may be flipped. Was it 59N 18E? If so, use x,y > notation: 'POINT(18 59)', which results in 'POINT(18.0000000000006 > 58.9999999999905)', which is close enough. > > Also keep in mind that you are outside the projection bounds: > http://spatialreference.org/ref/epsg/3021/ (just a bit too far east). > Whenever you are outside the projection bounds, the likelihood of > storage precision errors increase. To understand why this is, you can > think of taking the tangent of a two angles that are nearly a > right-angle (89.9991 and 89.9992) which have very different results > due to nature of the geometry. > > -Mike > > On 11 August 2010 13:10, Erik Rehn <e...@slagkryssaren.com> wrote: > > Hello Postgis Users! > > > > This is my first post on this list so I will start by asking > > a simple (and probably stupid) question. :) > > > > While using ST_AsKml() to produce an overlay for Google Earth I > > noticed that all my geometries where shifted slightly south-east. > > I figured this had something to do with the transformation between > > the projection that my geometries are stored in (SRID 3021) and WGS84 > (4326) > > that is outputted by ST_AsKml() > > > > Just to test I ran this: > > > > SELECT ST_AsText( > > ST_Transform( > > ST_Transform( > > ST_GeomFromText('POINT(59 18)',4326), > > 3021), > > 4326)); > > > > I input a point in WGS84 (59,18), transforms it to 3021 and then back to > > WGS84. The result I get is: > > POINT(58.8672757036296 18.0394763349359) > > > > Can anyone explain this? Am I missing something regarding ST_Transform()? > > > > Im running Postgis 1.5 on Windows. > > > > Thank you for any help! > > /Erik > > > > -- > > Erik Rehn > > Slagkryssaren > > e...@slagkryssaren.com > > www.slagkryssaren.com > > _______________________________________________ > > postgis-users mailing list > > postgis-users@postgis.refractions.net > > http://postgis.refractions.net/mailman/listinfo/postgis-users > > > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users >
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