[postgis-users] st_transform, irreversible?

[Ricardo Bayley]

very nice explanation Mike

On Thu, Aug 12, 2010 at 11:39 AM, Mike Toews <mwtoews at 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 <erik at 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
> > erik at slagkryssaren.com
> > www.slagkryssaren.com
> > _______________________________________________
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> > postgis-users at postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >
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