[postgis-users] st_transform, irreversible?

[Paul Ramsey]

Seems to be an underlying problem with proj4:

echo "59 21" | proj "+init=epsg:3021" | invproj "+init=epsg:3021"

58d49'47.733"E	21d2'54.745"N

And that's without doing the datum shift part.

P

On Wed, Aug 11, 2010 at 1:10 PM, 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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