[postgis-users] st_transform, irreversible?

[Paul Ramsey]

Removing the EPSG lookup from the equation changes nothing:

echo "59 21" | proj +proj=tmerc +lat_0=0 +lon_0=15.80827777777778 +k=1
+x_0=1500000 +y_0=0 +ellps=bessel +units=m +no_defs | invproj
+proj=tmerc +lat_0=0 +lon_0=15.80827777777778 +k=1 +x_0=1500000 +y_0=0
+ellps=bessel +units=m +no_defs

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

P.

On Wed, Aug 11, 2010 at 1:35 PM, Paul Ramsey <pramsey at opengeo.org> wrote:
> 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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>>
>