I think that is because many projections can indeed not be reversed, i.e.
the math is sometimes not straight formulas but iterative approximations and
you'll get rounding errors, etcetera...
--
Barend Köbben
Senior Lecturer
ITC University of Twente, Faculty of Geo-Information Science and
Earth Observation
PO Box 217, 7500AA Enschede, The Netherlands
+31 (0)53 4874253
On 11-08-10 22:39, "Paul Ramsey" <pram...@opengeo.org> wrote:
> 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 <pram...@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 <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
Faculty of Geo-Information Science and Earth Observation (ITC)
University of Twente
E-mail disclaimer
The information in this e-mail, including any attachments, is intended for the
addressee only. If you are not the intended recipient, you are hereby notified
that any disclosure, copying, distribution or action in relation to the content
of this information is strictly prohibited. If you have received this e-mail by
mistake, please delete the message and any attachment and inform the sender by
return e-mail. ITC accepts no liability for any error or omission in the
message content or for damage of any kind that may arise as a result of e-mail
transmission.
_______________________________________________
postgis-users mailing list
postgis-users@postgis.refractions.net
http://postgis.refractions.net/mailman/listinfo/postgis-users
