[postgis-users] st_transform, irreversible?

[Barend Köbben]

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" <pramsey at 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 <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
>>> _______________________________________________
>>> postgis-users mailing list
>>> postgis-users at postgis.refractions.net
>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>> 
>> 
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Faculty of Geo-Information Science and Earth Observation (ITC)
University of Twente

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