[Proj] tmerc projection and inversion don't match

[Jan Hartmann]

  I have taken the liberty to forward this question on the postgis list 
to the proj list. It's about a transformation that doesn't return the 
original values when inverted. I don't understand it either.

Jan

On 08/11/10 22:39, Paul Ramsey 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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