[Proj] tmerc projection and inversion don't match

[Hermann Peifer]

It looks to me that your input values are simply outside the valid area 
of EPSG:3021, which are:

--- snippet from http://epsg-registry.org/ ---
Bounding Box Boundary	 Value (Decimal Degrees)
West Bound Longitude     14.09
East Bound Longitude     16.94
North Bound Latitude     68
South Bound Latitude     56
--- snip ---

Here some other expamples:

echo 15 62 | proj +init=epsg:3021 | invproj +init=epsg:3021
15dE    62dN

echo 62 15 | proj +init=epsg:3021 | invproj +init=epsg:3021
61d46'4.814"E   15d4'41.775"N


Hermann



On 12/08/2010 13:08, Jan Hartmann wrote:
>    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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