[postgis-users] st_transform, irreversible?

[Paul Ramsey]

It should be, our spatial_ref_sys is generated from gdal which
generates from epsg. You have a pretty recent postgis, so I'm
surprised it's missing.
P

On Tue, Aug 17, 2010 at 1:57 AM, Erik Rehn <erik at slagkryssaren.com> wrote:
> Thank you guys! You were right, I had flipped the coordinates in the test I
> made.
>
> But that didn't explain the error I got in the KML. The solution to that
> was to add "+towgs84=414.1,41.3,603.1,-0.855,2.141,-7.023,0" to the
> spatial_ref_sys table for srid 3021. Why isn't that in there by default?
>
> erik
>
> On 2010-08-12 17:05, Ricardo Bayley wrote:
>>
>> very nice explanation Mike
>>
>> On Thu, Aug 12, 2010 at 11:39 AM, Mike Toews <mwtoews at gmail.com
>> <mailto:mwtoews at gmail.com>> wrote:
>>
>>    Hi,
>>
>>    Your coordinates may be flipped. Was it 59N 18E? If so, use x,y
>>    notation: 'POINT(18 59)', which results in 'POINT(18.0000000000006
>>    58.9999999999905)', which is close enough.
>>
>>    Also keep in mind that you are outside the projection bounds:
>>    http://spatialreference.org/ref/epsg/3021/ (just a bit too far east).
>>    Whenever you are outside the projection bounds, the likelihood of
>>    storage precision errors increase. To understand why this is, you can
>>    think of taking the tangent of a two angles that are nearly a
>>    right-angle (89.9991 and 89.9992) which have very different results
>>    due to nature of the geometry.
>>
>>    -Mike
>>
>>    On 11 August 2010 13:10, Erik Rehn <erik at slagkryssaren.com
>>    <mailto: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 <mailto:erik at slagkryssaren.com>
>>     > www.slagkryssaren.com <http://www.slagkryssaren.com>
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>
> --
> Erik Rehn
> Slagkryssaren
> erik at slagkryssaren.com
> www.slagkryssaren.com
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