[postgis-devel] Reprojection issues

[Nico Mandery]

Mark Cave-Ayland schrieb:
> Nico Mandery wrote:
>> Hello list,
>>
>> I got a problem reprojecting geometries from EPSG:3785 (the Google or 
>> Web Mercator projection) to WGS84 (EPSG:4236).
>> For example when I try to transform one point postgis gives me the 
>> following - correct - result:
>>
>> projects=# select 
>> astext(st_transform(GeomFromText('POINT(879650.162980923 
>> 6108535.16589579)',3785),4326));
>>                  astext
>> ------------------------------------------
>> POINT(7.90203186083991 48.0100993171653)
>> (1 row)
>>
>> When I try it again with exactly the same query, I get a wrong result 
>> and an error from libproj:
>>
>> projects=# select 
>> astext(st_transform(GeomFromText('POINT(879650.162980923 
>> 6108535.16589579)',3785),4326));
>> WARNING:  transform: -38 (failed to load NAD27-83 correction file)
>>                      astext
>> --------------------------------------------------
>> POINT(1.23892476139034e-06 7.52729195068025e-06)
>> (1 row)
>>
>> Every following query will also return wrong results, until I close 
>> the connection to the database. The first query after establishing a
>> new connection to postgresql returns a correct result again, every 
>> following raises the error again.
>> The NAD correction files exist. Transforming coordinates with cs2cs 
>> from libproj works without any problems.
>>
>> I tried it with Postgresql 8.2.4 and 8.3.3 and postgis 1.3.2 and 
>> 1.3.3. The proj4text in spatial_ref_sys for EPSG:3785 is
>> "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 
>> +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs"
>>
>> Thanks a lot,
>> Nico
>
>
> Hi Nico,
>
> I'm not sure about the +wktext part of your proj4text as I've never 
> seen that before - so maybe it is an invalid PROJ.4 string? What 
> happens if you try the transformation using the standard proj executable?
>
>
> ATB,
>
> Mark.
>

Hi Mark,

I'm using the same Proj.4 string in the epsg-file of the proj libray. As 
far as I can see the +wktext part does not seem to matter - the results 
stay the same with or without it. Reprojections using the cs2cs tool 
from proj are working correctly - whats also my workaround for this 
problem right now.


regards,
Nico