[postgis-users] Interesting Datum Conversion Issue

[Frank Warmerdam]

René F. Viancos S. wrote:
> Record added to spatial_ref_sys Table
> 
> srid : 50000
> auth_name : EPSG
> auth_srid: 50000
> srtext : Sad69 a wgs84
> proj4text : +proj=utm +zone=19 +south +ellps=GRS67 +towgs84=75,1,44 
> +unit=m +to +proj=utm +zone=19 +south +datum=WGS84 +no_defs

René,

The proj4text should not contain the "+to ..." portion.  It should
really read "+proj=utm +zone=19 +south +ellps=GRS67 +towgs84=75,1,44 +unit=m".

> To run this custom datum transformation we use the transform() function 
> build in a query inserted as parameter in the pgsql2shp command.
> 
> pgsql2shp -h localhost -p 5432 -u pgsql -P pgsql -f shapefile_name.shp 
> db_name
> "select transform(setsrid(the_geom
> ,29179),50000) as the_geom from nombre_tabla"

The above sets the source geometry to have SRID 29179 which I assume
translates as:
   +proj=utm +zone=19 +south +ellps=GRS67 +units=m +no_defs

And the request is to convert to SRID 50000.  I believe what you
should be doing is setting the source SRID to 50000, and requesting
a conversion to SRID 32719 which translates as "+proj=utm +zone=19
+south +ellps=WGS84 +datum=WGS84 +units=m".

I believe you end up having to reverse the signs because you have
the transformation backward.

So:
"select transform(setsrid(the_geom,50000),32719)"

Having tried to dig through this email and answer the questions, I
am not feeling that this problem is very familiar and that I may
have answered it before.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGF, http://osgeo.org