[postgis-users] NAD conversion problem

[Stefan Keller]

Hi,

I'm having problems with grid shift files and I suspect it's an
installation or compilation-config. problem:

This works:
# SELECT ST_AsText(ST_Transform('SRID=900913;POINT(977705
5981192)'::geometry, 21781))
But this...
# SELECT ST_AsText(ST_Transform('SRID=900913;POINT(977705
5981192)'::geometry, 4326))
.. gives following error:
> FEHLER:  transform: couldn't project point (977705 5.98119e+006 0): failed to load NAD27-83 correction file (-38)
> HINT:  PostGIS was unable to transform the point because either no grid shift files were found, or the point does not lie within the range for which the grid shift is defined. Refer to the ST_Transform() section of the PostGIS manual for details on how to configure PostGIS to alter this behaviour.

I installed Postgres 9.1alpha and PostGIS 2.0 on Windows XP
(experimental binaries) using the makepostgisdb.bat ( from
http://postgis.refractions.net/download/windows/pg91/experimental/postgis/postgis-pg91-binaries-2.0.0svn.zip).
Below you see the contents of srid=900913 in my spatial_ref_sys  table
(should be the most recent).

That's what I observed: Looking at the contents of this zip there is a
directory ".\share\contrib\postgis\proj". But looking at
makepostgisdb.bat this directory (and its content including files
calles nad*) is ignored. There is consequently also no proj files
under program files bin, lib, share (e.g. C:\Program
Files\PostgreSQL\9.1alpha1\ ). Under linux you wrote elsewhere that
the appropriate location of these files should be
/usr/local/share/proj and /usr/share/proj.

=> How can I make this transformation EPSG:900913 => 4326 work under Windows?

=> To me, either .\share\contrib\postgis\proj should be deleted from
.zip delivery (because it's compiled into dll?)
     or makepostgisdb.bat should install/copy this proj directory
somewhere into %PGBIN% or %PGLIB%.

Yours, S.

---
SELECT srid, auth_name, auth_srid, proj4text, srtext FROM
spatial_ref_sys WHERE srid=900913;
900913;"spatialreferencing.org";900913;"+proj=merc +a=6378137
+b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +units=m +k=1.0
+nadgrids=@null +no_defs";"PROJCS["Popular Visualisation CRS /
Mercator (deprecated)",GEOGCS["Popular Visualisation
CRS",DATUM["Popular_Visualisation_Datum",SPHEROID["Popular
Visualisation Sphere",6378137,0,AUTHORITY["EPSG","7059"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6055"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4055"]],UNIT["metre",1,AUTHORITY["EPSG","9001"]],PROJECTION["Mercator_1SP"],PARAMETER["central_meridian",0],PARAMETER["scale_factor",1],PARAMETER["false_easting",0],PARAMETER["false_northing",0],AUTHORITY["EPSG","3785"],AXIS["X",EAST],AXIS["Y",NORTH]]"


---

2009/9/28 Frank Warmerdam <warmerdam at pobox.com>:
> Peter N. Schweitzer wrote:
>>
>> So my hypothesis--please help me understand how wrong this is--is that the
>> @null has the effect of keeping proj going when I project a point that is
>> not in the areas covered by the nad27-to-nad83 conversion tables.
>
> Peter,
>
> Yes, this is the expected behavior of including null in a list of
> nadgrids. It is a fallback for quietly doing nothing if no other matching
> grid is found.  BTW the @ prefix means it's ok for the grid to not be
> found at all.   So if you wanted conus to be used for the continental
> US, and do nothing outside that region you could use +nadgrids=conus, at null
> while +nadgrids=@conus, at null means if the conus grid does not exist it is
> ok to do nothing everywhere.
>
>>I have
>>
>> to admit, as you probably suspect already, that I really don't know this
>> stuff very well.  And since the data are not highly accurate in any case,
>> the difference won't matter much in any analysis of the converted data.
>> But this is what I'm thinking.  Your counsel would be welcome!
>
> OK, as long as you understand the potential for error involved.
>
> Best regards,
> --
> ---------------------------------------+--------------------------------------
> 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    | Geospatial Programmer for Rent
>
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