[postgis-users] SRID 27700 problem even with datum shift correction

[johnvb]

I've applied the following script to my database to install support for the
Google Spherical Mercator projection, plus to fix the well known datum
missing for SRID 27700:
INSERT into spatial_ref_sys (srid, auth_name, auth_srid, srtext, proj4text)
values (900913 ,'EPSG',900913,'GEOGCS["WGS 84", DATUM["World Geodetic System 
1984", SPHEROID["WGS 84", 6378137.0,
298.257223563,AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich", 0.0,
AUTHORITY["EPSG","8901"]], NIT["degree",0.017453292519943295],
AXIS["Longitude", EAST], AXIS["Latitude", NORTH],AUTHORITY["EPSG","4326"]],
PROJECTION["Mercator_1SP"],PARAMETER["semi_minor", 6378137.0], 
PARAMETER["latitude_of_origin",0.0], PARAMETER["central_meridian", 0.0],
PARAMETER["scale_factor",1.0], PARAMETER["false_easting", 0.0],
PARAMETER["false_northing", 0.0],UNIT["m", 1.0], AXIS["x", EAST], AXIS["y",
NORTH],AUTHORITY["EPSG","900913"]] |','+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 +no_defs');

-- Update the OSGB systems (easting/northing and lat long OSGB36) so they
correctly use the OSGB36 datum
UPDATE spatial_ref_sys SET proj4text = '+proj=longlat +ellps=airy
+datum=OSGB36 +no_defs' WHERE srid=4277;
UPDATE spatial_ref_sys SET proj4text = '+proj=tmerc +lat_0=49 +lon_0=-2
+k=0.999601 +x_0=400000 +y_0=-100000 +ellps=airy +units=m +no_defs
+datum=OSGB36' WHERE srid=27700;

Now, even with this correction I can demonstrate a problem with the 27700
SRID by using the following SQL:

select '27700 to 900913 (incorrect)',
st_astext(st_transform(st_geomfromtext('POINT(400000
100000)',27700),900913))
union
select '27700 to 4326 then 900913 (correct)',
st_astext(st_transform(st_transform(st_geomfromtext('POINT(400000
100000)',27700),4326),900913))

This shows 2 different results, one by transforming the point from 27700 to
900913, then another by converting via 4326. The conversion via 4326 seems
to be correct, but the other conversion is wrong in a similar fashion to the
error I'd expect if the datum shift was not working. The output I get is:
"27700 to 4326 then 900913 (correct)";"POINT(-222791.172749668
6585914.69501315)"
"27700 to 900913 (incorrect)";"POINT(-222638.981586547 6585815.32217342)"

Any ideas why this discrepancy occurs? I'm using PostGIS 1.3.3 installed
using the Application Stack 
Builder on Windows, against PostgreSQL 8.3.
-- 
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