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

[johnvb]

Thanks - you pointed me in the right direction, but I've found a slightly
different, simpler problem. When I copied the query to insert the 900913
projection into PostGIS from the web, the carriage return in the middle of
the proj4text got inserted. This resulted in something that still seemed to
be valid so I didn't spot it, but everything after the carriage return was
ignored (including the nadgrids) hence the datum problem.

Thanks for your help.


Paul Ramsey-4 wrote:
> 
> By saying @nadgrids=null you are implicitly saying "please don't try
> to do spheroid conversions for me".  In cases where you are already in
> WGS84, this is good. In other cases, it isn't. The @nadgrids hack was
> invented by folks in North America, where their data is usually
> already in NAD83, which is close enough to WGS84 that nobody cares.
> 
> P.
> 
> On Tue, Feb 17, 2009 at 5:49 AM, johnvb
> <john.vanbreda at biodiverseit.co.uk> wrote:
>>
>> 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.
>> --
>> View this message in context:
>> http://www.nabble.com/SRID-27700-problem-even-with-datum-shift-correction-tp22056338p22056338.html
>> Sent from the PostGIS - User mailing list archive at Nabble.com.
>>
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