On Thu, Jul 14, 2011 at 5:24 PM, Mark Hammond <[email protected]> wrote:
> Using a Google base layer, projecting a WMS layer from a Geoserver install
> as a semi-transparent overlay. WMS layer sourced from our local Geoserver
> install.
>
> The Google layer is native ESPG:900913/ESPG:4326 projection
> The WMS layer is natively ESPG:27700
>
> The WMS layer does get re-projected, but at high zoom there is a noticeable
> disparity of a couple of hundred meters between the two layers.
>
It's likely an error in the way we perform the datum change from OSGB 1936
to
WGS84.
GeoServer can use the TOWGS84 parameter set, which does not make for
great accuracy, though it should be better than 200m error.
Wondering, which version of GeoServer are you using? Recent version (2.1.1)
should be working better at this than older ones (say, 2.0.2 or 2.0.3).
The set of conversion parameters that we use now should be these ones:
PROJCS["OSGB 1936 / British National Grid",
GEOGCS["OSGB 1936",
DATUM["OSGB 1936",
SPHEROID["Airy 1830", 6377563.396, 299.3249646,
AUTHORITY["EPSG","7001"]],
TOWGS84[446.448, -125.157, 542.06, 0.15, 0.247, 0.842, -20.489],
AUTHORITY["EPSG","6277"]],
PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]],
UNIT["degree", 0.017453292519943295],
AXIS["Geodetic longitude", EAST],
AXIS["Geodetic latitude", NORTH],
AUTHORITY["EPSG","4277"]],
PROJECTION["Transverse Mercator", AUTHORITY["EPSG","9807"]],
PARAMETER["central_meridian", -2.0],
PARAMETER["latitude_of_origin", 49.0],
PARAMETER["scale_factor", 0.9996012717],
PARAMETER["false_easting", 400000.0],
PARAMETER["false_northing", -100000.0],
UNIT["m", 1.0],
AXIS["Easting", EAST],
AXIS["Northing", NORTH],
AUTHORITY["EPSG","27700"]]
Check if you version of GeoServer reports the same in Demo/SRS list.
There are more accurate methods than the TOWGS84 parameters, they are called
grid based transformations and can give very good accuracy (centimeters).
We have been looking for quite a bit of time for someone
to sponsor the development to support those methods, unfortunately so far
nobody
showed up
Cheers
Andrea
--
-------------------------------------------------------
Ing. Andrea Aime
GeoSolutions S.A.S.
Tech lead
Via Poggio alle Viti 1187
55054 Massarosa (LU)
Italy
phone: +39 0584 962313
fax: +39 0584 962313
http://www.geo-solutions.it
http://geo-solutions.blogspot.com/
http://www.youtube.com/user/GeoSolutionsIT
http://www.linkedin.com/in/andreaaime
http://twitter.com/geowolf
-------------------------------------------------------
------------------------------------------------------------------------------
AppSumo Presents a FREE Video for the SourceForge Community by Eric
Ries, the creator of the Lean Startup Methodology on "Lean Startup
Secrets Revealed." This video shows you how to validate your ideas,
optimize your ideas and identify your business strategy.
http://p.sf.net/sfu/appsumosfdev2dev
_______________________________________________
Geoserver-users mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/geoserver-users
