On Sat, Jan 8, 2011 at 11:20 PM, Rick Workman <[email protected]> wrote: > I have imported a shape file into GeoServer (2.0.2 or 2.1-beta3) whose native > and declared SRS is EPSG:2019 (NAD27(76) / MTM zone 10 ). When I view the > layer from a WorldWind client whose WMS request uses EPSG:4236 (WGS 84) > overlaid on a MS virtual earth image layer, it appears to be offset from the > image. And I get the same effect using Google Earth as a client. > > Now if I use Quantum GIS to reproject the original shape file to EPSG:4326 > and import that shape file into Geoserver, the resultant layer seems to be > correctly aligned. > > If I go back to the layer definitions in the Geoserver admin interface, the > computed Lat/Lon Bounding Box for the two layers is slightly different: > > Original (EPSG:2019): -79.64, 43.578, -79.115, 43.854 > QGIS altered (EPSG:4326): -79.639, 43.581, -79.115, 43.855 > (correct values, as far as I can tell) > > The discrepancy between the two sets of values seems to correspond fairly > well with the offset displayed by the clients (WorldWind and Google Earth). > > So unless I'm missing some setting in the layer definition process, it looks > like Geoserver is not accurately projecting at least the EPSG:2019 SRS. > > On further investigation, it I can't find too many SRS systems that do seem > to work. For example, the same shape file converted (in QGIS) to a native SRS > of EPSG:26717 ("NAD27 / UTM zone 17N") has slightly larger errors: > > EPSG:26717: -79.643, 43.575, -79.113, 43.862 > > and EPSG:32190 ("NAD83 / MTM zone 10") seems to have correct native bounding > box lat/lon values, but the computed Lat/Lon bounding Box is nonsensical: > > EPSG:32190: -82.238, 0, -82.238, 0 > > > Any ideas what's going on?
This is odd, as you're the first to report systematic errors of this kind. In all the cases you're citing there is a datum transformation, which in GeoServer is performed by using the 7 params transformation driven by the towgs84 parameter set. It is a simple method that gives normally good results with some meters error (4 to 10). Maybe QGis is using under the hood a grid based datum transformation that GeoServer, as of now, does not have support for (there were talks about implementing it but nothing materialized). Grid based transformations normally give better results, with accuracies in the order of the centimeters (if properly setup). That said, there is something odd with the data you're providing. This EPSG:2019 is a projected coordinate system with ordinates expressed in meters that do range in the orders of millions of meters, a bbox like: Original (EPSG:2019): -79.64, 43.578, -79.115, 43.854 simply makes no sense in EPSG:2019, as you're still expressing the ordinates in degrees. (see http://spatialreferences.org/ref/epsg/2019/, look at projected bounds) If you have the actual original bounding box (expressed in epsg:2019) I can compare the transfromation results between GeoServer and libproj, the library that QGis uses under the hood, and probably figure out what's going on. Cheers Andrea PS: when datum transformations are concerned there is no "exact" result, there are just different transformation methods that provide different levels of accuracy -- Ing. Andrea Aime Technical Lead GeoSolutions S.A.S. Via Poggio alle Viti 1187 55054 Massarosa (LU) Italy phone: +39 0584962313 fax: +39 0584962313 http://www.geo-solutions.it http://geo-solutions.blogspot.com/ http://www.linkedin.com/in/andreaaime http://twitter.com/geowolf ----------------------------------------------------- ------------------------------------------------------------------------------ Gaining the trust of online customers is vital for the success of any company that requires sensitive data to be transmitted over the Web. Learn how to best implement a security strategy that keeps consumers' information secure and instills the confidence they need to proceed with transactions. http://p.sf.net/sfu/oracle-sfdevnl _______________________________________________ Geoserver-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/geoserver-users
