Hi,
not sure if people have been following what’s going on in reprojection land,
but we have two new high precision coordinate transformation methods coming
down the pipe* on trunk and probably later on 2.1.x
The two methods are NTv2** and NADCON***, which allow for datum transformations
with cm level precision (instead of the usual 4m, when all goes well).
Both will activate only if the grid files are present in the data dir, so by
default the mere presence of the method is harmless, but generally
speaking this is very good news for whoever needs to increase reprojection
accuracy.
Now, of course people will want to make sure the transformations are actually
using the grids, but to do that one needs to see what actual transformation steps
are being used.
So I rolled out a small transformation console that allows to pick two coordinate
systems and transform a single point back and forth:
The “show transformation details” dumps the actual transformation path in WKT syntax (yeah, there’s one
for transformations as well) allowing to see what exact values are used for the transformation:
If you have grids activated and available you may get something like the following instead, which uses a NADCON grid
to transform from EPSG:4138 to EPSG:4326:
CONCAT_MT[PARAM_MT[“Affine”,
PARAMETER[“num_row”, 3],
PARAMETER[“num_col”, 3],
PARAMETER[“elt_0_0”, 0.0],
PARAMETER[“elt_0_1”, 1.0],
PARAMETER[“elt_1_0”, 1.0],
PARAMETER[“elt_1_1”, 0.0]],
PARAM_MT[“NADCON”,
PARAMETER[“Latitude difference file”, “stgeorge.las”],
PARAMETER[“Longitude difference file”, “stgeorge.los”]],
PARAM_MT[“Affine”,
PARAMETER[“num_row”, 3],
PARAMETER[“num_col”, 3],
PARAMETER[“elt_0_0”, 0.0],
PARAMETER[“elt_0_1”, 1.0],
PARAMETER[“elt_1_0”, 1.0],
PARAMETER[“elt_1_1”, 0.0]]]
The page would be a demo one, and as long as it transforms a single point it is probably harmless.
If you want to transform more, like WTK geometries, GML or shapefiles, there is a WPS process to do that already.
Opinions? I was thinking to land this next week along with the ability to search NTv2 and NADCON grids
in $GEOSERVER_DATA_DIR/user_projections
Cheers
Andrea
*: most of the NTv2 work has been done by Oscar Fonts, I chimed to have the NADCON transformations
be wired up as well and to simplify the grid lookup plugin code.
The EPSG database already contains references to some 50+ official NADCON grids and a dozen
NTv2 grids, but afaik Oscar wants to add some way to override the database and use a custom
grid between any arbitrary couple of EPSG codes.
**: http://jgridshift.sourceforge.net/ntv2.html
***: http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html
–
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
mob: +39 339 8844549
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
