Jon, thanks for your answers
On Tue, Dec 13, 2011 at 1:42 PM, Jonathan Gregory <[email protected]> wrote: > Dear Etienne > > Thanks for your helpful email, and sorry for slow response. > >> Ok you mean that we could add new projections easily to the CF >> standard? That's great to know. > > Yes, you could do this with a trac ticket, like ticket 72, which proposes to > add the geos projection. See https://cf-pcmdi.llnl.gov/trac/query I would love to add a trac ticket, but unfortunately my repeated requests to webmaster to get a trac id have failed... who should I ask? > >> I'm not sure I understand your question - do you mean to ask if these >> additions to CF would be sufficient to describe most WKT definitions >> in pure CF metadata (without the WKT)? > > Yes, that's what I mean. I'm interested to know how many other elements of WKT > are used in the cases you deal with. > >> It seems that >> for many applications (especially at the scale most netcdf files are >> used for), TOWGS84 parameters are sufficient. A named datum would be >> nice, but there are quite a few different ways to identify datums (OGC >> vs ESRI). > > OK. If they are standardised lists, we could provide attributes to store > them in. This page is very informative on WKT specifications http://home.gdal.org/projects/opengis/wktproblems.html TOWGS84 corresponds to the Bursa Wolf parameters for datum shifts and is standard in CT 1.0 using (3 or 7 values). The CT 1.0 standard is relatively simple and fairly complete, I would suggest that CF should use it to fill the gaps in projection definitions. http://www.opengeospatial.org/standards/ct CT 1.0 WKT stores the datum's name, EPSG code as well as its relation to the WGS84 datum in TOWGS84. For example, the full WKT for EPSG:4618 (that uses SAD69 datum) is: GEOGCS["SAD69", DATUM["South_American_Datum_1969", SPHEROID["GRS 1967 Modified",6378160,298.25, AUTHORITY["EPSG","7050"]], TOWGS84[-57,1,-41,0,0,0,0], AUTHORITY["EPSG","6618"]], PRIMEM["Greenwich",0, AUTHORITY["EPSG","8901"]], UNIT["degree",0.0174532925199433, AUTHORITY["EPSG","9122"]], AUTHORITY["EPSG","4618"]] In the DATUM section you see the datum name "South_American_Datum_1969", EPSG code 6618 and TOWGS84[-57,1,-41,0,0,0,0] The Simple Features specification only stores datum_name, which can be problematic as some vendors (i.e. ESRI) use slightly different datum names than others (i.e. OGR, CADCorp). I prefer the later as it follows EPSG datum names more closely. => In light of all this, could we add datum_name, datum_code and towgs84 to grid_mapping? As maintainer of GDAL's netcdf driver, I see that the only thing missing in the WKT/CF transformation is named datum (and ideally datum code and towgs84), then there would not be any need to keep the entire WKT string - with some exceptions where the parameters are not 100% compatible (see the link below). For example, the same EPSG:4618 WKT translates to CF like this: crs:grid_mapping_name = "latitude_longitude" ; crs:longitude_of_prime_meridian = 0. ; crs:semi_major_axis = 6378160. ; crs:inverse_flattening = 298.249999999996 ; crs:spatial_ref = "GEOGCS[\"SAD69\",DATUM[\"South_American_Datum_1969\",SPHEROID[\"GRS 1967 Modified\",6378160,298.249999999996,AUTHORITY[\"EPSG\",\"7050\"]],TOWGS84[-57,1,-41,0,0,0,0],AUTHORITY[\"EPSG\",\"6618\"]],PRIMEM[\"Greenwich\",0],UNIT[\"degree\",0.0174532925199433],AUTHORITY[\"EPSG\",\"4618\"]]" ; crs:GeoTransform = "-80 0.25 0 -10 0 -0.25 " ; The spatial_ref is kept for now because CF lacks information on the named datum. It would be complete if we could have something like: crs:grid_mapping_name = "latitude_longitude" ; crs:longitude_of_prime_meridian = 0. ; crs:semi_major_axis = 6378160. ; crs:inverse_flattening = 298.249999999996 ; crs:datum_name = South_American_Datum_1969 ; crs:datum_code = 6618; crs:towgs84 = -57,1,-41,0,0,0,0 ; Perhaps there should be an additional parameter (like datum_authority) to specify that datum_code is from EPSG, or make it a string like 'EPSG:6618' > >> Here is a small compilation of the compatabilities between WKT (as >> GDAL sees it) and CF-1.5 projections, there are a few >> problems/unknowns with some projections: >> http://trac.osgeo.org/gdal/wiki/NetCDF_ProjectionTestingStatus > > If you identify errors or inadequacies with CF definitions from your detailed > analysis, you could propose they be corrected, again with a CF trac ticket, in > this case as a "defect" rather than an addition to the standard. I'll look into this when I have the time... and if I can get access to trac!!! > >> The following page lists WKT parameters: >> http://www.geoapi.org/2.0/javadoc/org/opengis/referencing/doc-files/WKT.html > > Yes, this is a useful page. > >> There are a few other parameters like VERT_CS, COMPD_CS and VERT_DATUM >> that some users may need. > > The VERT_CS and VERT_DATUM appear to be names (in the Newlyn example). Are > these names standardised? They are part of CT 1.0 , I suggest you take a look at the WKT keywords. Sorry but I don't know more to comment extensively. Thank you Jonathan! Etienne > > Best wishes and thanks for your help > > Jonathan _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
