OK, Firstly I should point out that until very recently I was one of the "flat earthers" who couldn't see why anyone would want anything more than two coordinate variables with longitude and latitude.
>From discussions with some of my colleagues who know a lot more about grid >remapping, supposedly there are libraries which can remap between two >arbitrary EPSG codes (when possible). Therefore, from the point of view of >software which uses these libraries an attribute such as crs:epsg = 4326 may >in fact be more "self-describing" than the complete information? Perhaps the solution is to allow only including an attribute with a simple epsg code, and recommend (but not require) adding all the other attributes to make it fully self-describing? Regards, Tim Hume Bureau of Meteorology ________________________________________ From: CF-metadata [[email protected]] On Behalf Of Etienne Tourigny [[email protected]] Sent: Tuesday, 31 July 2012 12:07 PM To: Kennedy, Paul Cc: [email protected] Subject: Re: [CF-metadata] How to include EPSG codes or WKT information in a CF file [SEC=UNCLASSIFIED] On Mon, Jul 30, 2012 at 10:56 PM, Kennedy, Paul <[email protected]> wrote: > Tim, > I could not agree more. Indeed, I would go even further and permit a simple > EPSG code as a suitable definition of the geodesy. EPSG:4326 is all you need > to specify WGS ellipsoid, datum etc. > > crs:epsg_code = 4326 ; //this would be the 99% case. This is how google > openlayers, wms, google, bing and others handle the tricky subject of geodesy > metadata. Unfortunately, the majority of CF community does not agree with the use of codes because they are not "self-describable". Have a look at the archives of this mailing list and you will see a load of threads on this subject. If you want EPSG:4326, there are parameters that describe it (see the link you submitted) In fact, it is quite compact in CF notation (see example 5/9 of the CF-1.6 document) int crs ; crs:grid_mapping_name = "latitude_longitude"; crs:longitude_of_prime_meridian = 0.0 ; crs:semi_major_axis = 6378137.0 ; crs:inverse_flattening = 298.257223563 ; > > WKT is also very handy, but harder to generate and parse. I suspect this is > the 1% case. Thus the need to write the necessary CF parameters to describe WGS84. This is easier than parsing WKT. Etienne > > http://spatialreference.org/ref/epsg/4326/ > > > regards > > Paul Kennedy > Technical Development Manager > Fugro Survey Pty Ltd > 24 Geddes St, Balcatta > Western Australia 6021 > ABN: 81 009 172 990 > Ph : +61 (0)8 6477 4400 > Direct: +61 (0)8 6477 4418 > Fax : +61 (0)8 6477 4499 > Mobile: +61 (0)439 518 265 > Email : [email protected] > Skype : p.kennedy.fugro.com > > > > -----Original Message----- > From: CF-metadata [mailto:[email protected]] On Behalf Of > Timothy Hume > Sent: Tuesday, 31 July 2012 9:34 AM > To: [email protected] > Subject: Re: [CF-metadata] How to include EPSG codes or WKT information in a > CF file [SEC=UNCLASSIFIED] > > Hi Jonathan, > > Thanks for the help. I can see that option 80 is probably better, but option > 69 is a lot easier for software developers to bolt onto existing NetCDF > files. Most of the people I talk to don't see why anyone is interested in > anything other than latitudes and longitudes on a spherical Earth; all the > stuff about shape of the Earth etc is irrelevant to their needs. The truth > is, because most of the NWP data I use is on a rather coarse grid, the grid > mapping information is not that important; two coordinate variables with the > longitudes and latitudes is sufficient for most purposes. > > Therefore, the path of least resistance is option 69 (with all its potential > pit falls); I think option 80 will be too much, and a lot of data writers > will simply decide to add no grid mapping information at all, a worse > situation than option 69. > > Cheers, > > Tim Hume > Bureau of Meteorology > ________________________________________ > From: CF-metadata [[email protected]] On Behalf Of Jonathan > Gregory [[email protected]] > Sent: Monday, 30 July 2012 7:10 PM > To: [email protected] > Subject: [CF-metadata] How to include EPSG codes or WKT information in a CF > file [SEC=UNCLASSIFIED] > > Dear Tim > > There are two open CF trac tickets that relate to this issue, proposing to > add new conventions to store OGC information in different ways in CF. Ticket > 80 https://cf-pcmdi.llnl.gov/trac/ticket/80 proposes several new attributes > which correspond to WKT elements that currently do not have grid_mapping > equivalents. Ticket 69 https://cf-pcmdi.llnl.gov/trac/ticket/69 proposed to > add an attribute to record the WKT information without translation. > Personally I prefer the former because I am concerned about duplication and > possible inconsistency of information, but please have a look at the tickets > and form your own view, and add comments if you like. So the answer to your > question is that there is not yet a CF standard way to do it, but there will > be. > > Best wishes > > Jonathan > > > ----- Forwarded message from Timothy Hume <[email protected]> ----- > >> Date: Mon, 30 Jul 2012 11:36:19 +1000 >> From: Timothy Hume <[email protected]> >> To: "[email protected]" <[email protected]> >> Subject: [CF-metadata] How to include EPSG codes or WKT information in a CF >> file [SEC=UNCLASSIFIED] >> >> Hi, >> >> I am adding grid mapping information to my NetCDF files. My data are on a >> lon-lat grid on a spherical Earth. I can put in the standard CF attributes, >> grid_mapping_name etc, but also want to include the EPSG and WKT codes to >> help software applications that might want these. I have not been able to >> find out how this is done in NetCDF-CF, so am using this format: >> = >> int crs ; >> crs:grid_mapping_name = "latitude_longitude" ; >> crs:earth_radius = 6371007.0 ; >> crs:prime_meridian = 0.0 ; >> crs:epsg_code = 4047 ; >> crs:wkt = "GEOGCS[\"Unspecified datum based upon the >> GRS 1980 Authalic Sphere\",DATUM[\"Not_specified_based_on >> _GRS_1980_Authalic_Sphere\",SPHEROID[\"GRS 1980 Authalic >> Sphere\",6371007,0,AUTHORITY[\"EPSG\",\"7048\"]],AUTHORITY[\"EPSG\",\ >> "6047\"]],PRIMEM[\"Greenwich\",0,AUTHORITY[\"EPSG\",\"8901\"]],UNIT[\" >> degree\",0.01745329251994328,AUTHORITY[\"EPSG\",\"9122\"]],AUTHORITY[\ >> "EPSG\",\"4047\"]]" ; >> >> What do other people do when including this extra grid information? >> >> Cheers, >> >> Tim Hume >> Centre for Australian Weather and Climate Research Bureau of >> Meteorology Melbourne Australia >> _______________________________________________ >> CF-metadata mailing list >> [email protected] >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > ----- End forwarded message ----- > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
