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 [cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Etienne Tourigny [etourigny.dev at gmail.com]
Sent: Tuesday, 31 July 2012 12:07 PM
To: Kennedy, Paul
Cc: cf-metadata at cgd.ucar.edu
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 <P.Kennedy at fugro.com.au> 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 : p.kennedy at fugro.com.au
> Skype : p.kennedy.fugro.com
>
>
>
> -----Original Message-----
> From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Timothy Hume
> Sent: Tuesday, 31 July 2012 9:34 AM
> To: cf-metadata at cgd.ucar.edu
> 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 [cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Jonathan Gregory [j.m.gregory at reading.ac.uk]
> Sent: Monday, 30 July 2012 7:10 PM
> To: cf-metadata at cgd.ucar.edu
> 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 <T.Hume at bom.gov.au> -----
>
>> Date: Mon, 30 Jul 2012 11:36:19 +1000
>> From: Timothy Hume <T.Hume at bom.gov.au>
>> To: "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>
>> 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
>> CF-metadata at cgd.ucar.edu
>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
> ----- End forwarded message -----
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