Jon,
thanks for your answers
On Tue, Dec 13, 2011 at 1:42 PM, Jonathan Gregory
<j.m.gregory at reading.ac.uk> 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
Received on Tue Dec 13 2011 - 10:35:12 GMT
