Using GrADS to retrieve HYCOM data

[Jennifer Adams]

Dear All,
If you do an ncdump on that OPeNDAP data set, you'll discover that the
lat and lon values of the grid vary in the X and Y dimensions. GrADS
doesn't handle this kind of native grid. To get this into GrADS you'd
have to use the abstract X and Y axes instead. With the descriptor
given below, you could open the file and look at grid values, but not
map them to locations on the globe. With the available lat/lon
variables, you might be able to build a file for the PDEF GENERAL
option, but with an opendap data set, this is not recommended because
the I/O goes point-by-point with PDEF and to do this via OPeNDAP is
just not viable. In any event, right now I am unable to extract any
data from the server. Even ncdump gave me the metadata, but hung on
returning any coordinate axis values.

Jennifer

# ncdump -h http://hycom.coaps.fsu.edu:8080/thredds/dodsC/glb_nrt_analysis
netcdf glb_nrt_analysis { // format variant: 64bit
dimensions:
         Y = 3298 ;
         X = 4500 ;
         MT = 4 ;
         Depth = 33 ;
variables:
         int Y(Y) ;
                 Y:point_spacing = "even" ;
                 Y:axis = "Y" ;
         int X(X) ;
                 X:point_spacing = "even" ;
                 X:axis = "X" ;
         float Latitude(Y, X) ;
                 Latitude:standard_name = "latitude" ;
                 Latitude:units = "degrees_north" ;
         float Longitude(Y, X) ;
                 Longitude:standard_name = "longitude" ;
                 Longitude:units = "degrees_east" ;
                 Longitude:modulo = "360 degrees" ;

This will open the file:

dset http://hycom.coaps.fsu.edu:8080/thredds/dodsC/glb_nrt_analysis
dtype netcdf
undef -9.99e8 _FillValue
xdef 4500 linear 1 1
ydef 3298 linear 1 1
zdef 33 linear 1 1
tdef 4 linear 31dec1900 1dy
vars 5
Longitude=>glon  0  y,x  longitude
Latitude=>glat  0  y,x  latitude
Depth=>depth  33  z depth in m positve down
MT=>tvals  0  t  time axis values
temperature=>t  33 t,z,y,x temperature
endvars



On Apr 30, 2009, at 9:40 PM, Dan Martin wrote:

> The full HYCOM data is on a curvilinear grid that I haven't been
> able to
> make work in grads. They set up a linear grid for grads users by
> cutting
> off pre-gridded section above 48 degrees north.
>
> http://hycom.coaps.fsu.edu/opendap/nph-dods/datasets/hycom/global/GLBa0.08_rect/data/
>
> The update schedule is a bit irregular and the server is not alway up,
> but nice data when it is there.
>
> If anyone has the curvilinear grid version working in grads, any tips
> would be nice.
>
> Dan
>
> Mark Hess wrote:
>> Has anyone out there used GrADS to retrieve and display data from the
>> HYCOM OPeNDAP near real-time data server
>> http://hycom.coaps.fsu.edu/thredds/dodsC/glb_nrt_analysis.html?
>>
>> I am hoping to use GrADS to retrieve mixed layer depth data from this
>> server, but not real clear on the http/OPeNDAP string I would use to
>> download the file (whether it be ASCII or binary).  If you have done
>> this and could send a sample script or command showing the http
>> line that would be much appreciated.
>>
>> Thanks,
>>
>> Mark
>>

--
Jennifer M. Adams
IGES/COLA
4041 Powder Mill Road, Suite 302
Calverton, MD 20705
jma at cola.iges.org



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