Re: [netcdf-java] Get data through lat lon coordinates

• To: netcdf-java@xxxxxxxxxxxxxxxx
• Subject: Re: [netcdf-java] Get data through lat lon coordinates
• From: Pablo Rozas Larraondo <p.rozas.larraondo@xxxxxxxxx>
• Date: Sat, 28 Jan 2012 13:40:48 +0100

Hello Martin,

Thank you very much for your help. Your proposal sounds very interesting to
me, I didn't know about the Geotoolkit project and it seems to solve a lot
of useful tasks like interpolation, transforms, etc.

I would like to try the toolkit in my project, I use eclipse IDE and I've
installed m2eclipse plugin for working with maven but I don't have much
experience with it... If you could give me some basic guidelines to start a
project I would really appreciate your help.

if you prefer you can reach me at my personal mail: p.rozas.larraondo(at)
gmail.com as this particular matter may be out of the netcdf-java mail list
purpose.

Thanks again.


Hello Pablo
>
> Le 27/01/12 10:14, Pablo Rozas Larraondo a écrit :
>
> I've succeeded in opening and reading Grib1 files, but I'm having problems 
> trying
> to find out how to access data through lat lon coordinates. In the ECMWF
> API there is a function called grib_find_nearest(gid, lat,lon) in which you
> give a geocoordinate point and the function returns the value of the closest,
> or the four closest grid points.
>
> Using the NetCDF API I can access the _CoordinateAxisType Attributes of
> the grib file which are GeoX and GeoY expressed in Km, but I wonder if
> there is a function for accessing values or translating points into
> geographic coordinates.
>
> I'm not sure if the NetCDF API provides API for this task. But one
> possible approach may be to use a library which sit on top of the UCAR
> NetCDF library. If I use as an example the library which I known (but you
> can find other projects doing similar jobs), you could use the
> Geotoolkit.org coverage module(http://www.geotoolkit.org/modules/coverage)
> like below:
>
> The read the image, the org.geotoolkit.coverage.io.CoverageIO static
> convenience methods should provide an easy start (there is also other
> ways if you need more control, for example specifying a subsampling or
> the geographic area you want to read).
>
>     GridCoverage coverage = CoverageIO.read("your_file.nc");
>
>
> It will use the UCAR library under the hood for reading the data. To get
> the nearest value of a geodetic coordinate, you can do as below:
>
>     float[] samples = null;
>     GeneralDirectPosition pos = new GeneralDirectPosition(numberOfDimensions);
>     // If you have to do a loop, do it here.
>     pos.setLocation(myOrdinateValues);
>     samples = coverage.evaluate(pos, samples);
>
>
> Thats it (if everything goes well - I'm actively working on improving the
> NetCDF support right now). If the position is not in the same coordinate
> reference system than the coverage, the library will do the reprojection
> for you. It is also possible to instruct the evaluate method do perform
> bilinear or bicubic interpolation automatically. There is also some other
> interesting stuff you can get automatically, like exporting the metadata
> found in your NetCDF file in an ISO 19139 compliant document.
>
> Please let me known if you wish to know more. I could then give a more
> concrete starting point depending on your environment (e.g. if you use
> Maven or not).
>
>     Martin
>
>