Re: netCDF and other things

• To: kevin.manross@xxxxxxxx
• Subject: Re: netCDF and other things
• From: Don Murray <dmurray@xxxxxxxxxxxxxxxx>
• Date: Mon, 18 Aug 2003 13:35:01 -0600

Kevin-

Kevin Manross wrote:

What follows are several questions I have been trying to figure out for a while with little or no success. (Yes, I'm still climbing the VisAD learning curve! ;-) I am very grateful to anyone who could shed light on my darkness!

First topic:

I am working with a netCDF file that has the following data:

(lat_index, lon_index) -> DEM

Where *_index is not the actually lat or lon, but Lat or Lon from the netCDF file (see below). Also DEM is the elevation from a Digital Elevation Model.

I would like to obtain the following:

latitude = corner_lat - lat_index*LatGridSpacing
longitude = corner_lon + lon_index*LonGridSpacing
elevation = DEM

I have all the data I need in the netCDF file, specifically, (lat_index = Lat, corner_lat = Latitude, LatGridSpacing = LatGridSpacing):

netcdf kiwaTerrain {
dimensions:
        Lat = 1079 ;
        Lon = 1298 ;
variables:
        float DEM(Lat, Lon) ;
                DEM:Units = "Meters" ;

// global attributes:
                :TypeName = "DEM" ;
                :DataType = "LatLonGrid" ;
                :Latitude = 37.7833333333342 ;
                :Longitude = -117.358333333342 ;
                :Height = -9.09494701772928e-10 ;
                :Time = 0 ;
                :FractionalTime = 0. ;
                :attributes = "" ;
                :LatGridSpacing = 0.00833333333333 ;
                :LonGridSpacing = 0.00833333333333 ;
                :MissingData = -99900.f ;
                :RangeFolded = -99901.f ;
data:

 DEM
  1764, 1766, 1792, 1769, 1769, 1748, 1744, 1718, 1713, 1705, 1689, 1667,
    1648, 1643, 1639, 1637, 1638, 1645, 1648, 1660, 1676, 1688, 1695, 1702,
    1727, 1757, 1826, 1878, 1805, 1812, 1822, 1751, 1715, 1690, 1666, 1655,

but would like to know how to extract the desired information from the netCDF file so I can complete the equations above. Does VisAD have the capability to extract such values?

The VisAD netCDF adapter does not do anything with global attributes.
You could use the netCDF classes in the VisAD distribution to get
a list of attributes:

     NetcdfFile ncFile = new NetcdfFile(filename, true);
     Attribute latSpace = ncFile.getAttribute("LatGridSpacing");
     float latS = latSpace.getNumericValue().floatValue();
     ...

I've thought about modifying Plain to create a Tuple from the
global attributes, but sometimes there are more attributes than
data in a netCDF file.  And, netCDF best practices would have
you use coordinate variables instead of global attributes to define
the lat/lon values at each point. ;-)

--------------

On another topic: I tried to create

Unit kilometer

    try
    { kilometer = visad.data.units.Parser.parse("kilometer"); }
    catch (ParseException P)
    { System.out.println("Parse exception: " + P); }

    r = RealType.getRealType("R", kilometer, null);
    h = RealType.getRealType("Height", kilometer, null);
    s = RealType.getRealType("SlantRange", kilometer, null);

<snip>

    xMap = new ScalarMap(s, Display.XAxis);
    yMap = new ScalarMap(h, Display.YAxis);


but the unit(s) displayed are:

SlantRange: xxx.xxxxx 1000.0 m
Height: yyy.yyyyy 1000.0 m

I followed Tom Whittaker's example for defining units, but perhaps I am missing something with the ScalarMap?

If you had:

   Unit km = kilometer.clone("km");

and called ScalarMap.setOverrideUnit(km), I think you'd get your
listings in as km instead of the unit definition.

--------------

topic 3: DataReferenceImpl

I would like to create a line with one anchored point *and of constant length*, basically a ray which the user can grab the outer end to trace a circle.

The only examples I have seen so far are a line which uses one "northing" point (Ugo's example in Section 6.4) and a line defined by two moveable points.

Can I create such a line as described?

(whew)

Yes, but I'm going to have to let someone else answer (time
limitations).

Thank you very much for your help!

Don
*************************************************************
Don Murray                               UCAR Unidata Program
dmurray@xxxxxxxxxxxxxxxx                        P.O. Box 3000
(303) 497-8628                              Boulder, CO 80307
http://www.unidata.ucar.edu/staff/donm
*************************************************************

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