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<H1><A NAME="SECTION0012180000000000000000"></A>
<A NAME="sec:netcdf"></A><A NAME="17985"></A>
<A NAME="17986"></A>
<BR>
4.18 Options for COARDS-compliant netCDF files
</H1>
<P>
When the netCDF file contains more than one 2-dimensional variable, <A NAME="tex2html1193"
HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> programs
will load the first such variable in the file and ignore all others. Alternatively,
the user can select the
required variable by adding the suffix ``?<I>varname</I>'' to the file name. For example,
to get information on the variable ``slp'' in file <U>file.nc</U>, use:
<PRE>
grdinfo "file.nc?slp"
</PRE>
Since COARDS-compliant netCDF files are the default, the additional suffix ``=nf'' can be omitted.
<P>
In case the named variable is 3-dimensional, <A NAME="tex2html1194"
HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> will load the first (bottom) layer. If another
layer is required, either add ``[<I>index</I>]'' or ``(<I>level</I>)'', where <I>index</I> is
the index of the third (depth) variable (starting at 0 for the first layer) and <I>level</I>
is the numerical value of the third (depth) variable associated with the requested layer.
To indicate the second layer of the 3-D variable ``slp'' use as file name: <U>file.nc?slp[1]</U>.
<P>
When you supply the numerical value for the third variable using ``(<I>level</I>)'',
<A NAME="tex2html1195"
HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> will pick the layer closest to that value. No interpolation is performed.
<P>
Note that the question mark, brackets and parentheses have special meanings on Unix-based platforms. Therefore,
you will need to either <I>escape</I> these characters, by placing a backslash in front of them, or place the
whole file name plus modifiers between single quotes or double quotes.
<P>
A similar approach is followed for loading 4-dimensional grids. Consider a 4-dimensional grid with the following
variables:
<PRE>
lat(lat): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
lon(lon): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
depth(depth): 0, 10, 20, 30, 40, 50, 60, 70, 80, 90
time(time): 0, 12, 24, 36, 48
pressure(time,depth,lat,lon): (5000 values)
</PRE>
To get information on the 10<IMG
WIDTH="16" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img83.png"
ALT="$\times$">10 grid of pressure at depth 10 and at time 24, one would use:
<PRE>
grdinfo "file.nc?pressure[2,1]"
</PRE>
or (only in case the coordinates increase linearly):
<PRE>
grdinfo "file.nc?pressure(24,10)"
</PRE>
<P>
The COARDS conventions set restrictions on the names that can be used for the units of the variables
and coordinates. For example, the units of longitude and latitude are ``degrees_east'' and ``degrees_north'',
respectively. Here is an example of the header of a COARDS compliant netCDF file (to be obtained using
<I>ncdump</I><A NAME="19024"></A>):
<PRE>
netcdf M2_fes2004 {
dimensions:
lon = 2881 ;
lat = 1441 ;
variables:
float lon(lon) ;
lon:long_name = "longitude" ;
lon:units = "degrees_east" ;
lon:actual_range = 0., 360. ;
float lat(lat) ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
lat:actual_range = -90., 90. ;
short amp(lat, lon) ;
amp:long_name = "amplitude" ;
amp:unit = "m" ;
amp:scale_factor = 0.0001 ;
amp:add_offset = 3. ;
amp:_FillValue = -32768s ;
short pha(lat, lon) ;
pha:long_name = "phase" ;
pha:unit = "degrees" ;
pha:scale_factor = 0.01 ;
pha:_FillValue = -32768s ;
</PRE>
This file contains two grids, which can be plotted separately using the names <U>M2_fes2004.nc?amp</U>
and <U>M2_fes2004.nc?pha</U>. The attributes <code>long_name</code> and <code>unit</code> for each variable are
combined in <A NAME="tex2html1196"
HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> to a single unit string. For example, after reading the grid <code>y_unit</code> equals
<code>latitude [degrees_north]</code>. The same method can be used in reverse to set the proper variable names
and units when writing a grid. However, when the coordinates are set properly as geographical or time axes,
<A NAME="tex2html1197"
HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> will take care of this. The user is, however, still responsible for setting the variable name and
unit of the z-coordinate. The default is simply ``z''.
<P>
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<ADDRESS>
Paul Wessel
2010-01-14
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