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<title>PDEF - Displayng Pre-Projected Data in GrADS</title>
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<p class="banner18">Use PDEF For Displaying Pre-Projected Data With GrADS</p>
<p class="plaintext"><a href="#about">Display pre-projected data with PDEF</a><br>
<a href="#syntax">PDEF Syntax</a><br>
<a href="#interp">How grid interpolation works</a><br>
<a href="#rotation">How wind rotation works</a><br>
<a href="#bilin">PDEF BILIN option</a><br>
<a href="#file">PDEF GENERAL option</a><br>
<a href="#file">PDEF FILE option</a></p>
<table width="650" border="0" cellspacing="0" cellpadding="0">
<tr>
<td><p class="plaintext"><a name="about" class="item12bold">Display Pre-Projected
Data with PDEF</a></p>
<p class="plaintext">Gridded data that are mapped onto a particular map
projection are called 'pre-projected.' An example of pre-projected data
is the output from a weather forecast model that is mapped onto a north
polar stereographic grid. </p>
<p class="plaintext">In order to display pre-projected data on a map in
GrADS, the descriptor file must contain a PDEF entry. A descriptor file
that contains a PDEF record describes two different grids. The first grid
is described by the PDEF record itself and is the "native" grid
for the pre-projected data in the file. The second
grid described in the desctiptor file is a rectilinear lat/lon grid, which
is defined by the XDEF and YDEF records. The PDEF record describes the
size of the native grid, and then describes how to convert from i/j of the native grid to the lat/lon values of the rectilinear grid described by XDEF and YDEF. The information in the PDEF entry describes the
projection of the grid along with the projection constants or providing
the mapping to the native grid in a supplementary data file. The rectilinear grid is used
by GrADS internally and can be any size or resolution -- it is completely
independent of the pre-projected or native grid. GrADS uses the information
about the two grids to interpolate from the PDEF-described native grid
to the XDEF/ YDEF-described rectilinear grid. All displays and analyses
are done using the interpolated data on the rectilinear grid. The virtue
of this approach is that all built in GrADS analytic functions (e.g.,
<a
href="/grads/gadoc/gradfuncaave.html">aave</a>, <a href="/grads/gadoc/gradfunchcurl.html">hcurl</a>...)
will work even though the data were not originally on a lon/lat grid.
The downside is that you are looking at interpolated data. </p>
<p class="plaintext">It is possible to view the pre-projected data on its
native grid. To do this, you omit the PDEF entry from the descriptor file,
and use the XDEF and YDEF entries to describe the shape of the native
grid. In this case, your displays must be drawn in i/j space without a
map projection (<a href="/grads/gadoc/gradcomdsetmpdraw.html">set mpdraw</a>
off). </p>
<p class="plaintext">When you do a <a
href="/grads/gadoc/gradcomddisplay.html">display</a> of a pre-projected vector
field, you must know whether the original vector components are defined
relative to the data grid or relative to the Earth. If the data are grid-relative,
they must be rotated to Earth-relative coordinates in order for the interpolation
to work properly. See the "Notes" under each particular projection
type for further information.</p>
<p class="item12bold"><a name="syntax"></a>PDEF Syntax</p>
<ul>
<table width="600" border="0" cellpadding="0" cellspacing="4" class="plaintext">
<tr bgcolor="#CCCCCC">
<td colspan="3"> <strong>PDEF <em>isize jsize</em> NPS<em> ipole jpole
lonref gridinc</em><br>
PDEF <em>isize jsize</em> SPS<em> ipole jpole lonref gridinc</em></strong></td>
</tr>
<tr bgcolor="ccdceb">
<td width="50" align="right">Example:</td>
<td colspan="2">PDEF 53 45 nps 27 49 -105 190.5</td>
</tr>
<tr bgcolor="b8c8d7">
<td align="right" valign="top">Args: </td>
<td colspan="2"> <table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr class="plaintext">
<td width="60" valign="top"><em>isize</em></td>
<td>The size of the native grid in the x direction </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>jsize</em></td>
<td>The size of the native grid in the y direction </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>ipole</em></td>
<td>the i coordinate of the pole referenced from the lower left
corner, assumed to be at (1,1)</td>
</tr>
<tr class="plaintext">
<td valign="top"><em>jpole</em></td>
<td>the j coordinate of the pole referenced from the lower left
corner, assumed to be at (1,1)</td>
</tr>
<tr class="plaintext">
<td valign="top"><em>lonref</em></td>
<td>reference longitude</td>
</tr>
<tr class="plaintext">
<td valign="top"><em>gridinc</em></td>
<td>distance between gripoints in km</td>
</tr>
</table></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top">Notes: </td>
<td colspan="2">Polar stereographic projections (N and S) are defined
as at NCEP. Wind rotation has also been added so that vector data
will be properly displayed. </td>
</tr>
<tr>
<td> </td>
<td colspan="2"> </td>
</tr>
<tr bgcolor="#CCCCCC">
<td colspan="3"><strong>PDEF <em>isize jsize</em> LCCR <em>latref
lonref iref jref Struelat Ntruelat slon dx dy<br>
</em>PDEF <em>isize jsize</em> LCC <em>latref lonref iref jref Struelat
Ntruelat slon dx dy</em><em> </em></strong></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right">Example:</td>
<td colspan="2">PDEF 103 69 lccr 30 -88 51.5 34.5 20 40 -88 90000
90000</td>
</tr>
<tr bgcolor="b8c8d7">
<td align="right" valign="top" bgcolor="b8c8d7">Args: </td>
<td colspan="2"> <table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr class="plaintext">
<td width="60" valign="top"><em>isize</em></td>
<td>The size of the native grid in the x direction </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>jsize</em></td>
<td>The size of the native grid in the y direction </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>latref</em></td>
<td>reference latitude</td>
</tr>
<tr class="plaintext">
<td valign="top"><em>lonref</em></td>
<td>reference longitude (in degrees, E is positive, W is negative) </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>iref</em></td>
<td>i of ref point </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>jref</em></td>
<td>j of ref point </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>Struelat</em></td>
<td>S true lat </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>Ntruelat</em></td>
<td>N true lat </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>slon</em></td>
<td>standard longitude</td>
</tr>
<tr class="plaintext">
<td valign="top"><em>dx</em></td>
<td>grid X increment in meters</td>
</tr>
<tr class="plaintext">
<td valign="top"><em>dy</em></td>
<td>grid Y increment in meters</td>
</tr>
</table></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top" bgcolor="ccdceb">Notes: </td>
<td colspan="2">Starting with version 1.9b4, the LCCR option supplements
the use of PDEF with data on the Lambert Conformal projection. With
LCCR, wind rotation has been implemented for data files with grid-relative
winds instead of Earth-relative winds. Use LCC if your vector components
are already Earth-relative. </td>
</tr>
<tr>
<td> </td>
<td width="9%"> </td>
<td width="82%"> </td>
</tr>
<tr bgcolor="#CCCCCC">
<td colspan="3"><strong>PDEF <em>isize jsize </em>ETA.U<em> lonref
latref dlon dlat</em></strong></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right">Example:</td>
<td colspan="2">PDEF 181 136 eta.u -97.0 41.0 0.38888888 0.37</td>
</tr>
<tr>
<td align="right" valign="top" bgcolor="b8c8d7">Args: </td>
<td colspan="2" bgcolor="b8c8d7"> <table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr class="plaintext">
<td width="60" valign="top"><em>isize</em></td>
<td>The size of the native grid in the x direction </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>jsize</em></td>
<td>The size of the native grid in the y direction </td>
</tr>
<tr class="plaintext">
<td valign="top">lonref</td>
<td>reference longitude (in degrees, E is positive, W is negative) </td>
</tr>
<tr class="plaintext">
<td valign="top">latref</td>
<td>reference latitude</td>
</tr>
<tr class="plaintext">
<td valign="top">dlon</td>
<td>grid longitude increment in degrees</td>
</tr>
<tr class="plaintext">
<td valign="top">dlat</td>
<td>grid latitude increment in degrees</td>
</tr>
</table></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top">Notes: </td>
<td colspan="2">The eta model native grid is awkward to work with
because the variables are on staggered <i>and</i> non-rectangular
grids. NCEP created "unstaggered" eta model fields, in which the
variables are placed on a common rectangular grid. Wind rotation
has also been added so that vector data will be properly displayed. </td>
</tr>
<tr>
<td> </td>
<td> </td>
<td> </td>
</tr>
<tr bgcolor="#CCCCCC">
<td colspan="3"><strong>PDEF <em>isize jsize</em> PSE <em>slat slon
ipole jpole dx dy sign</em></strong></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top">Example:</td>
<td colspan="2"> </td>
</tr>
<tr bgcolor="b8c8d7">
<td align="right" valign="top">Args: </td>
<td colspan="2"> <table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr class="plaintext">
<td width="60" valign="top"><em>isize</em></td>
<td>The size of the native grid in the x direction </td>
</tr>
<tr class="plaintext">
<td valign="top"><em>jsize</em></td>
<td>The size of the native grid in the y direction </td>
</tr>
<tr class="plaintext">
<td valign="top">slat</td>
<td>absolute value of the standard latitude </td>
</tr>
<tr class="plaintext">
<td valign="top">slon</td>
<td>absolute value of the standard longitude</td>
</tr>
<tr class="plaintext">
<td valign="top">ipole</td>
<td>the i coordinate of the pole referenced from the lower left
corner, assumed to be at (0,0)</td>
</tr>
<tr class="plaintext">
<td valign="top">jpole</td>
<td>the j coordinate of the pole referenced from the lower left
corner, assumed to be at (0,0)</td>
</tr>
<tr class="plaintext">
<td valign="top">dx</td>
<td>grid X increment in km</td>
</tr>
<tr class="plaintext">
<td valign="top">dy</td>
<td>grid Y increment in km</td>
</tr>
<tr class="plaintext">
<td valign="top">sign</td>
<td>1 for NH; -1 for SH</td>
</tr>
</table></td>
</tr>
<tr>
<td align="right" valign="top" bgcolor="ccdceb">Notes: </td>
<td colspan="2" bgcolor="ccdceb">The polar stereo projection used
by the original NMC models is not very precise because it assumes
the earth is round (eccentricity = 0). While this approximation
was reasonable for coarse resolution NWP models, it is inadequate
to work with higher resolution data such as SSM/I. <i>Wind rotation
has not been implemented!!! Use only for scalar fields.</i></td>
</tr>
<tr>
<td> </td>
<td> </td>
<td> </td>
</tr>
<tr bgcolor="#CCCCCC">
<td colspan="3"><strong>PDEF <em>isize jsize</em> OPS <em> latref
lonref xoff yoff iref jref dx dy </em></strong></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right">Example:</td>
<td colspan="2">PDEF 26 16 ops 40.0 -100.0 90000.0 90000.0 14.0 9.0
180000.0 180000.0 </td>
</tr>
<tr bgcolor="b8c8d7">
<td align="right" valign="top" bgcolor="b8c8d7">Args: </td>
<td colspan="2"> <table width="100%" border="0" cellpadding="0" cellspacing="0" class="plaintext">
<tr class="plaintext">
<td width="60"><em>isize</em></td>
<td>The size of the native grid in the x direction </td>
</tr>
<tr class="plaintext">
<td><em>jsize</em></td>
<td>The size of the native grid in the y direction </td>
</tr>
<tr class="plaintext">
<td>latref</td>
<td>reference latitude</td>
</tr>
<tr class="plaintext">
<td>lonref</td>
<td>reference longitude (in degrees, E is positive, W is negative) </td>
</tr>
<tr class="plaintext">
<td>xoff</td>
<td>lonref offset in meters</td>
</tr>
<tr class="plaintext">
<td>yoff</td>
<td>latref offset in meters</td>
</tr>
<tr class="plaintext">
<td>iref</td>
<td>the i coordinate of the reference point</td>
</tr>
<tr class="plaintext">
<td>jref</td>
<td>the j coordinate of the reference point</td>
</tr>
<tr class="plaintext">
<td>dx</td>
<td>grid X increment in km</td>
</tr>
<tr class="plaintext">
<td>dy</td>
<td>grid Y increment in km</td>
</tr>
<tr class="plaintext">
<td>dy</td>
<td>grid Y increment in km</td>
</tr>
</table></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top">Notes: </td>
<td colspan="2">The CSU RAMS model uses an oblique polar stereo projection.
<i>Wind rotation has not been implemented!!! Use only for scalar
fields.</i></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top" bgcolor="#FFFFFF"> </td>
<td colspan="2" bgcolor="#FFFFFF"> </td>
</tr>
<tr bgcolor="#CCCCCC">
<td colspan="3"><strong>PDEF <em>isize jsize</em> ROTLL<em> lonpol latpol dlon dlat lonll latll <br>
</em>PDEF <em>isize jsize</em> ROTLLR<em> lonpol latpol dlon dlat lonll latll </em></strong></td>
</tr>
<tr bgcolor="#CCCCCC"></tr>
<tr bgcolor="ccdceb">
<td align="right">Example:</td>
<td colspan="2">PDEF 500 330 rotllr -170.0 43.0 0.02 0.02 -5.5 -3.8</td>
</tr>
<tr bgcolor="b8c8d7">
<td align="right" valign="top" bgcolor="b8c8d7">Args: </td>
<td colspan="2"><table width="100%" border="0" cellpadding="0" cellspacing="0" class="plaintext">
<tr class="plaintext">
<td width="60"><em>isize</em></td>
<td>The size of the native grid in the x direction </td>
</tr>
<tr class="plaintext">
<td><em>jsize</em></td>
<td>The size of the native grid in the y direction </td>
</tr>
<tr class="plaintext">
<td>lonpol</td>
<td>Longitude of the rotated pole in degrees</td>
</tr>
<tr class="plaintext">
<td>latpol</td>
<td>Latitude of the rotated pole in degrees</td>
</tr>
<tr class="plaintext">
<td>dlon</td>
<td>grid spacing in longitudinal direction of the rotated grid in degrees</td>
</tr>
<tr class="plaintext">
<td>dlat</td>
<td>grid spacing in latitudinal direction of the rotated grid in degrees</td>
</tr>
<tr class="plaintext">
<td>lonll</td>
<td>longitude of the lower left corner, given in rotated space in degree</td>
</tr>
<tr class="plaintext">
<td>latll</td>
<td>latitude of the lower left corner, given in rotated space in degree</td>
</tr>
</table></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top">Notes: </td>
<td colspan="2" bgcolor="ccdceb"><p>(<font color="#990000">GrADS version 2.0</font>) The rotated lat/lon grid projection is described in the <a href="http://www.cosmo-model.org/content/model/documentation/core/cosmo_userguide_5.04.pdf" target="_blank">COSMO documentation</a>, Chapter 3.3. The lower left corner, i.e. the first element in the data array, has to be the southwest corner. It is not possible to use a mirrored grid by setting dlon or dlat to a negative value.</p> </td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top" bgcolor="#FFFFFF"> </td>
<td colspan="2" bgcolor="#FFFFFF"> </td>
</tr>
<tr bgcolor="ccdceb">
<td colspan="3" align="left" valign="top" bgcolor="#CCCCCC"> <p><strong>PDEF<em>
isize jsi</em>ze BILIN <em>format byteorder fname</em></strong></p></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top">Example:</td>
<td colspan="2">PDEF 100 100 BILIN sequential binary-big ^mygrid.interp.values </td>
</tr>
<tr bgcolor="b8c8d7">
<td align="right" valign="top" bgcolor="b8c8d7">Args:</td>
<td colspan="2"> <table width="100%" border="0" cellpadding="0" cellspacing="0" class="plaintext">
<tr class="plaintext">
<td width="60"><em>isize</em></td>
<td>The size of the native grid in the x direction </td>
</tr>
<tr class="plaintext">
<td><em>jsize</em></td>
<td>The size of the native grid in the y direction </td>
</tr>
<tr>
<td><em>format</em></td>
<td>Must be either STREAM (direct access) or SEQENTIAL (fortran
formatted)</td>
</tr>
<tr>
<td valign="top"><em>byteorder</em></td>
<td>If set to BINARY, byte odering is assumed to be same as
local machine<br>
If set to BINARY-BIG, byte ordering is assumed to be big-endian<br>
If set to BINARY-LITTLE, byte ordering is assumed to be little-endian</td>
</tr>
<tr>
<td valign="top"><em>fname</em></td>
<td>The name of the supplementary file </td>
</tr>
</table></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top">Notes:</td>
<td colspan="2">The supplementary file contains three lat-lon floating-point
grids: i values, j values, and wind rotation values. The native grid is assumed to have a corner (i,j) value of (1,1). The size of
these grids must match the XDEF and YDEF entries in the descriptor
file. </td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top" bgcolor="#FFFFFF"> </td>
<td colspan="2" bgcolor="#FFFFFF"> </td>
</tr>
<tr align="left" bgcolor="#CCCCCC">
<td colspan="3" valign="top"><strong>PDEF<em> isize jsize</em> GENERAL <em>num
format byteorder fname</em></strong></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top">Example:</td>
<td colspan="2">PDEF 182 149 general 4 sequential binary-big ^mygrid.interp.values<br>
PDEF 15238 1 general 1 stream binary ^gtd.filepdef</td>
</tr>
<tr bgcolor="b8c8d7">
<td align="right" valign="top">Args:</td>
<td colspan="2"><table width="100%" border="0" cellpadding="0" cellspacing="0" class="plaintext">
<tr>
<td><em>isize</em></td>
<td>The size of the native grid in the x direction </td>
</tr>
<tr>
<td><em>jsize</em></td>
<td>The size of the native grid in the y direction </td>
</tr>
<tr>
<td width="60"><em>num</em></td>
<td>number of sets of interpolation grids supplied</td>
</tr>
<tr>
<td><em>format</em></td>
<td>Must be either STREAM (direct access) or SEQENTIAL (fortran
formatted)</td>
</tr>
<tr>
<td valign="top"><em>byteorder</em></td>
<td>If set to BINARY, byte odering is assumed to be same as
local machine<br>
If set to BINARY-BIG, byte ordering is assumed to be big-endian<br>
If set to BINARY-LITTLE, byte ordering is assumed to be little-endian</td>
</tr>
<tr>
<td valign="top"><em>fname</em></td>
<td>The name of the supplementary file; it may be mixed case. </td>
</tr>
</table></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top" bgcolor="ccdceb">Notes:</td>
<td colspan="2" bgcolor="ccdceb"><p>(<span class="style1">GrADS version 2.0.a3 and later</span>) The syntax and behavior of PDEF GENERAL is exactly like PDEF FILE, except that the convention for the native grid offset values in the pdef file is the same for all data formats. The offsets should be 1-based; the first grid point is assumed to have (i,j) values of (1,1), and valid offset values are > 0 and <= <em>isize</em>*<em>jsize</em> .</p>
<p>Native grid offset values of -999 indicate not to use
an input point for that portion of the calculation (thus you can apply
less than the "<em>num</em>" number of interpolation points for some
of the points). </p>
<p>See additional notes in the paragraphs below.</p></td>
</tr>
<tr align="left" bgcolor="#CCCCCC">
<td colspan="3" valign="top" bgcolor="#FFFFFF"> </td>
</tr>
<tr align="left" bgcolor="#CCCCCC">
<td colspan="3" valign="top"><strong>PDEF<em> isize jsize</em> FILE <em>num
format byteorder fname</em></strong></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top">Example:</td>
<td colspan="2">PDEF 182 149 file 4 sequential binary-big ^mygrid.interp.values<br>
PDEF 15238 1 file 1 stream binary ^gtd.filepdef</td>
</tr>
<tr bgcolor="b8c8d7">
<td align="right" valign="top">Args:</td>
<td colspan="2"><table width="100%" border="0" cellpadding="0" cellspacing="0" class="plaintext">
<tr>
<td><em>isize</em></td>
<td>The size of the native grid in the x direction </td>
</tr>
<tr>
<td><em>jsize</em></td>
<td>The size of the native grid in the y direction </td>
</tr>
<tr>
<td width="60"><em>num</em></td>
<td>number of sets of interpolation grids supplied</td>
</tr>
<tr>
<td><em>format</em></td>
<td>Must be either STREAM (direct access) or SEQENTIAL (fortran
formatted)</td>
</tr>
<tr>
<td valign="top"><em>byteorder</em></td>
<td>If set to BINARY, byte odering is assumed to be same as
local machine<br>
If set to BINARY-BIG, byte ordering is assumed to be big-endian<br>
If set to BINARY-LITTLE, byte ordering is assumed to be little-endian</td>
</tr>
<tr>
<td valign="top"><em>fname</em></td>
<td>The name of the supplementary file; it may be mixed case. </td>
</tr>
</table></td>
</tr>
<tr bgcolor="ccdceb">
<td align="right" valign="top">Notes:</td>
<td colspan="2" bgcolor="ccdceb"><p>For GrADS v2.0.a2 and earlier, <em>jsize</em> was fixed to be 1, and <em>isize</em> was the size of the native grid expressed as a vector (i.e., all gridpoints in the x-y grid). This mode for describing the native grid will continue to work with v2.0.a3+, but only if the native grid is in GRIB or binary format. For NetCDF and HDF formats, the <em>isize</em> and <em>jsize</em> args must match the X and Y dimensions of the native grid. </p>
<p><span class="style1">WARNING: The use of PDEF FILE may be incorrect!</span> A long-standing bug and incomplete documentation has led to different conventions for the native grid offset values in the pdef file for GRIB and non-GRIB data formats. For GRIB (1&2), the offsets must be 0-based; the first grid point is assumed to have (i,j) values of (0,0). For all other data types, the offsets must be 1-based; the first grid point is assumed to have (i,j) values of (1,1). Thus:<br>
for GRIB format: valid offset values are >= 0 and < <em>isize</em>*<em>jsize</em><br>
for other formats: valid offset values are > 0 and <= <em>isize</em>*<em>jsize</em><br>
To maintain backward compatibility, the bug will remain in GrADS as a feature, but the use of PDEF FILE has been deprecated as of version 2.0.a3 and a warning message will be displayed when a data set is opened that uses PDEF FILE. Note that if you use <a href="http://www.cpc.ncep.noaa.gov/products/wesley/grib2ctl.html">grib2ctl</a> or <a href="http://www.cpc.ncep.noaa.gov/products/wesley/g2ctl.html">g2ctl </a>to generate your pdef file, the offsets are correct. </p>
<p>Native grid offset values of -999 indicate not to use
an input point for that portion of the calculation (thus you can apply
less than the "<em>num</em>" number of interpolation points for some
of the points). </p>
<p>See additional notes in the paragraphs below.</p></td>
</tr>
</table>
</ul>
<p class="item16"> <span class="item12bold"><strong><a name="interp"></a>How
PDEF Grid Interpolation Works</strong></span><br>
<span class="plaintext">To illustrate how the data is interpolated from
the native grid to the rectilinear grid, let's consider an example. Here
are a set of relevant records from a descriptor file: <br>
PDEF 100 100 nps ...<br>
XDEF 181 linear -180 1<br>
YDEF 90 linear 0 1<br>
These three entries describe data on a native 100x100 North Polar stereographic
projection and a rectilinear lat/lon grid that is 181 by 90 and has an
interval of 1 degree in both lat and lon. Consider one point within the
rectilinear grid, the point -90,40. GrADS calls an internal routine to
calculate the i and j values in the native grid that correspond to this
lat/lon point. Let's say we get i,j values of 31.24 and 67.88. To do the
interpolation to the lat/lon point -90,40, GrADS uses the data values
from the following four native grid points: 31,67 - 31,68 - 32,67 - 32,68.
Bi-linear interpolation is used within this grid box to get down to the
position 31.24,67.88. The interpolation is linear within the i,j grid.
When a descriptor file is opened that contains a PDEF record, GrADS calculates
the i/j values in the native grid that correspond to the lat/lon pair
for each gridpoint in the rectilinear grid. </span></p>
<p class="item16"> <span class="item12bold"><strong><a name="rotation"></a>How
PDEF Wind Rotation Works</strong></span><br>
<span class="plaintext">There is a third value calculated for every lat/lon
point, and that is the wind rotation value. With some "pre-projected"
or native grids, the winds are given relative to the i/j space and not
the lat/lon space. To insure correct interpolation, the winds must be
rotated to lat/lon space. This is done by determining a rotation amount
for each lat/lon point. When u or v wind components are displayed, the
values are not just interpolated but also rotated. </span></p>
<p class="plaintext">To do the wind rotation properly, GrADS requires both
the u and v components. Even if you are just displaying u, GrADS has to
retrieve (internally) both the u and v component in order to do the rotation
calculation. GrADS determines how to match u and v variables by checking
the<em> units</em> field of the variable record in the descriptor file.
The u variable must have a <em>units</em> value of 33, and the v variable
must have a <em>units</em> value of 34. (This is the GRIB convention).
If there are more than one u/v pairs, secondary <em>units</em> values
are used. For example:
<ul>
<table width="600" border="0" cellpadding="0" cellspacing="0" class="plaintext">
<tr>
<td width="30">u</td>
<td width="15" align="right">18</td>
<td width="50" align="center"> <p>33,100</p></td>
<td > U-Wind Components on Pressure Levels</td>
</tr>
<tr>
<td>v</td>
<td align="right">18</td>
<td align="center">34,100</td>
<td> V-Wind Components on Pressure Levels</td>
</tr>
<tr>
<td>u10</td>
<td align="right">0</td>
<td align="center"> 33,105</td>
<td>10 Meter U Wind</td>
</tr>
<tr>
<td>v10</td>
<td align="right">0</td>
<td align="center">34,105</td>
<td>10 Meter V Wind</td>
</tr>
</table>
</ul>
<p class="plaintext">might be some variable records in the descriptor file.
If wind rotation is called for, u and v would be paired, and u10 and v10
would be paired (since the secondary values would be checked, ie, the
105,100 values). </p>
<p class="item16"><span class="item12bold"><strong><a name="bilin"></a>The
PDEF BILIN Option</strong></span><br>
<span class="plaintext">When a descriptor file is opened that contains
a PDEF record, we have explained that GrADS internally generates three
grids, each one the size of the rectilinear lat/lon grid. The first two
grids contain the i and j values (respectively) from the native grid that
correspond to each grid point in the rectilinear grid; the third grid
contains wind rotation values. But this only works for a small set of
well-defined native grids. GrADS will generate these three internal grids
automatically for polar stereographic, lamber conformal, and some eta
grids. If the native grid for your data is not one of the predefined projections,
it is still possible for GrADS to handle the data. All you have to do
is supply these three grids to GrADS with a supplementary data file and
use the bilin option in your PDEF record. </span></p>
<p class="plaintext">The supplementary file will contains three lat-lon
floating-point grids sized according to the XDEF and YDEF records in the
descriptor file.The three grids contain: i values, j values, and wind
rotation values. A value of -999 in the i-value grid indicates not to
interpolate to that lat-lon point (will end up missing on output) and
a value of -999 in the wind-rotation grid indicates not to do wind rotation
for that point. If the wind-rotation grid is all -999 values, no rotation
is ever done and a flag is set not to even attempt rotation.</p>
<p class="item16"><span class="item12bold"><strong><a name="file"></a>The
PDEF GENERAL Option (and the PDEF FILE option)</strong></span><br>
<span class="plaintext">All of the PDEF examples discussed so far involve
the same method for grid interpolation: a grid point value in the rectilinear
grid is calculated by finding the four neighboring grid points in the
native grid and averaging them, with weights applied bi-linearly according
to their proximity to the to rectilinear grid point. The PDEF GENERAL option
and the PDEF FILE option generalize this method so that an arbitrary number of native grid point
values and their weights are averaged to generate the interpolated rectilinear
grid point values. The index values for the native grid values that are
to be used and their weights are specified by the user in a supplementary
data file (<em>fname</em>). The FILE and GENERAL options are identical except for one detail: they have different conventions for the native grid offset values in the supplementary file (see the "Notes" in the syntax tables above for specifics). <br>
</span></p>
<p class="plaintext">The <em>num</em> argument in the PDEF FILE entry specifies
the number of native grid points that will be used to calcuate each interpolated
rectilinear grid point value. For each <em>num, </em>the supplementary
data file will contain two grids -- both will be the size of the rectilinear
grid (as defined by XDEF and YDEF). The first grid contains the index
or offset values that point to the native grid value that will be used
in the interpolation; the second grid contains the weights for those native
grid values. The first grid contains integer values, the second grid contains
floating-point values. Finally, the supplementary data file must also
contain one grid of floating-point wind rotation values. Thus if <em>num</em>
equals 1, there will be 3 grids in <em>fname</em>. If <em>num</em> equals
3, there will be 7 grids in <em>fname</em> (3 sets of 2 grids plus the
wind rotation grid). </p>
<p class="plaintext">To do the grid interpolation, GrADS will first read
the data in the native grid (vector) along with the values in the supplementary
grids. To calculate the interpolated value for a particular lat-lon point,
GrADS will get <em>num</em> native grid point values, multiply each by
their weight, sum over all the weighted native grid points, and divide
by the sum of the weights. </p>
<p class="plaintextbold">An Example: </p>
<p class="plaintext">The original data are set up as a vector of land points
only, taken from a 1-degree lat/lon grid. There are 15238 land points
in the native grid (vector). We use the PDEF FILE option to repopulate
a rectilinear lat/lon grid with the land values, leaving the ocean grid
points as missing. In this case, ther eis no interpolation done. The PDEF
option is used simply to convert a vector of land points into a 2D grid
with ocean points missing. The descriptor file looks like this:
<ul>
<span class="plaintext"> DSET ^gswp_vegetation_parameters.nc<br>
DTYPE netcdf <br>
TITLE Monthly Vegetation parameters at 1x1 degree<br>
UNDEF 1.e+20<br>
PDEF 15238 1 file 1 stream binary ^gswp.filepdef<br>
XDEF 360 linear -179.5 1<br>
YDEF 150 linear -59.5 1<br>
ZDEF 1 linear 1 1<br>
TDEF 204 linear 00Z01jan1982 1mo<br>
VARS 1<br>
NDVI=>ndvi 0 t,x Monthly vegetation index<br>
ENDVARS</span>
</ul>
<p class="plaintext">The supplementary file gtd.filepdef contains three
grids -- the first contains the index values that associate each location
in the lat/lon grid with it's point in the vector. All of the ocean points
will have a missing value of -999. The second grid will contain the weights,
which will be 1 for land points, 0 for ocean points. The third grid will
contain all missing values since wind rotation is not a issue in this
example. Here is a descriptor file for the supplementary data file (a
useful strategy for making sure you've got everything written out correctly):</p>
<ul>
<p class="plaintext">DSET ^gswp.filepdef<br>
TITLE PDEF file for GSWP Vegetation Parameters<br>
UNDEF -999<br>
XDEF 360 linear -179.5 1<br>
YDEF 150 linear -59.5 1<br>
ZDEF 1 linear 1 1 <br>
TDEF 1 linear 00z01jul1982 3hr<br>
VARS 3<br>
i 0 -1,40,4 Index Values<br>
w 0 99 Weights<br>
r 0 99 Wind Rotation Values<br>
ENDVARS</p>
<p></p>
<p> </p>
</ul>
</td>
</tr>
</table>
<p class="plaintext"> </p>
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