[GEMPAK #QRQ-364226]: GDPOINT

["Unidata GEMPAK Support" <support-gempak@xxxxxxxxxxxxxxxx>]

Hi Evan-

Sorry for the delay in responding.  Steve is no longer at Unidata
and others are filling in until we have a replacement.

> Full Name: Evan Lowery
> Email Address: emlowery@xxxxxxxxxxxx
> Organization: Plymouth State University
> Package Version: Gempak 5.11.1
> Operating System:
> Hardware:
> Description of problem: Steve,
> 
> Currently, I'm creating a simple GFS MOS which needs forecast equations for 3900 locations.  Each location has a lat/lon.  I am able to get all interpolated weather variables for each location.  My question is:
> 
> How does GDPOINT interpolate values to lat/lon locations?  Does it take the 4 closest grid points and do a basic linear interpolation?
> 
> I tested Laguardia temp@2%hght and got the 4 closest grid point values as well.  When doing a linear interpolation in Excel, my values were not the same as GDPOINT.
> 
> I haven't found any specific documentation of this in help or through the support pages.

Looking through the code, GDPOINT creates a set of 9 points (3x3) grid
around the location (+/- .01 lat/lons):

C
C*      Set a new output reference navigation using gpoint as the center
C*      of a 9 point grid. This allows vector components to be interpolated
C*      to the desired grid point first within DG_GRID before the quantity is
C*      calculated, rather than interpolated from surrounding points after
C*      the computation is done. This will improve calculations involving
C*      vector components.
C*
C*      Note: This isn't good for functions such as HIGH(), LOWS(), GWFS(),
C*      SM5S(), SM9S(), which use an area of points
C               

The grid values are then interpolated to the 9 grid points using a bilinear
interpolation.  Then it just takes the center value.

Hope that helps.

Don Murray

Ticket Details
===================
Ticket ID: QRQ-364226
Department: Support GEMPAK
Priority: Normal
Status: Open