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[GEMPAK #QRQ-364226]: GDPOINT

Hi Evan-

> Thanks for your help Don. That answers my question.  

glad I could help.

>It's too bad Steve is no longer with Unidata.  He was an excellent source of 
>information whenever Gempak questions came up.

Yes, he will be missed.   But we are well on our way to replacing
that position and will try to continue the good support.  For me,
it means dredging up stuff buried deep in my brain. ;-)

Don

> -----Original Message-----
> From: Unidata GEMPAK Support [mailto:address@hidden]
> Sent: Tuesday, February 05, 2008 11:56 AM
> To: address@hidden
> Cc: address@hidden
> Subject: [GEMPAK #QRQ-364226]: GDPOINT
> 
> 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: address@hidden
> > 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
> 
> 
> 
> 


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