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20040818: 20040817: No Interpolation with GDPLOT2



Charles,

The GEMPAK grid display programs expect a regulary spaced grid
of points, so you will have to do something to provide data
at every point in the grid. That is the only way the GEMLIB routines 
will be able to compute any quantity that involves a finite
difference such as gradients and advections. There are WSETA postprocessing
programs that produce a standard grib instead of staggared, but again
this would involve interpolation to some grid.

It sounds like you need the nagrib routine (or any interpolation routine)
to not do a forward difference when a point is not surrounded by
data. Eg, only interpolate when data exists on all sides of
the point to be interpolated. 

The other function you can use in GEMPAK plotting programs to help is the 
MASK function. With MASK, you can use the LAND/SEA field you have to
mask the temperature field, so that you can assign missing value over water
instead of having the interpolated value. This might look like:
   MASK ( TMPK, SGT(LAND,0))

Steve Chiswell
Unidata User Support




>From: "Charles J. Alonge" <address@hidden>
>Organization: SAIC - NASA Goddard Space Flight Center
>Keywords: 200408181357.i7IDvcaW028995

>Hello Support,
>
>   Thanks that worked!!  Now for another issue.  Is there anyway to run 
>nagrib without the interpolation.  I notice with some of the temperature 
>fields (output from the NOAH land surface model on the ETA-12km Arakawa 
>E grid where values over water = -9999.00) are interpolated on the coast 
>line.  Is there anyway I can circumnavigate this problem?  Would I need 
>to change the nagrib source code to not interpolate or is there perhaps 
>a better way to plot this data?
>
>Thanks,
>Charles
>
>Unidata Support wrote:
>
>>Charles,
>>
>>I'm assuming you have converted the grid to GEMPAK using NAGRIB.
>>The NAGRIB decoder that converts from the staggared grid to
>>the regular grid will interpolate the alternating grid points.
>>Those values would be interpolated before any plotting routine
>>(which you would see with GDLIST. The intepolation in NAGRIB is 
>>usingcenetered differences (except on the edges).
>>
>>If this is the case, you can use the SGT/SLT (or BOOL/MASK) functions
>>to establish a value above which you treat the value as "1"
>>(by dividing by the value (eg QUO ( SGT ( LAND, .5 ), LAND ).
>>
>>Steve Chiswell
>>Unidata User Support
>>
>>  
>>
>>>From: "Charles J. Alonge" <address@hidden>
>>>Organization: SAIC - NASA Goddard Space Flight Center
>>>Keywords: 200408172042.i7HKgGaW009114
>>>    
>>>
>>
>>  
>>
>>>Hello Support,
>>>
>>>   I am trying to plot vegetation type, soil type, and land/sea
>>><>masks gridded data sets on the native Arakawa E grid (ETA).  When I 
>>>use gdplot2 I continuously get an interpolated data set.  That would be 
>>>fine for most of the fields, except for the above because they consist 
>>>of integer class values and should not be interpolated.  I want to fill 
>>>the entire grid box with its value,
>>>instead of doing any interpolation from grid box to grid box.  I thought
>>><>that setting type="Z" would achieve this, but interpolation between 
>>>grid values still exists when mapped by gdplot2.  Any assistance will be 
>>>greatly appreciated.
>>>
>>>Charles
>>>
>>>-- 
>>>
>>>Charles J. Alonge
>>>SAIC - NASA Goddard Space Flight Center
>>>Hydrological Sciences Branch
>>>NASA-GSFC Mail Code 974.1
>>>Greenbelt, MD 20771
>>>
>>>Office: Building 22, Room 008
>>>Phone:  (301) 286-8272
>>>Fax:    (301) 286-8624
>>>Email:  address@hidden
>>>
>>>    
>>>
>>--
>>**************************************************************************** 
>>Unidata User Support                                    UCAR Unidata Program 
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>>address@hidden                                   Boulder, CO 80307 
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>>  
>>
>
>-- 
>
>Charles J. Alonge
>SAIC - NASA Goddard Space Flight Center
>Hydrological Sciences Branch
>NASA-GSFC Mail Code 974.1
>Greenbelt, MD 20771
>
>Office: Building 22, Room 008
>Phone:  (301) 286-8272
>Fax:    (301) 286-8624
>Email:  address@hidden
>
--
NOTE: All email exchanges with Unidata User Support are recorded in the
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