Re: your mail : extracting data at given points

[Jaison Kurian <jaison@xxxxxxxxxxxxxxxxxx>]

Hi Ansley,
            Yes, you are right, SAMPLEXY works fine for irregular axis
if it is not flagged as MODULO. Actually it was SAMPLEIJ which fails for
irregular axis/axes even when the axis/axes are not flagged as MODULO.
I came to this when tried SAMPLEIJ as an alternative for SAMPLEXY : while  
we have to sample "only on" grid points and one axis is irregular and 
MODULO (the querry from Jagadish few days back). The solution is to define
a regular "index" axis so that the I or/and J indices remains the same as 
in the original grid, define an intermediate variable defined over this 
regular axis, then use this intermediate variable for sampling (since i 
or/and j positions remains same, we will get the data sampled at the 
desired points itself).

 Here is an example....

 With Regards 

 Jaison 

!-----sampleij_irreg.jnl-----------------
\ cancel mode verify

! create some data defined on an "irregular" axis (say ..X)

   let xdata = XSEQUENCE({0,5,10,15,20,25,30,40,50,60,70,80,90})
   define axis/x/from_data xirreg=xdata 
   define axis/y=1:13:1    yreg
   let var = x[gx=xirreg]/10 + y[gy=yreg]/10 

   LET ipts = XSEQUENCE({2,9,13})   
   LET jpts = XSEQUENCE({3,4,12})   
  
   LET var_ij = SAMPLEIJ(var,ipts, jpts)

   list var_ij

   list/nohead var[i=2,j=3], var[i=9,j=4], var[i=13,j=12]

! failed !!!! what about regular axis ?? just do @ASN to a regular
!    abstract axis so that the "i" index remains the same (1 to 13).

    define axis/x=1:13:1 xreg
    let var_reg = var[gx=xreg@ASN]
    
    LET var_reg_ij =  SAMPLEIJ(var_reg,ipts, jpts)

    list var_reg_ij

!----end of sampleij_irreg.jnl------------------

 

On Tue, 23 Nov 2004, Ansley Manke wrote:

> Hi Sudheer and Jaison,
> The SAMPLEXY function works fine if the data is on irregular axes, but it
> does have the restriction that if the axis is MODULO then it must be 
> REGULAR.
> (I imagine this restriction could be lifted with some clever 
> programming, but we
> have not yet made the function that smart.)
> 
> Ansley
> 
> Jaison Kurian wrote:
> 
> >Hi Sudheer,
> >            You can use SAMPLEXY function to sample a variable 
> >at desired lat-lon locations. From the usermanual 
> >(Ch3 Sec2.3.34.  SAMPLEXY )
> >
> >   SAMPLEXY(DAT_TO_SAMPLE,XPTS,YPTS) Returns data sampled at a 
> >   set of (X,Y) points, using linear interpolation. 
> >  
> >   SAMPLEXY is a "grid-changing" function; its output grid is 
> >   different from the input arguments.  Therefore it is best 
> >   to use explicit limits on the first argument rather than a 
> >   SET REGION command.
> >
> > I think SAMPLEXY works well only for regular (equaly spaced)
> >X & Y axis (especially if the LONGITUDE axis is flagged as 
> >MODULO). Anyway if you are planning to use Levitus Climatology
> >you don't have to worry about this. 
> >
> > Please have a look at the example given below.
> >
> > Hope this helps
> >
> > With Regards 
> >
> > Jaison
> >
> >...........samplexy_lev.dat...........
> >LAT   LON
> >===   ===
> > 9.0  68.4
> >10.2  72.6
> > 8.5  70.2
> > 7.2  65.5
> >...............................
> >!---------samplexy_lev.jnl-----------
> >
> >! open climatology dataset
> >
> >      set data levitus_climatology
> >
> >! open ascii file containing lat-lon points
> >!    use "/SKIP" to skip header lines if any
> >
> >      file/skip=2/var="lat,lon"  samplexy_lev.dat
> >
> >! do the sampling using SAMPLEXY function
> >
> >      let temp_lev = SAMPLEXY(temp[d=1], lon[d=2], lat[d=2])
> >
> >      list/k=1:5 temp_lev
> >
> >! assign proper title and units if you are saving this to a file
> >
> >      set var/title="Temperature Extracted from Levitus Climatology"/\
> >               units="^oC"  temp_lev
> >      set var/title="Latitude  of data points"/units=latitudes  lat
> >      set var/title="Longitude of data points"/units=longitudes lon
> > 
> >      sp rm -f samplexy_lev.nc
> >      save/file=samplexy_lev.nc/append temp_lev
> >      save/file=samplexy_lev.nc/append lon,lat
> >
> >!-----------------------------------------------
> >
> >
> >
> >On Tue, 23 Nov 2004, Sudheer Joseph wrote:
> >
> >  
> >
> >>Dear Users ,
> >>                 Is there any way to extract data from a netcdf file at 
> >>specicifed( irregular) lat lon positions read from a file,
> >>essentially I needed the following
> >>file/var= lat,lon      ll.dat
> >>get temperature data from levitus data.cdf @  lat log
> >>
> >>is there a method to get this data form the specified lat  lon
> >>thanks,
> >>sudheer
> >>
> >>***************************************
> >>Dr. Sudheer Joseph
> >>Indian National Centre for Ocean Information Services
> >>Gajularamaram B.O., IDA Jeedeemetla S.O.
> >>Via Pragathi Nagar, Kukatpally, Hyderabad
> >>Pin:5000 55
> >>Tel:+91-40-23044600(R)
> >>Tel:+91-40-9440832534(Mobile)
> >>Tel:+91-40-25503595(O)
> >>Fax:+91-40-25503596(O)
> >>E-mail:sjo@rediffmail.com;
> >>callsjo@yahoo.com;
> >>sjo@incois.gov.in.
> >>Web- http://oppamthadathil.tripod.com
> >>***************************************
> >>
> >>_________________________________________________________________
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> >>
> >>    
> >>
> >
> >  
> >
> 

-- 
___________________________________________________

    Jaison Kurian                           
    Centre for Atmospheric and Oceanic Sciences
    Indian Institute of Science
    B A N G A L O R E   560 012
    Ph: +91-80-3942505
        +91-80-3600450
    Fax:+91-80-3600865
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