# [ncl-talk] grid_area_avg

Mary Haley haley at ucar.edu
Thu Oct 12 12:41:24 MDT 2017

```Kunal,

I created an example for you, showing three ways you can subset a WRF grid
(since I already had a similar example).  It shows how to use the
getind_latlon2d function.

You have to be careful with subsetting curvilinear data, because specifying
a lat/lon box to take an average over can mean two different things. You
will be able to see the difference when you look at the graphic.

See example wrf_gsn_10.ncl at:

http://www.ncl.ucar.edu/Applications/wrfgsn.shtml#ex10

--Mary

On Wed, Oct 11, 2017 at 12:03 PM, Kunal Bali <kunal.bali9 at gmail.com> wrote:

> Dear NCL users,
>
> Single point coordinates values of 23.0 N and 77.0 E is extracted or print
> by using the script below
>
> ----------------------------------------------------------------------
> ; Main code
> ;----------------------------------------------------------------------
>   dir    = "./"
>   fnames = systemfunc("ls " + dir + "maiactaot*.nc")
> print(fnames)
>   data   = short2flt(a->Optical_Depth_055_grid1km)
>   nFill  = num(ismissing(data))
>   lat2d  = a->GridLat_grid1km
>   lon2d  = a->GridLon_grid1km
>
> ;---grid points of interest
>   latv = (/23.0/)
>   lonv = (/77.0 /)
>
>   nm = getind_latlon2d (lat2d,lon2d, latv, lonv)
>
>   do k=0,dimsizes(latv)-1
>      n = nm(k,0)
>      m = nm(k,1)
>      print(lat2d(n,m)+"   "+lon2d(n,m))   ; grid point location
>      print("     "+data(:,n,m))                   ; nearest grid point at
> all time steps
>      print("-----")
>   end do
>
>
> NOW, I want to extract the square grid box of 100 km around the
> coordinates (23N,77E) such as
>    latv = (/22, 24.0, 24.0, 22.0, 22.0/)
>    lonv = (/76, 76.0, 78.0, 78.0, 76.0 /)
>
>
> after that I want to do the area mean of the grid box. So that I can get
> one single point value of that grid box.
> So when I replacing
>   latv = (/23.0/)
>   lonv = (/77.0/)
>
> with
>
>    latv = (/22, 24.0, 24.0, 22.0, 22.0/)
>    lonv = (/76, 76.0, 78.0, 78.0, 76.0 /)
>
> then It's not giving mean of the square grid box.
>
> It shows the something like that (given below), which is incorrect. So
> please let me know how can I get the square grid box mean and then extract
> as asciiwrite or print?
>
> (0)    23.76603307545667   75.53995171540016
> (0)         0.179
> (1)         0.142
> (2)         -28672
> (3)         0.169
> (4)         -28672
> (5)         -28672
> (6)         -28672
> (0)    -----
> (0)    24.00322458681621   75.99551564353467
> (0)         0.171
> (1)         0.16
> (2)         -28672
> (3)         0.17
> (4)         -28672
> (5)         -28672
> (6)         -28672
>
> etc..etc..
>
>
>
> regards
> Kunal Bali
>
>
>
>
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