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Re: [ferret_users] Store an array of georeferenced values for climate model : Masking

Hi Eowyn,
You can set up a variable in Ferret and use it to define a set of masks that can be combined to form the mask you want. Use the same grid as the climate model. It's a little tedious, but not too hard, really.

You could use the grid from your climate model, or just define axes having the same grid. For example,

yes? define axis/x=1:360:1/units=degrees/modulo xlon
yes? define axis/y=-90:90:1/units=degrees ylat

Then define a set of masks. I'll define them to have values of 1 or 0, and then at the end these can be used to define your regions with the values N=300 or N=100. Remember that adding two masks gets us an OR operation (mask1 or mask2) , and multiplying gets an AND (mask1 and mask2). Try this and you'll see how it goes.
define axis/x=1:360:1/units=degrees/modulo xlon
define axis/y=-90:90:1/units=degrees ylat
let testvar = 1 + 0*x[gx=xlon] + 0*y[gy=ylat]   ! apply the mask to this

(Or start with a file:
  use my_data.nc
let testvar = 1+0*var_from_file using x[gx=testvar] etc below)

let Alon1 = if x[gx=xlon] gt 20 then 1 else 0
let Alon2 = if x[gx=xlon] lt 120 then 1  else 0
let Alat1 = if y[gy=ylat] gt -60 then 1 else 0
let Alat2 = if y[gy=ylat] lt -20 then 1 else 0
let Alonmask = Alon1 * Alon2
let Alatmask = Alat1 * Alat2
let Alonlatmask = Alonmask* Alatmask  ! creates a 2-D mask

shade Alonlatmask * testvar; pause  ! to check what you've done

let Blon1 = if x[gx=xlon] gt 120 then 1 else 0
let Blon2 = if x[gx=xlon] lt 145 then 1  else 0
let Blat1 = if y[gy=ylat] gt -10 then 1 else 0
let Blat2 = if y[gy=ylat] lt 20 then 1 else 0
let Blonmask = Blon1 * Blon2
let Blatmask = Blat1 * Blat2
let Blonlatmask = Blonmask* Blatmask

shade Blonlatmask * testvar; pause

shade (Alonlatmask+Blonlatmask) * testvar; pause  ! both rectangles

let Clon1 = ...

! And so forth. When all done, add them all up, and where the
! sum > 0 the mask is your value of 300, else 100. This works
! even if some of the rectangles overlap each other.

LET all_mask = if (Alonlatmask+Blonlatmask+...) GT 0 then 300 else 100

! Multiply by test_var to define the final mask over the whole grid:
shade all_mask * testvar; pause

LET n = all_mask * testvar
save/file=mask_file.nc n

Eowyn.Connolly-Brown@xxxxxxxx wrote:
I have searched the mail archives, but have been unable to find a solution to my problem. I want to create a netCDF file that contains an array of values that are referenced by latitude and longitude. Specifically, I have 4 regions in which I want to set N=300 and set N=100 otherwise. This array will then be supplied to a climate model. The mask=if ... else command would be perfect, except it requires me to already have a data set from which I take a variable to analyze. The set region/x=/y= command would be perfect, except you can't "add" nonadjacent regions the way you can add masks. Has anyone done this before?
Thank you very much!!

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