Re: [ferret_users] Problem with masking

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

 Hi -
The IF statements used to define masks are logically True/False statements, but the mask variable is a numeric variable consisting of 0's and 1's, with 0 equivalent to false and 1 equivalent to true.

See the documentation for IF,
http://porter.pmel.noaa.gov/Ferret/home/documentation/users-guide/commands-reference/IF, and continue to scroll down for section Ref Sec 14.2, "IF-THEN-ELSE logic for masking"

You could use this expression for the mask, and I think it will give the result you want:

   yes? let mask = if (sossheig or sossheig eq 0) then sossheig

-Ansley

On 9/7/2010 2:59 AM, Bruno Levier wrote:
> Hi Ferreters
>
> I'm trying to mask a 2D field from a NetCDF file. The first example works fine :
>
>
>      NOAA/PMEL TMAP
>      FERRET v6.2
>      Linux(g77) 2.6.9-22. - 05/19/09
>       7-Sep-10 11:05
>
> yes? use PSY2V4_2D.nc
> yes? let mask = if sossheig LT 0.01 then sossheig
> yes? list /j=120 /i=24:26 sossheig, mask
>              DATA SET: ./PSY2V4_2D.nc
>              X: 23.5 to 26.5
>              Y: 120
>              TIME: 01-MAY-2009 00:00
>  Column  1: SOSSHEIG is Sea Surface Height (m)
>  Column  2: MASK is IF SOSSHEIG LT 0.01 THEN SOSSHEIG
>           SOSSHEIG    MASK
> 24   / 24: -0.01500 -0.01500
> 25   / 25:  0.00000  0.00000
> 26   / 26:  0.01400     ....
>
>
> But not the second one:
>
>
> yes? let mask = if sossheig then sossheig
> yes? list /j=120 /i=24:26 sossheig, mask
>              DATA SET: ./PSY2V4_2D.nc
>              X: 23.5 to 26.5
>              Y: 120
>              TIME: 01-MAY-2009 00:00
>  Column  1: SOSSHEIG is Sea Surface Height (m)
>  Column  2: MASK is IF SOSSHEIG THEN SOSSHEIG
>           SOSSHEIG    MASK
> 24   / 24: -0.01500 -0.01500
> 25   / 25:  0.00000     ....
> 26   / 26:  0.01400  0.01400
>
>
> Why is the "0" value masked in this case ?
>
> Here's a ncdump of the file :
>
>     short sossheig(time_counter, y, x) ;
>         sossheig:units = "m" ;
>         sossheig:missing_value = 32767.f ;
>         sossheig:valid_min = -9999s ;
>         sossheig:valid_max = 9999s ;
>         sossheig:add_offset = 0.f ;
>         sossheig:scale_factor = 0.001f ;
>         sossheig:long_name = "Sea Surface Height" ;
>         sossheig:standard_name = "sea_surface_height_above_geoid" ;
>         sossheig:short_name = "sossheig" ;
>         sossheig:online_operation = "N/A" ;
>         sossheig:axis = "TYX" ;
>         sossheig:interval_operation = 86400 ;
>         sossheig:interval_write = 86400 ;
>         sossheig:associate = "time_counter nav_lat nav_lon" ;
>
>
> Thanks in advance,
>
> Bruno Levier