Setting a missing value
How can I set a missing value over land ?
Consider the following problem: You have a global data set with values over ocean and over land but currently you're only interested in quantities over the ocean. Replacing all land values with a correct missing value is necessary if you want to use Ferret transformations such as @ave on only ocean data.
! Use a topography dataset (for example etopo120) to define ! a ocean mask. This mask must be first interpolated onto the ! correct grid i.e. ! topography data set set data etopo120 ! file containing my_var set data my_file ! set an explicit region for example define region/x=100W:30E/y=10S:90N NA set region/@NA ! define an ocean mask LET ocean = if rose[d=etopo120,gxy=my_var] le 0 then 1 ! Note: in this IF construct the "ELSE" clause is omitted. This tells Ferret ! that those values which would satisfy the ELSE condition are undefined ! (i.e. are missing in ocean). ! define a new variable that has a missing values over land define variable/title="var_ocean" var_ocean = ocean * my_var ! make a two dimensional plot of the values over the ocean only shade var_ocean ! since there is now a correct missing value over land you can make a plot ! showing the averaged over the ocean only. i.e. plot var_ocean[x=@ave]
Contributed by Christof Appenzeller of Universitat Bern
Last modified: Nov 6,1996