[ferret_users] question about replace a value on a grid

[Liya Jin <lj@xxxxxxxxxxxxxxx>]

Dear Ferreters,

There is a data file (sst.nc) like this:

(ncdump -h sst.nc)
dimensions:                                                                                                         

       x = 182 
;                                                                                                   

       y = 149 
;                                                                                                   

       deptht = 31 
;                                                                                               

       time = UNLIMITED ; // (73 
currently)                                                                        

variables:                                                                                                          

       float nav_lon(y, x) 
;                                                                                       

               nav_lon:long_name = "Longitude" 
;                                                                  
               nav_lon:units = "degrees_east" 
;                                                                   
               nav_lon:standard_name = "longitude" 
;                                                              
       float nav_lat(y, x) 
;                                                                                       

               nav_lat:long_name = "Latitude" 
;                                                                   
               nav_lat:units = "degrees_north" 
;                                                                  
               nav_lat:standard_name = "latitude" 
;                                                               
       float deptht(deptht) 
;                                                                                      

               deptht:long_name = "Vertical T levels" 
;                                                           
               deptht:units = "m" 
;                                                                                

       double time(time) 
;                                                                                         

               time:units = "days since 999-01-03 12:00" 
;                                                        
       float sosstsst(time, y, x) 
;                                                                                

               sosstsst:long_name = "Sea Surface temperature" 
;                                                   
               sosstsst:units = "C" 
;                                                                              

               sosstsst:coordinates = "nav_lon nav_lat" 
;                                                         
               sosstsst:valid_min = 1.00000002004088e+20 
;                                                        
               sosstsst:valid_max = -1.00000002004088e+20 
;                                                       
               sosstsst:short_name = "sosstsst" 
;                                                                 
               sosstsst:online_operation = "ave(x)" 
;                                                             
               sosstsst:interval_operation = 5760. 
;                                                              
               sosstsst:interval_write = 432000. ;

I would like to replace the values of some grids of the variable 
(sosstsst) with new values, e.g., replace sosstsst(time, *88*,160) with 
sosstsst(time, *89*,160), or with the interpolated values of surrounding 
grids (e.g., (sosstsst(time, *89*,160)+sosstsst(time, *90*,160))/2). 
Then the sosstsst(time, *88*,160) will have a new value in sst.nc.

How can I do this with Ferret? Thanks for your help!

Regards,
Liya