Hi,
If I am understanding correctly, you should do this manually by listing each coordinates' rain and elevation value to a text file and then reading it later. Following are few steps:
(1) let m_rain=if mean_rainfall ge 0 then mean_rainfall else (-1)
! To make bad values or missing values as -1. Later use -1 as bad
(2) list/clobber/file=Elevation_rain.dat/nohead/form=(f9.3,x,f9.3,x,f9.3,x,f9.3,) topo,m_rain,x[gx=topo],y[gy=topo]! ^ Last x and y are just to keep track of lat and long in case required in future.
(3) Come out of ferret session and restart new session and read the file by : file/var=ele,m_r,lon,lat Elevation_rain.dat ; sh da ;
By this you can do rain vs elevation analysis.