Hi ferret_users, I try to get anomalies: yes? let h_time=h[t=16-jan-1971:16-dec-1980] yes? let h_clim=h_time[gt=month_reg@mod] yes? let h_a=h-h_clim[gt=cl@asn] It works fine with original data. But it seems not work with those data averaged or integrated over space: yes? list h_a **ERROR: unknown defining grid: GT=Cl@ASN Does anybody know why? Thanks in advance. Wendy Wang