gird problem

["Min, Dughong" <min@xxxxxxxxxxxxxx>]

Hello ferret users;

In order to the potentail vorticity, I am trying to get each depth of two
isopycnal surfaces in x-y domain, and then the layer thickness between two
like
 ......
let high_layer_dep = ann_rho[z=@loc:1025.0]
let  low_layer_dep = ann_rho[z=@loc:1026.0]
let thickness = low_layer_dep - high_layer_dep
.....

But it show " **ERROR: inconsistent sizes of data regions: Y axis that may
cause from the different position of the outcrop line between two isopycnal
surface. That is, the outcrop line of 1026.0 isopycnal surface is further
extended to the north than that of 1025.0 which is not valid in these
regions. How can I deal with this problem? 
Or could anyone give me a scrip file for estimating the potential vorticity
field in the open ocean using the density data ?
I am appreciated of your help in advance.
Thanks for your time.

Duke Min