function a = mk3D(b,c)

a=c;

if isa(b,'gcmfaces');
   %go from 2D field to 3D field
   for kk=1:size(a.f1,3)
      for iFace=1:a.nFaces;
         iF=num2str(iFace);
         eval(['a.f' iF '(:,:,kk)=b.f' iF '(:,:);']);
      end;
   end;
elseif isa(b,'double');
   for kk=1:size(a.f1,3)
      for iFace=1:a.nFaces;   
         iF=num2str(iFace);
         eval(['a.f' iF '(:,:,kk)=b(kk);']);
      end;
   end;
else
   error('indexing not supported by gcmfaces objects')
end