GMDP is a struct
GMDP.Ploc is a field of GMDP
GMDP.Ploc{1} is a cell containing a 3D array
GMDP.Ploc{1}(:,:,1) is the first 2D slice along the third dimension
repmat([0 1],4,1) repeats the array [0 1] 4 times along the first dimension and once along the second dimension.
