Calculate offset into data array. Only uses to test correctness
            of the formula.

def calcOffset(self, x, y):
        """Calculate offset into data array. Only uses to test correctness
            of the formula."""
        # Datalayout
        # X = longitude
        # Y = latitude
        # Sample for size 1201x1201
        #  (   0/1200)     (   1/1200)  ...    (1199/1200)    (1200/1200)
        #  (   0/1199)     (   1/1199)  ...    (1199/1199)    (1200/1199)
        #       ...            ...                 ...             ...
        #  (   0/   1)     (   1/   1)  ...    (1199/   1)    (1200/   1)
        #  (   0/   0)     (   1/   0)  ...    (1199/   0)    (1200/   0)
        #  Some offsets:
        #  (0/1200)     0
        #  (1200/1200)  1200
        #  (0/1199)     1201
        #  (1200/1199)  2401
        #  (0/0)        1201*1200
        #  (1200/0)     1201*1201-1
        return x + self.size * (self.size - y - 1)