def convert_field_format(fieldAsArray):
    """
    Description:
        converts the polygon given as nested arrays
        to nested tuples.
    Parameters:
        fieldAsArray: [ [ [x, y], ... ], ... ]
    Return:
        ( ( (x, y), ... ), ... )
    """
    return tuple( tuple( tuple(vertex) for vertex in ring) for ring in fieldAsArray )