def triangle_area(pt1, pt2, pt3):
    """Return the area of a triangle.

    Parameters
    ----------
    pt1: (X,Y) ndarray
        Starting vertex of a triangle
    pt2: (X,Y) ndarray
        Second vertex of a triangle
    pt3: (X,Y) ndarray
        Ending vertex of a triangle

    Returns
    -------
    area: float
        Area of the given triangle.

    """
    a = 0.0
    a += pt1[0] * pt2[1] - pt2[0] * pt1[1]
    a += pt2[0] * pt3[1] - pt3[0] * pt2[1]
    a += pt3[0] * pt1[1] - pt1[0] * pt3[1]
    return abs(a) / 2
