def area(poly):
    """
    calculate the area of polygon

    Keyword arguments:
    poly -- polygon geojson object

    return polygon area
    """
    poly_area = 0
    points = poly['coordinates'][0]
    j = len(points) - 1
    count = len(points)
    for i in range(0, count):
        p1_x = points[i][1]
        p1_y = points[i][0]
        p2_x = points[j][1]
        p2_y = points[j][0]
        poly_area += p1_x * p2_y
        poly_area -= p1_y * p2_x
        j = i
    poly_area /= 2
    return poly_area
