def epsg_4326_to_900913(lon, lat):
    x = lon * 20037508.34 / 180
    y = (math.log(math.tan((90 + lat) * math.pi / 360)) / (math.pi / 180)) * (20037508.34 / 180)
    return x, y


def epsg_900913_to_4326(x, y):
    lon = x * 180 / 20037508.34
    lat = (360 / math.pi) * math.atan(math.exp(y * math.pi / 20037508.34)) - 90
    return lon, lat