def calc_gsd_cross(altitude,
                   focal_length,
                   pixel_dim_cross):
    """
    ground sample distance (gsd) is the distance between pixel centers measured
    on the ground.
    https://en.wikipedia.org/wiki/Ground_sample_distance

    Returns
    -------
    double : meters per pixel
    """

    magnification = altitude / focal_length
    return magnification * pixel_dim_cross