def intersection_spectrail_arcline(spectrail, arcline):
    """Compute intersection of spectrum trail with arc line.

    Parameters
    ----------
    spectrail : SpectrumTrail object
        Instance of SpectrumTrail class.
    arcline : ArcLine object
        Instance of ArcLine class

    Returns
    -------
    xroot, yroot : tuple of floats
        (X,Y) coordinates of the intersection.

    """

    # approximate location of the solution
    expected_x = (arcline.xlower_line + arcline.xupper_line) / 2.0

    # composition of polynomials to find intersection as
    # one of the roots of a new polynomial
    rootfunct = arcline.poly_funct(spectrail.poly_funct)
    rootfunct.coef[1] -= 1
    # compute roots to find solution
    tmp_xroots = rootfunct.roots()

    # take the nearest root to the expected location
    xroot = tmp_xroots[np.abs(tmp_xroots - expected_x).argmin()]
    if np.isreal(xroot):
        xroot = xroot.real
    else:
        raise ValueError("xroot=" + str(xroot) +
                         " is a complex number")
    yroot = spectrail.poly_funct(xroot)

    return xroot, yroot