def docustom(lon, lat, alt, gh):
    """
    Passes the coefficients to the Malin and Barraclough
    routine (function pmag.magsyn) to calculate the field from the coefficients.

    Parameters:
    -----------
    lon  = east longitude in degrees (0 to 360 or -180 to 180)
    lat   = latitude in degrees (-90 to 90)
    alt   = height above mean sea level in km (itype = 1 assumed)
    """
    model, date, itype = 0, 0, 1
    sv = np.zeros(len(gh))
    colat = 90. - lat
    x, y, z, f = magsyn(gh, sv, model, date, itype, alt, colat, lon)
    return x, y, z, f