def convert_lat(lat):
    """Convert a single latitude value to a floating point number.

    Input latitude can be string or float and in -24 or 24S format. 

    """

    lat = str(lat)

    if 'N' in lat.upper():
        lat = float(lat[:-1])
    elif 'S' in lat.upper():
        lat = float('-'+lat[:-1])
    else:
        lat = float(lat)

    return lat