Convert alphanumeric British National Grid references to/from easting northing integer pairs

bng.py

import numpy as np

myriad_codes = np.array((
    None, None, None, None, "HP", None, None,
    None, None, None, "HT", "HU", None, None,
    None, "HW", "HX", "HY", "HZ", None, None,
    "NA", "NB", "NC", "ND", None, None, None,
    "NF", "NG", "NH", "NJ", "NK", None, None,
    "NL", "NM", "NN", "NO", None, None, None,
    None, "NR", "NS", "NT", "NU", None, None,
    None, "NW", "NX", "NY", "NZ", "OV", None,
    None, None, "SC", "SD", "SE", "TA", None,
    None, None, "SH", "SJ", "SK", "TF", "TG",
    None, "SM", "SN", "SO", "SP", "TL", "TM",
    None, "SR", "SS", "ST", "SU", "TQ", "TR",
    "SV", "SW", "SX", "SY", "SZ", "TV", None))

myriad_codes = np.flipud(myriad_codes.reshape(13, 7))


def myriad_alpha_code_from_x_y(x, y):
    """Given a British National Grid 2-digit myriad x and y e.g. (4,5)
    returns the myriad code e.g. 'SE'"""
    try:
        return myriad_codes[y][x]
    except IndexError:
        raise IndexError("Invalid British National Grid myriad (single " +
                         "digit) eastings and northings.")


def myriad_x_y_from_alpha_code(code):
    """Given a British National Grid myriad code e.g. 'SE' returns the
    2-digit myriad x and y e.g. (4,5)"""
    try:
        x = np.where(myriad_codes == code)[1][0]
        y = np.where(myriad_codes == code)[0][0]
    except IndexError:
        raise IndexError("Invalid British National Grid myriad code.")
    return (x, y)


def grid_ref_str_to_easting_northing(grid_ref_str):
    """Converts British National Grid ref in form 'SE 12345 12345' to 12-digit
    (easting, northing) tuple."""
    code, easting, northing = grid_ref_str.split(' ')
    easting = myriad_x_y_from_alpha_code(code)[0] * 100000 + float(easting)
    northing = myriad_x_y_from_alpha_code(code)[1] * 100000 + float(northing)
    return (easting, northing)