Convert British National Grid References to Easting and Northing coordinates.

bng2en.py

#!/usr/bin/env python3

"""
Convert British National Grid References to Easting and Northing coordinates.

Prints the Easting and Northing coordinates of the provided grid reference. Prints south
west point coordinates of reference box by default. You can specify the point of the box
to print by passing the point as the second argument. Accepted values are: "SW", "NW",
"NE", "SE", "MID", "SWNE".

Disclaimer: Adapted to Python from the original Perl code by Ben Soares for EDINA
national datacentre. For original implementation, please refer to:
https://digimap.edina.ac.uk/help/files/resource-hub/downloads/ngconv.pl

Example: python bng2en.py NZ20NE SW

"""

# Copyright (c) 2022 Bubo.AI

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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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from typing import Tuple, Union
import re
import sys

__version__ = "0.1.0"
__author__ = "Furkan Tektas, Ben Soares"
__copyright__ = "Copyright 2022, Bubo.AI"
__license__ = "MIT"
__credits__ = ["EDINA national datacentre"]


def bng2en(
    grid_ref: str, point: str = "SW"
) -> Union[Tuple[str, str], Tuple[str, str, str, str]]:
    """Convert British National Grid References to Easting and Northing coordinates.
    Adapted to Python from the original Perl code by Ben Soares for EDINA national
    datacentre.
    Reference: https://digimap.edina.ac.uk/help/files/resource-hub/downloads/ngconv.pl


    Args:
        grid_ref (str): National Grid Reference
        point (str, optional): Convert to specified point(s) on box. Accepted values are
        "SW", "NW", "NE", "SE", "MID", or "SWNE". Defaults to "SW".

    Returns:
        Union[Tuple[str,str], Tuple[str,str,str,str]]: Easting and Northing coordinates
        if point is not SWNE, otherwise South, West, North, East coordinates.
    """

    assert point in ["SW", "NW", "NE", "SE", "MID", "SWNE"], "Invalid point"
    matches = re.match(
        r"^([A-Z])([A-Z])\s*((\d{2}){0,5})\s*([NS])?([EW])?$", grid_ref.upper()
    )

    assert matches, "Invalid grid reference"
    l1, l2, d, _, q1, q2 = matches.groups()

    digits = 5
    d_length = len(d) // 2
    if d:
        dx = int(d[:d_length]) * 10 ** (digits - d_length)
        dy = int(d[d_length:]) * 10 ** (digits - d_length)
    else:
        dx = 0
        dy = 0
    grid1 = ((ord(l1) - 1) if (ord(l1) > ord("I")) else ord(l1)) - ord("A")
    grid2 = ((ord(l2) - 1) if (ord(l2) > ord("I")) else ord(l2)) - ord("A")

    grid1_x = grid1 % 5
    grid1_y = 4 - grid1 // 5

    grid2_x = grid2 % 5
    grid2_y = 4 - grid2 // 5

    # put SV as origin
    grid1_x -= 2
    grid1_y -= 1

    precision = 10 ** (digits - d_length)

    quadrant_x = 1 if (q2 == "E") else 0
    quadrant_y = 1 if (q1 == "N") else 0

    if q1 and q2:
        precision //= 2

    easting = (grid1_x * 500_000) + (grid2_x * 100_000) + dx + (quadrant_x * precision)
    northing = (grid1_y * 500_000) + (grid2_y * 100_000) + dy + (quadrant_y * precision)

    if point == "SW":
        return (easting, northing)
    elif point == "NW":
        return (easting, northing + precision)
    elif point == "SE":
        return (easting + precision, northing)
    elif point == "NE":
        return (easting + precision, northing + precision)
    elif point == "MID":
        return (easting + (precision // 2), northing + (precision // 2))
    elif point == "SWNE":
        return (easting, northing, easting + precision, northing + precision)
    else:
        raise ValueError("Invalid point value")


def main() -> None:
    assert 2 <= len(sys.argv) <= 3, "Usage: bng2en.py <grid_ref> [point]"
    grid_ref = sys.argv[1].strip().upper()
    point = sys.argv[2].strip().upper() if len(sys.argv) == 3 else "SW"
    coordinates = bng2en(grid_ref=grid_ref, point=point)
    formatted_coordinates = ", ".join(map(str, coordinates))  # seperate by comma
    print(formatted_coordinates)


if __name__ == "__main__":
    main()