bng 1.0.2

Convert between BNG grid refs (e.g. NT123456) and OSGB36 (EPSG:27700) coords

BNG

Convert between BNG grid refs (e.g. NT123456) OSGB36 (EPSG:27700) coords

Coordinates in the Ordnance Survey National Grid or British National Grid are often defined by alphanumeric grid references. These are based on the osgb36 (EPSG:27700) coordinate reference system. This module contains Python functions to convert osgb36 coordinates to/from 4, 6, 8, or 10 figure alphanumeric grid references.

This code was originally published on the Easily change coordinate projection systems in Python with pyproj blog post. See it for more information and for details on converting between coordinate systems using Python.

Installation

BNG can be installed for Python 2.7 or Python 3 using pip:

pip install bng

Instructions

The to_osbg36 and from_osbg36 functions are used to convert between tuples of OSGB36 (x, y) coordinates and alphanumeric grid references.

to_osbg36

BNG grid references can be converted to osbg36 coordinates as follows.

Single values:

import bng
bng.to_osgb36('NT2755072950')
# (327550, 672950)

For multiple values, use Python's zip function and list comprehension:

import bng
gridrefs = ['HU431392', 'SJ637560', 'TV374354']
x, y = zip(*[bng.to_osgb36(g) for g in gridrefs])
x
# (443100, 363700, 537400)
y
# (1139200, 356000, 35400)

from_osbg36

osbg36 coordinates can be converted to BNG grid references as follows.

Single values:

import bng
bng.to_osgb36('NT2755072950')
# (327550, 672950)

The number of figures in the grid reference can be specified.

For multiple values, use Python's zip function and list comprehension:

import bng
x = [443143, 363723, 537395]
y = [1139158, 356004, 35394]
[bng.from_osgb36(coords, figs=4) for coords in zip(x, y)]
# ['HU4339', 'SJ6356', 'TV3735']

Converting grid references to GPS coordinates

BNG can be combined with pyproj (see blog post) to convert grid references to many different coordinate systems.

BNG grid references can be converted to lat/lon as used by GPS systems (EPSG:4326) as follows:

import bng
import pyproj

# Define coordinate systems
wgs84=pyproj.Proj("+init=EPSG:4326") # LatLon with WGS84 datum used by GPS units and Google Earth
osgb36=pyproj.Proj("+init=EPSG:27700") # UK Ordnance Survey, 1936 datum

# Transform
x, y = bng.to_osgb36('NT2755072950')
pyproj.transform(osgb36, wgs84, x, y)
# (-3.1615548588213667, 55.944109545140932)

GPS coordinates can be converted to BNG grid references as follows:

import bng
import pyproj

# Define coordinate systems
wgs84=pyproj.Proj("+init=EPSG:4326") # LatLon with WGS84 datum used by GPS units and Google Earth
osgb36=pyproj.Proj("+init=EPSG:27700") # UK Ordnance Survey, 1936 datum

# Transform
lon = -3.1615548588213667
lat = 55.944109545140932
x, y = pyproj.transform(wgs84, osgb36, lon, lat)
bng.from_osgb36((x, y))
# 'NT275729'

Note that for surveying work (i.e. < 1 m) it is necessary to make a geoid correction. The OSTN2 transformation model used to do this is available on the Ordnance Survey website. Proj is able to use grid correction files in NTv2 format.

For Developers

Install developer dependencies:

pip install -r requirements.txt

Run tests:

pytest -vs test_bng.py