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