The
British national grid reference system is a system
of geographic grid references commonly used in Great Britain
, different from using latitude and longitude.
The
Ordnance
Survey
(OS) devised the national grid reference system, and it is heavily used
in their survey data, and in maps (whether published by the
Ordnance Survey or commercial map producers) based on those
surveys. Grid references are also commonly quoted in other
publications and data sources, such as guide books or
government planning documents.
Two such
systems exist for the British Isles
: this article describes the one used for Great Britain and its outlying islands (including the Isle of Man
); a similar system, used throughout Ireland
(including
Northern
Ireland
), is the Irish grid reference system
(used jointly by the Ordnance
Survey of Ireland and Ordnance Survey of Northern
Ireland).
General
The grid is based on the
OSGB36 datum (Ordnance Survey Great Britain 1936, based
on the
Airy 1830
ellipsoid), and was introduced after the
retriangulation of
1936–1962.
The Airy ellipsoid is a regional best fit for Britain; more modern
mapping tends to use the GRS80 ellipsoid used by the
GPS (the Airy ellipsoid assumes the Earth to be about
1 km smaller in diameter than the GRS80 ellipsoid, and to be
slightly less flattened).
The maps adopt a Transverse Mercator
projection with an origin at 49° N, 2° W (an offshore point in the
English
Channel
which lies between the island of Jersey
and the
French port of St.
Malo
). Over the Airy ellipsoid a straight line
grid, the National Grid, is placed with a new false origin (to
eliminate negative numbers), creating a 700 km by 1300 km
grid. This false origin is located south-west of the Isles of
Scilly. The distortion created between the OS grid and the
projection is countered by a scale factor in the longitude to
create two lines of longitude with zero distortion rather than one.
Grid north and true north are only aligned on the 400 km
easting of the grid which is 2° W (OSGB36) and approx. 2° 0'
5" W (
WGS 84).
OSGB 36 was also used by
Admiralty
nautical charts until 2000 after
which
WGS 84 has been
used.
A
geodetic transformation between OSGB 36
and other terrestrial reference systems (like
ITRF2000,
ETRS89, or
WGS 84) can become quite tedious if
attempted manually. The most common transformation is called the
Helmert datum
transformation, which results in a typical 7 m error from
true. The definitive transformation from ETRS89 that is published
by the OSGB is called the National Grid Transformation OSTN02. This
models the detailed distortions in the 1936–1962 retriangulation,
and achieves backwards compatibility in grid coordinates to
sub-metre accuracy.
Datum shift between OSGB 36 and WGS 84
The difference between the coordinates on different datums varies
from place to place. The
longitude and
latitude positions on OSGB 36 are the same
as for
WGS 84 at a point in
the Atlantic Ocean well to the west of Great Britain.
In Cornwall
the WGS 84 longitude lines are about 70
metres east of their OSGB 36 equivalents, this value rising
gradually to about 120 m east on the east coast of East Anglia
. The WGS 84 latitude lines are about
70 m south of the OSGB 36 lines in South Cornwall, the difference diminishing to zero in the Scottish Borders, and then increasing to
about 50 m north on the north coast of Scotland
. (NB. If the
lines are further
east, then the longitude
value of any given point
is further
west.
Similarly, if the lines are further south,
the values will give the point a more northerly latitude.) The
smallest datum shift is on the west coast of Scotland and the
greatest in Kent
.
Datum shift between OSGB 36 and ED 50
These two
datums are not really both in general use in any one place, but for
a point in the English
Channel halfway between Dover
and Calais
, the
ED50 longitude lines are about 20 m east
of the OSGB36 equivalents, and the ED50 latitude lines are about
150 m south of the OSGB36 ones.
Grid letters
For the first letter the grid is divided into squares of size 500
km by 500 km. There are four of these which contain significant
land area within Great Britain: S,T,N and H. (The "O" square
contains a tiny area of
North
Yorkshire, almost all of which lies below mean high
tide.)
For the second letter, each large square is subdivided into 25
squares of size 100 km by 100 km, each with a letter code from A to
Z (omitting I) starting with A in the north-west corner to Z in the
south-east corner. The accompanying map shows the resultant grid,
with the squares containing land lettered, and the central meridian
marked in red.
It would
be possible to extend the grid system over Ireland, completing
the S and N squares and introducing what would become the R and M
squares (with the arrangement of first letters following the same
pattern as for the second letter). However, there is no
motion for this at the moment, and the accuracy of the projection
would start to diminish in the west of Ireland, more than 8 degrees
from the central meridian.
Theoretically, the system extends far over
the Atlantic
Ocean
and well into Western
Europe with square AA near Iceland and square ZZ in northern
Italy. In fact, Rockall
is mapped by the Ordnance Survey, but is usually
shown as an inset without gridlines on a mainland sheet.
However, the grid can be extended to put Rockall in grid square MC
as shown in this
1:50,000 mockup.
Grid digits
Within each square,
eastings and
northings from
the origin (south west corner) of the square are given numerically.
For example, NH0325 means a 1 km square whose south-west corner is
3 km east and 25 km north from the south-west corner of square NH.
A location can be indicated to varying resolutions numerically,
usually from two digits in each coordinate (for a 1 km square)
through to five (for a 1 m square); in each case the first half of
the digits is for the first coordinate and the second half for the
other. The most common usage is the
six figure grid
reference, employing three digits in each coordinate to
determine a 100 m square.
For example, the grid reference of the 100 m
square containing the summit of Ben Nevis
is . (Grid references may be written with or
without spaces, e.g. also NN166712.)
All numeric grid references
Grid references may also be quoted as a pair of numbers: eastings
then northings in metres, measured from the southwest corner of the
SV square.
Note that 13 digits may be required for
locations in Orkney
and further
north. For example the grid reference for
Sullom Voe oil terminal may be given as or
.
Another, distinct, form of all-numeric grid reference is an
abbreviated alphanumeric reference where the letters are simply
omitted, e.g. 166712 for the summit of Ben Nevis. Unlike the
numeric references described above, this abbreviated grid reference
does not contain enough information to specify a 100m square
uniquely without additional context, and is therefore less useful.
However, it is often used informally when the context already
limits the location to within an area of less than 100 km in each
direction. For example, within the context of a location known to
be on
OS Landranger
sheet 41 (which extends from NN000500 in the south-west to
NN400900 in the north-east) the abbreviated grid reference 166712
is equivalent to NN166712.
Summary parameters of the coordinate system
- Datum: OSGB36,
- Map projection: Transverse Mercator,
- True Origin: 49°N,2°W,
- False Origin: 400 km west, 100 km
north of True Origin,
- Scale Factor: 0.9996012717,
- EPSG Code: EPSG:27700
- Ellipsoid: Airy 1830
- Semi-major axis a: 6 377 563.396
m
- Semi-minor axis b: 6 356 256.910
m
- Flattening (derived constant):
1/299.3249646
Notes
- OS Net, The true
origin
- Standing, 2006
References
use of GPS datasets with Ordnance Survey mapping; accessed 18 Oct
2009.
See also
External links
Wikis
Quiz
Map
Search