The ORDNANCE SURVEY NATIONAL GRID REFERENCE SYSTEM is a system of
geographic grid references used in Great Britain, different from using
The
A number of different systems exist that can provide grid references
for locations within the
CONTENTS * 1 General * 1.1 Datum shift between OSGB 36 and WGS 84 * 1.2 Datum shift between OSGB 36 and ED 50 * 2 Grid letters * 3 Grid digits * 4 All-numeric grid references * 5 Summary parameters of the coordinate system * 6 See also * 7 Notes * 8 References * 9 External links GENERAL Grid square TF. The map shows
The grid is based on the _OSGB36_ datum (
The Airy ellipsoid is a regional best fit for Britain; more modern
mapping tends to use the GRS80 ellipsoid used by the Global
Positioning System (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 British maps adopt a Transverse Mercator
projection with an origin (the "true" origin) at 49° N , 2° W (an
offshore point in the
OSGB 36 was also used by
A geodetic transformation between OSGB 36 and other terrestrial
reference systems (like ITRF2000 ,
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
DATUM SHIFT BETWEEN OSGB 36 AND ED 50 These two datums are not both in general use in any one place, but
for a point in the
GRID LETTERS For the first letter, the grid is divided into squares of size 500 km by 500 km, outlined in dark grey on the map to the right. 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 500 km 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. These squares are outlined in light grey on the map, with those containing land lettered. The central (2° W) meridian is shown 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,
GRID DIGITS Within each square, _eastings_ and _northings_ from the 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
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 14 digits may be required for locations in Orkney
and further north. For example, the grid reference for
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 is incomplete; it gives the location relative to an OS 100×100 km square, but does not specify which square. It is often used informally when the context identifies the OS 2-letter square. 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. If working with more than one Landranger sheet, this may also be given as 41/166712. Alternatively, sometimes numbers instead of the two-letter combinations are used for the 100×100 km squares. The numbering follows a grid index where the tens denote the progress from West to East and the units from South to North. In the north of Scotland, the numbering is modified: the 100 km square to the north of 39 is numbered N30; the square to the north of 49 is N40, etc. SUMMARY PARAMETERS OF THE COORDINATE SYSTEM Datum :
SEE ALSO *
NOTES * ^ "OSGB 1936 / British National Grid:
REFERENCES *
EXTERNAL LINKS * |