The National
Grid is the map reference system used on all British Ordnance Survey maps
to identify the position of any feature. Other nations have their own
systems.
How it works:
The National
Grid breaks Great Britain down into progressively smaller squares
identified first by letters and then numbers.
The largest unit of the grid is 500km squares each
designated by a prefix letter alphabetically from A-Z omitting I - the
first letter to be quoted in today's National Grid Reference. Great
Britain is covered only by four of these squares: H, N, S and
T.
The 500km squares are then further broken down in to
twenty-five 100km squares which are identified by a letter, again A - Z
omitting I ( the second letter quoted in a reference).
These squares are divided into smaller squares by grid
lines representing 10 km spacing each numbered 0 - 9, from the south-west
corner in an easterly and northerly direction. You can thus identify a
10km grid square by quoting two grid letters and the eastings and
northings; for example, TQ 6 3.
On OS Landranger Maps, you will find the two grid
letters on the legend or the corner of the map. The 10km grid is then
further broken down into 1km grid squares.
By estimating the eastings and northings to one tenth
of the grid interval, you can quote a full six figure grid reference that
is accurate to 100m on the ground. For example, the Tower of London's grid
reference is TQ 336805.
Geographical origin of the National
Grid:
The National Grid applies to all Ordnance
Survey maps at all scales. Northern Ireland and the Republic of Ireland
use their own National Grid system. It covers the whole of Great Britain
and extends 700 km to the east and 1300 km to the north of the false
origin. It is based on the Transverse Mercator Projection.
The True origin latitude and longitude coordinates of
the national grid are 49 Degrees North: 2 degrees West. The False origin
latitude and longitude or coordinates are 49 degrees 45 minutes and 58
seconds North: 7 degrees 33 minutes 23 seconds West.
The False origin which lies slightly southwest of the
Isles of Scilly was devised to ensure that all National Grid coordinates
were positive (that is, to the east and north of origin 00) 400km are
added to all eastings coordinates and 100km subtracted from all northings
coordinates. If coordinates were calculated from the true origin, the
positions lying west of the central meridian would be negative and the
northings, although positive would exceed 1000km for some points in
northern Scotland.
Datum height differences:
The difference
between the Liverpool and Newlyn Datums varies across the country. This is
due to levelling of the day only being fit to form a framework to control
lower order levelling and contouring.
The original Liverpool levelling was started in
1840 using a bench mark on St John's Church. In 1844 the datum was changed
to the tidal pole in Victoria Dock and tidal observations taken place over
a nine day period.
Due to the imperfections with the levelling, it
was decided to undertake a second geodetic levelling (1912 to 1921). It
was at this time that mean sea level was fixed at Newlyn in Cornwall.
Fixed points throughout the country were established fundamental bench
marks (FBMs).
In 1950 it was decided to undertake a third
geodetic levelling, still based on the Newlyn tidal observations. This
started in 1951 and was completed in 1956.
The conversion factors between the two datums vary
sometimes between kilometre squares. The rule of thumb is that the
conversion factor is negative in the south of the country and positive in
the north. The conversion figures are only given as a guide and cannot be
given exactly, but can be given with sufficient accuracy for most
practical purposes to one decimal place of a foot.
In remoter islands such as Shetland and Hebrides
and so on, datums are based on a local determination of mean sea level.
Being based on short observations, the figures should be regarded as only
approximate.