Grid References and Tetrads
Know Your Place!
all BTO surveys now rely heavily on the use of Ordnance
Survey grid references yet to the uninitiated they can be
In 1791 the Ordnance Survey was founded
as Britain's national mapping organisation, mostly to
produce maps for military purposes. In 1799 Ordnance Survey
rose to the challenge of invasion from France and the entire
OS staff were sent to the English south coast to survey the
counties thought at most risk. This lead to the production
of the first Ordnance Survey map in 1801 of Kent.
So it was that the Ordnance Survey began,
though we would not recognise it until following the second
World War when the recommendations of the Davidson Committee
brought about the metric National Grid. Prior to the
National Grid many counties had different projections and
matching maps at county boundaries was notoriously
difficult. In 1947 the first map was published bearing the
National Grid on its cover. This grid provided the first
truly national map referencing system by which any place in
the country could be given
a unique reference.
A complete grid reference consists of
three parts - an alphabetic prefix, followed by numeric
eastings and then numeric northings. Britain and Ireland are
split into 100km by 100km squares (termed 100-km squares).
In Britain these are given a two-letter alphabetic reference
whereas in Ireland they are given a single letter reference
(rarely given as IB, IC etc). Figure 1 shows the National
grid with 100-km squares labelled with the alphabetic
prefix. Note that, because the grid is square and is imposed
on our round earth, the grid is not necessarily parallel to
lines of latitude or longitude.
By referring to the map's key you can
find a diagram showing which 100-kms are shown where on the
map sheet - on some maps the alphabetic prefix is also
printed in the corners of the map and wherever the 100-km
changes (e.g. the pink 1:50,000 OS Landranger Series).
Whenever you give a grid reference you should always include
this prefix - sending in a sighting of a flock of Tree
Sparrows at '7233' means we cannot determine whether they
were in Norfolk, on the Isle of Mull or on Snowdonia.
A grid reference always refers to the
south-west or bottom left-hand corner of a square but many
people get confused by which numbers to give first - do you
use the ones along the bottom of the map or those up the
side first? The answer is the eastings come first - those
are the numbers running along the bottom of the map and
determining how far east you are. Next come the northings,
the numbers up the side of the map. You can come up with
your own way of remembering them. I think of them as Across
first and Up second because A is before U in the alphabet.
The number of figures in the grid
reference determines how accurately you define a point:
a 2 figure grid
reference, e.g. TF73, identifies a single 10-km
a 4 figure grid
reference, e.g. TF7233, identifies a single 1-km
a 6 figure grid
reference, e.g. TF722336, identifies a point to the nearest
Reading the grid
Figure 2 shows extracts from a road atlas
and Ordnance Survey Pathfinder sheet 839 covering an area
where I saw a flock of Tree Sparrows (shaded square). On the
left the 10-km square is shown in bold with its constituent
100 1-km squares.
To give the grid reference, start with
the alphabetic prefix - I know I was in north-west Norfolk
so from Figure 1 the 100-km square must be 'TF'
(also printed on the cover of the Pathfinder map). The
eastings, the numbers running across the
bottom, read '72' for the bottom left-hand corner of the
square the birds were in. The northings, the numbers
up the side, read '33' for the bottom left-hand
corner. To give a 2 figure grid reference give the first
digit from each - so the 10-km square is TF73. The 4 figure
grid reference giving the 1-km square is TF7233. To be more
precise, give a 6 figure grid reference indicating where, to
within 100m, the birds were within the 1-km square. The
dotted lines overlaid on the right-hand map show where the
flock was - it was 2 tenths east of the
grid line and 6 tenths north of the grid
line, so the 6 figure reference is TF722336.
surveys use tetrads rather than 1-km squares. A tetrad,
after the Greek tetras meaning four, is a group of
four 1-km squares arranged into a 2km by 2km square. There
are 25 tetrads within each 10-km square and they are
identified by an alphabetic suffix after the 10-km square's
reference. The 25 tetrads in any 10-km square are labelled
from A to Z (excluding O to prevent confusion) as shown in
Figure 3. The Tree Sparrow flock I saw in Norfolk was in the
10-km square TF73 and by overlaying the grid from Figure 3
onto the 10-km square in Figure 2 the tetrad letter must
have been G - so the full tetrad code was
For more information on the Ordnance
Survey and its maps visit www.ordsvy.gov.uk.