Tetrads
are squares measuring 2x2 kilometres. They are defined using the National Grid,
which is printed as pale blue lines on all Ordnance Survey Maps. They are
designated by two letters, two numbers and a final letter. The first two letters
define a 100-km square on the Grid, the two numbers define a 10-km square within
that larger square, and the last letter defines the Tetrad within that 10-km
square.
The
first two letters can be found in the OS Map Key under 'How
to give a grid reference'. In
To
Locate A Given Tetrad, Using SJ
43 C As An Example
The
two letters show the Tetrad is in the northern half of
The
smallest squares marked on the map are 1-km squares. There are 100 of these in
each 10-km square (10 across and 10 up). Four
of these smaller 1-km squares make up a Tetrad, and there are 25 Tetrads in each
10-km square. Tetrads are indicated
by letters. The letter “O” is
omitted to prevent confusion with zero. The
first Tetrad 'A' is in the bottom left hand corner and the final Tetrad letter
'Z' in the top right. The letters go
bottom to top, and move across in columns left to right. The edges of Tetrads
always follow even numbered lines and the centres are where the odd numbered
lines cross. The table below shows
the layout of the Tetrads within a 10-km square and gives the reference of their
centre points.
For
the example SJ43C, take the centre reference for Tetrad C
from the table (i.e. number 15) and insert it into the 10-km square
number. ie 41 35. Where the line 41 (from bottom) crosses the line 35 (from
side) will be the centre point of the Tetrad. The Tetrad is the four small 1-km
squares that surround this point. If
you follow this on the map you will find that Tetrad SJ43C contains the Mere
Ellesmere.
To
Put A Place In
|
E |
J |
P |
U |
Z |
|
19 |
39 |
59 |
79 |
99 |
|
D |
I |
N |
T |
Y |
|
17 |
37 |
57 |
77 |
97 |
|
C |
H |
M |
S |
X |
|
15 |
35 |
55 |
75 |
95 |
|
B |
G |
L |
R |
W |
|
13 |
33 |
53 |
73 |
93 |
|
A |
F |
K |
Q |
V |
|
11 |
31 |
51 |
71 |
91 |
First
locate the small 1-km square that contains the vast majority of the pool.
Then write down the appropriate two letters, SJ or SO (SJ in this case,
which covers all of
The
square will be bordered by an odd numbered line and an even numbered line along
either side and along the top and bottom. Where
the two odd numbered lines cross will be the centre of the Tetrad (in this case
about 1 kilometre east of Cross Houses). Follow
this point down to the bottom (or top) of the map and you should find number 55.
The first 5 will be the first part of the 10-km square number SJ5 and the
second 5 will be the first part of the Tetrad identity.
Next trace the line to the side of the centre point and you should find
number 07. The 0 is the final part
of the 10-km square number ie SJ50 and the 7 completes the Tetrad identity 57.
From the chart 57 = Tetrad N, so the full Tetrad reference is SJ50N.
Alternatively
Using
'How To Give A Grid Reference To The
Nearest 100 Metres', which can be found in the Key at the side of the map,
write down the six figure grid reference. For the island in Venus Pool this is
SJ549062.
The
letters plus the first and fourth of the six numbers define the 10-km square
i.e. SJ50.
The
second and fifth numbers define which 1-km square contains the grid reference
– in this case 46. This is the Grid number of the bottom left hand corner of
the 1-km square, with the first number being at the bottom of the map and the
second number being up the side.
Use
the Tetrad letter key (reproduced
from the last Shropshire Breeding Bird Atlas, p177) to look up which Tetrad
letter contains this number, or has it at the bottom left hand corner of the
Tetrad. In this case 46 is the bottom left hand corner of Tetrad “N”, so the
full Tetrad reference is SJ50N.