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This page attempts to explain how map references identify any spot in or around the British Isles, and to help you find the spot for which you have been given a reference. It will also hopefully help you to understand what "10 kilometre squares", "Hectads", and "Tetrads" are.
Although the grid which defines the position of any map reference appears on all British Ordnance Survey maps and most other maps such as Road Atlases we suggest that for our purposes you should have the 1:25,000 scale OS maps covering our area (e.g. for the area around Havant either the single new EXPLORER series sheet 120 (CHICHESTER, SOUTH HARTING AND SELSEY) or the two PATHFINDER series sheets (1304 'Portsmouth & Havant' and 1285 'Horndean') which are now superceded by the Explorer sheet.
You also need something that is called a 'Romer' (a piece of clear plastic on which scales have been engraved to the correct size for the scale of your map - there must be two scales at right angles to each other so that you can measure across and up and down the map at the same time). A map reading ('orienteering') compass mounted in a clear plastic base will incorporate these scales so that you can lay it on a map and read off the precise map reference of any spot on which the top right corner of the appropriate scale is placed. You will probably find that your compass base or Romer has three different scales, one for each of the standard map scales, and you must always use the scale which has exactly ten divisions for the length and height of the smallest grid squares on the map you are using (these squares are usually drawn in thin blue lines across the whole map surface, with the lines numbered along the edges of the map).
Even if you never bother to master grid references the maps and compass will prove to be one of your best aids to finding wildlife - just as vital as your field guides to identify species.
An optional extra which is helpful when planning walks is a device which has a tiny wheel at the base, attached by hidden gears to a dial which gives the distance you would have walked for whatever route you run the wheel over the map. I don't know what is the proper name for this device but you should have no difficulty in getting one, together with the compass and maps, from any large camping/walking equipment shop.
The new Explorer map is bulkier than the old Pathfinder maps but is more up to date and has a number of advantages, among them an illustrated guide to the working of the National Grid which I attempt to explain below, but note that where that guide asks you to estimate the tenths of a kilometre between the blue grid lines on the map I am suggesting that you use a Romer of a compass which comes with an inbuilt exact scale, allowing you to put accurate values to both the figures that you would otherwise have to estimate, just by laying the compass on the map with the top right corner of the appropriate pair of scales on the point for which you want a reference - you then read both numbers directly off the scales where they cross the nearest vertical and horizontal grid lines. Another advantage of the new Explorer map series is that it does not stop precisely at the ten kilometre square boundary but gives you an overlap of 1 kilometre with the next map, and a further advantage in our case is that the new map includes Selsey for which you previously had to buy a third map.
Map references are a precise way of locating a spot on the map by stating the number of metres that spot is west of, and north of, a fixed reference point. As the grid covers the whole British Isles the number of metres from the origin of the grid would be a large, unwieldy, number so the grid is broken down into squares having sides 100 km long which are identified by two letters (Havant happens to be in the SU square but South Hayling is just within the SZ square to the south).
The allocation of two letter combinations for the whole of mainland Britain is shown in the National Grid section of the explanatory notes printed on each of the new EXPLORER or OUTDOOR LEISURE series maps, but you should be aware that there is another numerical way of indicating 100km squares which is used by some computerised systems (such as the 'Streetmap' system popular with online users of the internet). This alternative system adds an additional single digit to the front of the numbers used with the two letter system, e.g. the map reference for Havant railway station can be expressed either as SU 718 066 or as 4718 1066. You can find the extra digits used by this alternative system printed (in a smaller font size) as the initial digits of the line numbers printed at the extreme bottom left corner of your map, and/or when the two digits printed at either the bottom or the left side of the map are 00 (indicating a change of 100 km square)
Each 100 km square is divided into 100 squares each having 10 kilometre sides and these 10 Km squares are referred to by a two digit number (e.g. '70') which measures the number of tens of thousands of metres west and north of the 'origin' (i.e. bottom left corner) of the 100 km square - thus SU 70 is a 10 Km square with its origin lying 70,000 metres east of, and 00,000 metres north of, the orgin of the 'SU' 100 Km square. You may sometimes find people (especially botanists) who call 10 km squares 'HECTADS'
This process of breaking down each larger square into 100 small ones can be continued indefinitely but for practical purposes map references are usually four, six or eight digits long (preceded by the two letters defining the 100 km square - the same reference is of course repeated in each 100 km square). If you take the Portsmouth map and find the line marked 71 along the top or bottom, then follow that vertically to the point where it intersects with the horizontal line numbered 06 along either side of the map you will come to the bottom left corner of the square in which Havant railway station lies, and this whole 1 km square is known as SU 7106. If you have a map reading compass lay it with its top right corner on the middle of the oblong marking the station and read off the numbers on the plastic ruler where the scales cross the nearest grid lines. These numbers tell you the number of 100s of metres that the centre of the station is east of the 71 line and north of the 06 line - I make these numbers to be 8 and 6 respectively, and the six digit reference for the station is thus SU 718 066. To be more precise move the corner of the compass to the west end of the station (50 metres west of the centre) and the more accurate eight digit reference for that point can be read as SU 7175 0660.
If you do not have a map reading compass get a small piece of card or paper and lay it anywhere around the edge of the map between any two numbers, then mark 100 metre units on the card using the scale markers which divide the distance between the two numbers into ten sections of 100 metres. You can then use this home made scale to measure the distance from the reference point to each of the nearest grid lines in turn - clumsier and less accurate, but sufficient!
One other piece of knowledge about maps that you will probably need in connection with wildlife surveys is an understanding of 'TETRADS'. A Tetrad is an area 2km by 2km formed by four adjacent 1 km squares, with the bottom left of these four squares having an even numbered line defining its western and southern boundary. A better way of understanding them is to consider the 100 1km squares in a 10km square, for example SU 71. Now take the four 1km squares forming the bottom left corner of the 10 km square and call them the first Tetrad in the 10km square. This group of four will have the 70 line running up their west side and the 10 line along their southern edge. Immediately above them comes the second Tetrad bounded by the 70 and 12 lines, and so on to the fifth tetrad bounded by the 70 and 18 lines. As this takes us to the top of the square the sixth Tetrad is at the bottom of the next column (bounded by the 72 and 10 lines). Repeating this process you will end up with 25 Tetrads in a 10 km square, the last one being bounded in our example by the 78 and 18 lines. To complete the picture you have to know that these 25 Tetrads are sometimes referred to by single letters, so what we have called the first Tetrad would be called SU 71A. At this point, since there are 26 letters of the alphabet and only 25 Tetrads, a further complication is introduced by omitting, not the 'Z' but the 'O' - to avoid confusion with the digit zero. The end result for SU 71 (or any other square if you alter the digits along the edges) is the following pattern of letter designated Tetrads in a 10 km square...
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