The Ordnance Survey
Ordnance Survey is the organisation that is responsible for the mapping of Great Britain. It was initially formed in 1791 - in the face of a possible French invasion it was deemed necessary to map the country for military purposes. The Ordnance Survey is so called because the Board of Ordnance (the government department responsible for ammunition, fortresses, and the artillery and engineer corps) was the department which commissioned the first maps. During the First and Second World Wars, the Ordnance Survey prepared maps of Continental Europe for the armed forces, which were extensively used during the wars. Since, the Ordnance Survey has become a civilian organisation which is an Executive Agency of the government, under the Department for Communities and Local Government.
Series
The ordnance survey produces two series of maps which are used by walkers and hikers:
Explorer: This is a map suited for the outdoors, with a scale of 4cm to 1km, or 1:25000. The series contains 403 maps, which covers all of Great Britain. It is the most detailed map that Ordnance Survey produce, and has been specifically designed with walkers and cyclists in mind.Landranger: This is a larger scale map, at 1:50000. The series covers all of Great Britain, over 204 different maps. This is a more general purpose map, and although it still shows footpaths and other relevant detail for walkers, is not as detailed as the explorer series.
The National Grid
The Ordnance Survey National Grid reference system is a method of geographic grid references used within Great Britain (Ireland uses a different system). This is comprised of two letters, denoting a grid square, followed by a series of numbers which denote the location within the grid square. The most common form is a six figure grid reference, which can be used to represent a 100m by 100m square.
In order to form the first letter, Great Britain is divided into four squares, each 500km by 500km and lettered S, T, N and O. For example, Snowdonia is within grid area S.To form the second letter, each square is subdivided into 25 squares, each 100km by 100km. Each has a letter code, A through Z (missing I), and left to right from top to bottom. For example, Snowdonia is in grid square SH.
Six Figure Grid References
Once we have used the National Grid system to locate a 100km grid square, we can then pinpoint a location within the grid square more accurately. Each grid square is split into 100 smaller squares of 10km each. These can be referenced by using the system of eastings (how far east) and northings (how far north), given in that order, in the form of numbers.
Therefore, we can use the following example:
The grid square SH is 100km by 100km. Areas within that grid square of 10km by 10km can be pinpointed using eastings and northings. For example, grid square SH12 is 1 square (equal to 10km) to the East and 2km to the North from the origin. SH28 is 20km to the East, and 80km to the North. SH71 is 70km to the East and 10km to the North.The grid square SH84 is 10km by 10km. In exactly the same method, squares of 1km by 1km can be pinpointed, and referred to using a 4 figure grid reference. For example, grid square SH8395 is 3 squares (equal to 3km) to the East of the origin, and 5km to the North, within grid SH89.
Six Figure Grid References Six figure grid references can be used to locate an area of 100m by 100m on the map. However, there are no grid lines, so distances must be estimated - split the grid axes into tenths, and imagine how many tenths away the point being described is. Alternatively, use a romer for a more accurate measurement.
Remember to give the grid reference with the eastings followed by the northings. Some map symbols may occupy more than one 6 figure grid reference, therefore remember that the following rules apply:
Double line symbols: half way between linesCircular or rectangular symbols (churches, railway stations): the centreVertical symbols (radio masts, lighthouses): the centre of the base
Marginal Information
The marginal information is the information situated in the margins, or around the sides, of the map. It contains information that can be vital to the reading of the map, and should always be examined first when looking at a map.
Basic information: At the top right of the sheet is some basic information about the map. This will show the title, sheet number, name of area covered, and give a representative fraction showing scale.Conventional signs: This shows the abbreviations, signs and symbols used in the map, and this part is vital to reading the map.Technical information: This will show information on the three different north points, and the relevant angles for conversion between the three on that particular map. It will also give a reference guide for giving grid references.Map scale: The scale of the map is shown at the bottom centre of the map. It is shown as a representative fraction (RF), and as a scale bar as kilometres, statute miles, and nautical miles. More information about scale, and the ways this can be represented, can be found here.Top margin: This will show information on the three north points. It will show an arrow indicating magnetic north, a star indicating true north, and a line indicating grid north. There will also be information on the Grid Magnetic Angle, and annual change.
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