The Nature of Geographic Information

23. National Grids


The Transverse Mercator projection provides a basis for existing and proposed national grid systems in the United Kingdom and the United States.

In the U.K., topographic maps published by the Ordnance Survey refer to a national grid of 100 km squares, each of which is identified by a two-letter code. Positions within each grid square are specified in terms of eastings and northings between 0 and 100,000 meters. The U.K. national grid is a plane coordinate system that is based upon a Transverse Mercator projection whose central meridian is 2 West longitude, with standard meridians 180 km west and east of the central meridian. The grid is typically related to the Airy 1830 ellipsoid, a relationship known as the National Grid (OSGB36®) datum. The corresponding UTM zones are 29 (central meridian 9° West) and 30 (central meridian 3° West). One of the advantages of the U.K. national grid over the global UTM coordinate system is that it eliminates the boundary between the two UTM zones.

A similar system has been proposed for the U.S. by the Federal Geographic Data Committee. The proposed "U.S. National Grid" is the same as the Military Grid Reference System (MGRS), a worldwide grid that is very similar to the UTM system. As Phil and Julianna Muehrcke (1998, p.p. 229-230) write in the 4th edition of Map Use, "the military [specifically, the U.S. Department of Defense] aimed to minimize confusion when using long numerical [UTM] coordinates" by specifying UTM zones and sub-zones with letters instead of numbers. Like the UTM system, the MGRS consists of 60 zones, each spanning 6° longitude. Each UTM zone is subdivided into 19 MGRS quadrangles of 8° latitude and one (quadrangle from 72° to 84° North) of 12° latitude. The letters C through X are used to designate the grid cell rows from south to north. I and O are omitted to avoid confusion with numbers. Wikipedia offers a good entry on the MGRS here.

Try This!

Fun Demo of U.K. National Grid

A kids-friendly information sheet about the U.K. National Grid is published by the U.K. Ordnance Survey. You can find it in the National Grid for Schools link on their website.

A less-kids-friendly video can be seen below:

The National Grid
Click Here for Transcript of The National Grid Video
If you have ever used an ordnance survey paper map and been given a grid reference and located it using the numbers along the edge of the map then you'll have used the National Grid if you're not familiar with reading grid references please look at the education section of our website however if you look at one of our paper maps you will notice that apart from the grid numbers there are also letter pairs around the edge these point to the wider scale of the national grid here we'll use the example of our head office in Southampton to illustrate this the National Grid starts out as a series of 25 500 kilometer by 500 kilometer squares each given a letter of the alphabet from A to Z missing out I each of these 500 kilometer squares is then subdivided into a series of 25 100 kilometer by 100 kilometre squares again each with a letter of the alphabet A to Z excluding I this then results in a series of 100 kilometer squares with references such as SX sy and s set in terms of products this is how one 225-thousand scale color raster tiles are referenced these 100 kilometre squares can then be divided again into 110 kilometers by 10 kilometre squares these are each given a number from 0 0 to 99 so using our office as an example it falls within su-31 this is our products such as 1 to 25000 scale color raster and ho aspect amount district tiles are referenced these 10 kilometer squares can also be merged into a sets of 4 to form 25 20 kilometer squares in our example our office falls in square su 20 this is how one 250-thousand scale color raster tiles are referenced these 10 kilomettre tiles can be divided into one of two ways either into five kilometres by five kilometres squares or into one kilometer by one kilometre squares with the five kilometre squares the ten kilometre square is split into four and is referenced as northwest northeast southeast or Southwest so for example with su-31 five kilometre square where our offices would be su-31 northeast this five kilometre grid is held products os Street View and one to ten thousand scale raster tiles are referenced alternatively the ten kilometer squares can be split into one kilometer squares so with our office the one kilometer grid reference would be su-37 one five this one kilometer grid is how OS master map imagery tiles are referenced to find out more please visit our website.
Credit: Ordnance Survey
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