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What's A Grid Square?

One of the things which makes The SETI League unique among grass-roots science organizations is that we have hundreds of members around the world, dozens of amateur observatories in operation, hundreds more under construction, and a grand goal of perhaps 5,000 participating stations early in the next decade. Our size, scope and span demand a method for uniquely identifying each of our active stations, preferably by geographical location. After extensive discussion on our various email lists, our members have decided to adopt six-character Grid Square designations for this purpose.

Grid Squares have recently become popular among radio amateurs to identify the location of their stations. First proposed at a conference in Maidenhead, England in 1980 (and hence sometimes called Maidenhead Locators), grid squares allow low-precision (four character) and high-precision (six character) expression of a station's latitude and longitude. Each grid in the low-precision case consists of a region one degree of latitude high by two degrees of longitude wide. Such a region is expressed by a "grid" identified by two letters of the alphabet (generally shown in capitals), and a "square" consisting of two numeric characters. For example, SETI League headquarters is in location FN20.

For higher precision, each grid square may be further divided into subsquares, expressed as two lower-case alphabetical characters. Since each subsquare encompasses 2.5 minutes of latitude by 5 minutes of longitude, a station's location is thus identified to within better than 5.6 nautical miles anywhere on the surface of the Earth. Our headquarters building is at FN20xv in this nomenclature.

Click here to download WinGrid version 3.2 (, contributed by Stacey Mills, W4SM. This program calculates grid squares from latitudes and longitudes, the reverse, and calculates distances and headings from two sets of lat/long or grid squares. The program also saves your station location information. Click on "DISTANCE" to convert from miles to kilometers. WinGrid is written in Visual C++ as true "standalone" routine, compiled for the Microsoft Windows platform and not requiring any DLL or OCX helper files. The file is in zip format to save time in downloading. The program runs best at 1024 x 768 screen resolution or higher in "hi-color" with small fonts, but will run in other resolutions as well.

A more detailed explanation of grid squares, including still more downloadable software to enable you to compute yours, may be found on the American Radio Relay League website. The ARRL site also contains an online calculator into which you may enter your latitude and longitude, and which will return your grid square in the six-character format.

The Radio Amateur Satellite Corporation (AMSAT, a SETI League affiliate society) also has an excellent HTML grid square calculator on the web at You can enter a lat/lon and get the corresponding grid square, or vice-versa, directly from your web browser.

When you register your Project Argus station, you will be asked to identify it by your grid square. If you don't know your grid square, please supply your latitude and longitude in degrees/minutes/seconds, as precisely as you are able to identify them, and we will compute your grid square for you, and assign it to you as your unique station identifier.

Eastern Australia regional coordinator Noel Welstead notes that not all grid square locator programs compute Southern Hemisphere locations correctly. He recommends that our members in his hemisphere check their location on a map, and then ask themselves if the results from a given grid square calculation program seem reasonable.

OS Grid Reference Method

Although the Maidenhead grid square system described above originated in England, many radio amateurs in the UK use a different system based upon Ordnance Survey maps. Adrian Kingsley-Hughes, The SETI League's volunteer regional coordinator for Wales, elaborates:

As mapped by the Ordnance Survey, Great Britain is covered by 100 x 100 km grid squares identified by two letters (91 in all - 7 across, 13 upwards). The origin of this grid is about 110 km west and 20 km south of Land's End. On the Ordnance Survey maps these squares are further divided by grid lines representing 10 km spacing, numbered 0 to 9 from the southwest corner in an easterly (left to right) and northerly (upwards) direction. Using this system you can identify a 10 km square grid. On the more detailed maps (Landranger) you can further divide up the grid into 1 km squares.

Using this method you get an Ordnance Survey grid reference. It takes the following format:

SH551725 (my station's location)
The first two letters are the sheet identifier. The next three digits give the eastings along the grid and the final three digits give the northings.

For those finding it difficult to remember that eastings come before northings, then remember to go "along the hall and up the stairs".

The OS system is a very good system indeed. An advantage of it is that if you want to find the distance between two points then you can use Pythagoras' theorem.

Pythagoras' theorem states that in a right angled triangle, the square of the length of the hypotenuse equals the sum of the squares of the other two sides. As eastings lines are perpendicular to northings, it can be used to calculate the distance between the two points (ie the hypotenuse) by just calculating the difference in eastings by subtracting the smaller figure from the larger, and then calculating the difference in northings by subtracting the smaller figure from the larger. Now square the two results and add together, now the square root of this result is the length of the hypotenuse. Using this method you can get a value in meters.

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this page last updated 3 January 2004
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