Introduction to Maidenhead Grid squares
“W1AW this is N4EGA and my grid square is Echo Mike Seven Six X-ray Bravo (EM76xb).”
As Amateur Radio operators, many of have used or heard this type of exchange before. A great number of us even know what our “Maidenhead” grid square is. But how many of us really know what it means? This is a question that I am asked by hams and non hams alike. When hams new to Amateur Radio see a Maidenhead map, their first question is usually about all the grid squares, letters and numbers.
Before I began designing maps for hams, I too was aware of Maidenhead but not really the how and why. Developing the Maidenhead Grid square layer in the Geographic Information System/Software (GIS), a few things became very apparent that I had never thought of.
As Earth is an ellipsoid (squished sphere), representing a system of grid squares requires the Earth to be flattened (also known as a projection) in order to understand the system. Thinking back to high school geometry, Maidenhead is much like the typical Cartesian coordinates that are represented on a +/- quadrant graph. As we only want to think in terms of positive, this flattened projection of Earth is placed into the +/+ quadrant, the upper right as in Figure 1.
Typically, when we speak of coordinates, we speak of latitude and longitude as it begins at 0 degrees north or south and 0 degrees east or west, which is just off the coast of the central Western Africa. In the case of Maidenhead, our origin is at 180 (or 0) degrees West and 90 degrees South which is the South Pole. This is to ensure that all grid squares only advance up the alphabet.
As such, all grid squares designations from west to east start with “A” and end in “R.” Each grid square from South to North also begins with the letter “A” and ends in “R.” West to east boundaries of the grid square are 20 degrees wide while South to North boundaries are 10 degrees. Therefore, the origin grid square has a designation of “AA” (lower left) and the last grid square is “RR” (upper right). Figure 2
The grid squares are not really squares, but rectangles. Yes, because of the way Earth is shaped. As an ellipsoid, lines of longitude are spaced far apart at the equator and converge at the poles. Latitude lines are evenly spaced from pole to pole. This actually causes the grid squares to be shaped as trapezoids. When viewed on a globe or any map projection that causes curvature, this becomes very apparent. Also keep in mind that there are 360 degrees of longitude while there are only 180 total degrees of latitude.
Reading the grid.
A very simple way to remember how to read these grids is that each letter is a direction to read. The first letter is read from left to right and the second is up. So if a ham tells you “EM”, then you would read on your map from left to right until you find “E” and then you would read up until “M”. Right then up. For you military members out there familiar with the Military Grid Reference System (MGRS), it is the same concept. Figure 3
Now that the largest grid squares have been laid out it’s time to work on the sub grids, the “Numbers.” These are created nearly identically to the larger grid squares in that they are placed in a positive ascending manner from west to east and south to north. However, the sub grids are only 2 degrees wide from west to east and 1 degree from south to north for a total of 100 sub grids. Keeping with the same logic as the main grid squares, these sub grids are also read from west to east and then south to north, but this time with ascending numbers beginning with “00” and ending with “99.” Figure 4
So now you have been given a Maidenhead grid of “EM76” and you want to find it on the map. You will read to the right to E and then up to M. Now that you are in grid square EM, start at 00 and read to the right until see 7 and then read up until you find 6. Right then up and will land on EM76!
The last part of EX76xb is a little bit different in that the smaller sub grids are 5 minutes from west to east and 2.5 minutes (2 minutes and 30 seconds) south to north. So now we are back to letters but use lower case instead.
Why are these sub grids using 5 minutes by 2.5 minutes? One degree is made up of 60 minutes. If the units were divided by the same base 10 unit, our grids would be 12 minutes wide and 6 minutes tall. Making the grids 5 minutes by 2.5 minutes allows for a greater number of grid squares which in turn gives us a greater precision for location. Therefore, these sub grids are divided into a 24×24 letter pattern. The result is an origin of “aa” and ending in “xx.” Figure 5
Looking at your map again, you are now ready to find EM76xb. Each first character in the grouping is read from west to east and then the second character is read from south to north. Right then Up. Read right to “E” and then Up to “M.” Now read Right to “7” and then Up to “6.” Lastly, read Right to “x” and up to “b.” There you have it, the QTH of N4EGA!
With a 6 character grid square, a location can be found to within a 5′(minute) by 2.5′ (minute) square. What does that mean? A couple things:
1) in the continental US, based on an Equal Albers Conic Projection (a projection is the Earth is flattened on a flat surface), each 6 character grid square covers about 12 square miles, or about 4 miles by 3 miles. This is fine for chasing grids but too coarse if you wish to locate a street address. For that, you will need to extend into fourth and fifth characters.
2) As these are grid squares, you should really think of them as two dimensional boundaries and not singular points as with traditional Latitude and Longitude. Each six character grid square encloses an area that is 5′ by 2.5′. Your position could be within any location inside of that grid square which is about 12 square miles in the continental US. Even increasing your precision to a 4 or 5 character grouping still places you within an area, not a point. It would be as if telling someone you are in a specific park, but not which part of the park. And then you tell them you are in a specific building, but not which room and so on. With latitude and longitude, you are telling someone that you are sitting in a chair, in a room, in house, in a city, in a state in a country on Earth.
Obtaining your Maidenhead grid square is fairly easy now. There are online resources for direct conversion from latitude and longitude. As web address change often, simply do an online search for “Maidenhead Converter” which will result in many good site.
QRZ.com has a database of nearly every Amateur Radio in the US and DX. Look up your callsign or one you hear on the air, and you will normally find their Grid Square listed along with other information about them.
For you rovers out there, numerous Global Positioning System (GPS) devices will provide Maidenhead grid squares to a high degree of precision. Just be sure to set your map datum to the official WGS84.
73! DE N4EGA