Editing Maidenhead Locator System (section)

==Description of the system==
[[Image:Maidenhead Locator System explained.svg|thumb|right]]
A Maidenhead locator compresses [[latitude]] and [[longitude]] into a short string of characters, which is similar in concept to the [[World Geographic Reference System]] or GEOREF.  This position information is presented in a limited level of precision to limit the number of characters needed for its transmission using voice, [[Morse code]], or any other operating mode.<ref>
{{cite magazine
 |first=Edmund, N5JTY |last=Tyson
 |date=January 1989
 |title=Conversion between geodetic and grid locator systems
 |pages=29–30, 43
 |magazine=QST Magazine
 |publisher=[[American Radio Relay League]]
 |place=Newington, CT
 |url=http://www.arrl.org/files/file/protected/Group/Members/Technology/tis/info/pdf/18929.pdf
 |url-access=subscription |access-date=2018-03-09
}}
</ref>

The chosen coding uses alternating pairs of letters and digits, like so:

* BL11BH16

In each pair, the first character encodes longitude and the second character encodes latitude.<ref>
{{cite web
 |first=Bruce, KK5DO |last=Paige
 |title = Maidenhead grid squares
 |year = 2000
 |publisher = [[AMSAT]]
 |website=amsat.org
 |access-date=10 September 2012
 |url = http://www.amsat.org/amsat/articles/houston-net/grids.html
}}
</ref> These character pairs also have traditional names, and in the case of letters, the range of characters (or "encoding base number") used in each pair does vary.

[[Image:Maidenhead Locator Map.png|thumb|The world is divided into 324 (18×18) Maidenhead fields.]]
To avoid negative numbers in the input data, the system specifies that latitude is measured from the [[South Pole]] to the [[North Pole]], and longitude measured eastward from the [[antimeridian]] of [[Greenwich]], giving the [[prime meridian]] a [[Projected coordinate system#Easting and northing|false easting]] of 180° and the [[equator]] a [[Projected coordinate system#Easting and northing|false northing]] of 90°.

To simplify manual encoding, the base for the first pair of letters—traditionally called a ''field''—was chosen to be 18, thus dividing the globe into 18 zones of longitude of 20° each, and 18 zones of latitude 10° each. These zones are encoded with the letters "A" through "R".

[[Image:Maidenhead grid over Europe.svg|thumb|Fields are divided into 100 squares each.]]
The second pair of numbers, called a ''square'' and placed after the first pair of letters, uses a base number of 10, and is encoded using the digits "0" to "9".  This is where the alternative name "grid squares" comes from. Each of these squares represents 1° of latitude by 2° of longitude.
For additional precision, each square can optionally be sub-divided further, into ''subsquares''.  These are encoded into a second pair of letters, which should be presented in uppercase,<ref>https://www.rsgbcc.org/vhf/VHF_Handbook_V6_12.pdf Section 5.13</ref> but are sometimes (incorrectly) presented in lowercase as a legacy from the old QRA. The error has unfortunately been incorporated into various software packages, several examples of which can be seen on this page. Again, to make manual calculations from degrees and minutes easier, 24 was chosen as the base number, giving these subsquares dimensions of 2.5' of latitude by 5' of longitude.  The letters used are "A" through "X".

The resulting Maidenhead subsquare locator string is hence composed of two letters, two digits, and two more letters.  To give an example, W1AW, the [[American Radio Relay League]]'s [[Hiram Percy Maxim]] Memorial Station in [[Newington, Connecticut]], is found in grid locator&nbsp;{{Coor Maidenhead|41.71463|-72.72713|yes}}. Two points within the same Maidenhead subsquare are always less than {{cvt|10.4|km|mi}} apart, which means a Maidenhead locator can give adequate precision from only six easily transmissible characters.

For even more precise location mapping, two additional digits were proposed and ratified as an ''extended locator'', making it altogether eight characters long, and dividing ''subsquares'' into even smaller ones with dimensions 15" of latitude by 30" of longitude.  Such precision has uses in very short communication spans.  Beyond this, no common definition exists to extend the system further into even smaller squares.  Most often the extending is done by repeating alternating subsquare and square rules (base numbers 24 and 10 respectively).  However, other bases for letter encodings have also been observed, and therefore such ''extended extended'' locators might not be compatible.

To summarise:
* Character pairs encode [[longitude]] first, and then [[latitude]].
* The first pair (a ''field'') encodes with base 18 and the letters "A" to "R".
* The second pair (''square'') encodes with base 10 and the digits "0" to "9".
* The third pair (''subsquare'') encodes with base 24 and the letters "A" to "X".
* The fourth pair (''extended square'') encodes with base 10 and the digits "0" to "9".

(The fifth and subsequent pairs are not formally defined, but recursing to the third and fourth pair algorithms is a possibility, e.g.: BL11BH16OO66)

On [[shortwave]] frequencies, positions are reported at ''square'' precision, and on VHF and UHF, ''subsquare'' precision is used.  At high microwave frequencies ''extended square'' and ''extended subsquare'' precision is often used.