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{{Short description|Geocode system used by radio amateurs}}
{{EngvarB|date=September 2013}}
{{Use dmy dates|date=June 2024}}
The '''Maidenhead Locator System''' (a.k.a. '''QTH Locator''' and '''IARU Locator''') is a [[geocode#Geocode system|geocode system]] used by [[amateur radio]] operators to succinctly describe their [[geographic coordinate system|geographic coordinates]], which replaced the deprecated [[QRA locator]], which was limited to European [[Contact (amateur radio)|contact]]s.<ref name="G4FTJ">{{cite book |first=R.J., G4FTJ |last=Eckersley |year=1985 |title=Amateur Radio Operating Manual |edition=third |pages=64–66 |publisher=[[Radio Society of Great Britain]] |isbn=0-900612-69-X|location=Potters bar, UK}}</ref> Its purpose is to be concise, accurate, and robust in the face of [[Interference (communication)|interference]] and other adverse transmission conditions. The ''Maidenhead Locator System'' can describe locations anywhere in the world.

Maidenhead locators are also commonly referred to as ''QTH locators'', ''grid locators'' or ''grid squares'', although the "squares" are distorted on any non-[[equirectangular projection|equirectangular]] [[cartographic projection]]. Use of the terms ''QTH locator'' and ''QRA locator'' was initially discouraged, as it caused confusion with the older QRA locator system. The only abbreviation recommended to indicate a Maidenhead reference in Morse code and radio teleprinter transmission was '''LOC''', as in '''LOC KN28LH'''.<ref name="G4FTJ"/>

John Morris [[Amateur radio#Call signs|G4ANB]] originally devised the system and it was adopted at a meeting of the [[International Amateur Radio Union|IARU]] VHF Working Group in [[Maidenhead]], England in 1980.<ref name=QRZ.RU-Locator_System/>

==History==
[[Amateur radio]] [[contesting|contests]] on [[VHF]] and [[UHF]] are often scored based on the distance of [[Contact (amateur radio)|contact]]s, typically 1 point per kilometre,<ref name="SM5AGM-Maidenhead">{{Cite web |title=The Locator System |first=Folke, SM5AGM |last=Rosvall |website=jonit.com |url=http://www.jonit.com/fieldlist/maidenhead.htm |access-date=2017-01-30}}</ref> so there is a need for amateurs to exchange their locations over the air. To facilitate this, following the growth of the sport in the 1950s, the German [[QRA locator]] system was adopted in 1959.<ref name=QRZ.RU-Locator_System/> The QRA locator system was limited to describing European coordinates, and by the mid-1970s there was growing need for a global locator system.<ref name=SM5AGM-Maidenhead/>

By the time of their April&nbsp;1980 meeting, in [[Maidenhead, England|Maidenhead]], England, the VHF Working Group had received twenty different proposals to replace the QRA locator grid. That devised by John Morris (G4ANB) was deemed to be the best.<ref name=SM5AGM-Maidenhead/>

At the 1999 [[International Amateur Radio Union|IARU]] Conference in Lillehammer it was decided that the latitude and longitude to be used as a reference for the determining of locators should be based on the [[World Geodetic System|World Geodetic System 1984]] (WGS-84).<ref name=QRZ.RU-Locator_System>
{{cite web
 |title=The Locator System
 |website=qrz.ru
 |url=http://www.qrz.ru/vhf/qth_h.pdf
 |access-date=2014-07-13
}}
</ref>

==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.

==Adoption and use==
Like the QRA system before it, Maidenhead locators were enthusiastically adopted by radio amateurs beyond contesting, and it is now in widespread use.

Maidenhead locators are still used as part of the formulas for scoring in many [[VHF]] amateur radio contests and as the basis of earning awards like the American Radio Relay League's [[VHF/UHF Century Club]], URE TTLOC, etc. operating contests.

Under [[International Amateur Radio Union|IARU]] Region&nbsp;1 rules, VHF distance calculations are carried out between Maidenhead subsquare centres, assuming a ''spherical'' Earth. This results in a small error in distance, but makes calculations simpler and, given the inherent imprecision in the input data used, it is not the biggest error source.
Until the adoption of [[WGS 84]] as the official [[geodetic datum]] of the Maidenhead locator system in 1999,<ref name=QRZ.RU-Locator_System/> operators had usually specified their location based on their local national datum. Consequently, stations very near the edges of squares (at denoted precision) may have changed their locators when changing over to the use of WGS&nbsp;84.

The relatively new [[WSJT (amateur radio software)#FT8|FT8]] narrowband digital mode transmits Maidenhead locator square as part of standard messages, with the 4&nbsp;character locator square being efficiently represented within 15&nbsp;bits of the transmitted string.

In 2019{{Citation needed|date=September 2023}} the [[International Amateur Radio Union|IARU]] clarified the latest position on use of the IARU locator at various levels of precision, including a fifth pair of characters and that all letters should be uppercase.<ref>
{{cite conference
 |first=Andy, G4JNT |last=Talbot
 |date=16–23 September 2017
 |chapter=Clarification and extension of the IARU locator system
 |pages=42–45, 33–34
 |id=Appendix A, paper LA17 C5 17
 |editor-first=Dennis, ZS4BS |editor-last=Green
 |title=Report of the 24th IARU Region&nbsp;1 General Conference
 |conference=24th IARU Region&nbsp;1 General Conference
 |place=Landshut, DE
 |publisher=[[International Amateur Radio Union]] Region&nbsp;1
 |url=https://www.iaru-r1.org/wp-content/uploads/2019/12/GC_2017_Landshut_-Conf-Rep.pdf
 |access-date=19 September 2020
}}
</ref>

==Hardware and software support==
In 1985, the Radio Society of Great Britain published a small set of [[BASIC]] language routines to convert from locator references to geographical coordinates (latitude and longitude) for further processing.<ref>
{{cite book
 |first=John, GM4ANB |last=Morris
 |year=1985
 |title=Amateur Radio Software
 |pages=129–161
 |publisher=Radio Society of Great Britain
 |isbn=0-900612-71-1
 |place=Potters Bar, UK
}}
</ref> A complete program in BASIC called ''Universal Gridlocator'' was made available the following year by ARRL for a nominal cost of US$3.<ref>
{{cite magazine
 |first=Wayne |last=Overbeck
 |title = A universal grid-locator program for your personal computer
 |date=December 1986
 |magazine=QST Magazine
 |pages = 30–31
 |publisher=[[American Radio Relay League]]
 |place=Newington, CT
}}
</ref>

Many other utilities exist to convert latitude and longitude to locators, as this is a favourite [[Hack (technology slang)|hack]] for programmers who are also radio amateurs. [[Perl]] supports conversion between geographical coordinates and Maidenhead locators in module Ham::Locator by Andy Smith, available on [[CPAN]].<ref>
{{cite web
 |title=Ham Locator v&nbsp;0.1000
 |series=CPAN module
 |url=http://search.cpan.org/~meh/Ham-Locator-0.1000/lib/Ham/Locator.pm 
 |website=cpan.org
}}
</ref>

The Python maidenhead module is on pypi.org for installation via pip.<ref>
{{cite web
 |title=maidenhead – PyPI
 |url=https://pypi.org/project/maidenhead/
 |website=pypi.org
}}
</ref>

Many{{which|date=April 2024}} commercially available general purpose (civil) [[Global Positioning System]] (GPS) receivers (e.g. Garmin GPS-12) have the option to display positions in Maidenhead Locator format.<ref>
{{cite web
 |first=L.A., N7CFO |last=Burlingame
 |year=2010
 |title=GPS units that support the Maidenhead grid system
 |website=n7cfo.com
 |url=http://www.n7cfo.com/vhf/gps/~gps.htm
 |access-date=9 September 2012
}}
</ref>{{update inline|date=April 2024|reason=The GPS-12 has been out of production for about 15 years.}}

==See also==
* [[Military Grid Reference System]]
* [[Ordnance Survey National Grid]]
* [[QRA locator]]
* [[United States National Grid]]
* [[World Geographic Reference System]] (GEOREF)
* [[Open Location Code]]

==References==
{{reflist}}

===Maps with Maidenhead Locator grid===
{{refbegin}}
* {{cite book
 |title=ARRL World Grid Locator Atlas
 |quote=Containing all 32,400 Maidenhead Locator Squares
 |publisher=[[American Radio Relay League]]
 |place=Newington, CT
 |year=2007
}}
* {{cite book
 |title=IARU Locator of Europe
 |publisher=[[Radio Society of Great Britain]]
 |place=Potters Bar, UK
 |year=1984
}}
* {{cite book
 |title=IARU Locator of Western Europe
 |quote=(scale 1:2,000,000)
 |publisher=[[Radio Society of Great Britain]]
 |place=Potters Bar, UK
 |year=1985
}}
* {{cite book
 |title=ARRL Amateur Radio Map of North America
 |publisher=[[American Radio Relay League]]
 |place=Newington, CT
 |year=2003
}}
{{refend}}

==External links==
* {{cite web
 |title=Amateur Radio Maidenhead Grid Square Locator and Distance Calculator
 |website=karhukoti.com
 |url=https://www.karhukoti.com/grid.aspx
}}
* {{cite web
 |title=View full dynamic Maidenhead locator grid in Google Maps
 |website=sotamaps.org
 |url=http://www.sotamaps.org
 |quote=Click on the [Position] control in the map area to activate.
}}
* {{cite web
 |title=View Maidenhead locator grid overlay in OpenTopoMap
 |website=opentopomap.org
 |url=https://opentopomap.org
 |accessdate=2022-08-18
}} "Click on the [QTH Locator] checkbox in upper right corner menu to activate."
* {{cite web
 |title=ABC of Grid Locator System
 |website=HamRadio India
 |url=https://www.hamradio.in/projects/grid-locator-system
}}
* {{cite web
 |title=Grid Locators and Grid Squares
 |website=ARRL
 |url=http://www.arrl.org/locate/gridinfo.html
}}
<!--
 * {{cite web
 |url=http://www.arrl.org/locate/grid.html
 |website=ARRL
 |title=Calculate Grid Square
 }}
 -->
* {{cite web
 |title=An explanation of the [Maidenhead] system and how it came into being.
 |series=From the field hunter's web page
 |website=jonit.com
 |url=http://www.jonit.com/fieldlist/maidenhead.htm
}}
* {{cite web
 |title=Map grid square
 |website=levinecentral.com
 |url=http://www.levinecentral.com/ham/grid_square.php
 |quote=[Maidenhead] grid square for any location or amateur radio call sign
}}
* {{cite web
 |title=Generate a KML file that outlines a grid position
 |website=w8an.net
 |url=http://www.w8an.net/cgi-bin/grid.pl
}}
* {{cite web
 |title=Maidenhead locator
 |website=dev.unclassified.de
 |url=http://dev.unclassified.de/source/maidenheadlocator
 |quote=C# class to convert between geographic coordinates and Maidenhead locators, including distance calculation
}}
* {{cite web
 |title=View Maidenhead grid on Google Earth
 |website=earthpoint.us
 |url=http://www.earthpoint.us/grids.aspx
 |url-access=subscription
}}

{{geocoding-systems}}

[[Category:Amateur radio]]
[[Category:Geographic coordinate systems]]
[[Category:Maidenhead]]
[[Category:Science and technology in Berkshire]]
[[Category:1980 in radio]]
[[Category:Geocodes]]