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We have a question about the algorithm for conversion from coordinates into grid squares here, and this is an extension of that.

I've noticed that there are several web-sites (for example here, or here, or here) and Android applications which will happily show me my 8 or 10 character locator. However, I've been unable to find an exact specification for that extended system.

So my question is: Is there a publicly available specification for the 8 and 10 character locator systems, and if yes, where?

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The Maidenhead locator system (which you correctly tagged) is described in detail in this Wikipedia page.

A short summary from that page is:

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 recycling the third and fourth pair algorithms is one possible definition: BL11bh16oo66

On shortwave frequencies, positions are reported at square precision, and on VHF and UHF, subsquare precision is used. More precise position reports are very rarely used.

Note that this means the formal standard does not define anything past the first eight characters (four pairs), but that people tend to extend the system used in the third and fourth pair if they need to be more accurate.

Also note that this extension (only as far as five pairs, or ten characters) is the one used at the APRS website.

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  • $\begingroup$ But, from what I can see on the Wikipedia page, there's only the reference to the 6 character system there. I don't see a reference to the 8 or 10 character versions. $\endgroup$
    – AndrejaKo
    May 26, 2019 at 10:38
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    $\begingroup$ @AndrejaKo the quoted material covers 8 characters and discusses the possibility of more. $\endgroup$
    – hobbs - KC2G
    May 28, 2019 at 21:02
  • $\begingroup$ For the fifth pair, did anybody consider 25x25 squares? I.e. AA through YY? 24x24 for the third pair is practical (dividing a range consisting of 60 units gives better result when dividing by multiple of 6 rather than 5). However, since the 5th pair is a division of the angular seconds fraction, 25x25 sounds more appropriate. Although, from practical point of view, the difference would be negligible - absolute accuracy of 5th pair is about 25-50 meters. $\endgroup$
    – Jindrich Vavruska OK4RM
    Jun 30, 2020 at 7:07
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The Maidenhead system breaks down a grid into increasingly smaller chunks. The fourth pair is defined as an evenly spaced 100 square grid, or 10x10. This site has an excellent image that breaks that down for you:

enter image description here

In this picture, the black box outline is MK80ht. You can see this box was further subdivided, and the user's location of Kalpathy, India is located in MK80ht80.

As noted by the Wikipedia article, there is no formal definition for anything smaller than that, however some users apparently recycle the formula for the third and fourth pairs if needed:

  • 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 recycling the third and fourth pair algorithms is one possible definition
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A recommendation for a formal definition of the 10 character system is available in the IARU-R1 VHF Handbook. Here's the relevant quote:

3.7 Proposals to Clarify and Standardise the IARU Locator, including higher Accuracy positioning

  • The definition of the existing 8-character scheme should be extended by adding a further division into 24 lettered squares to give a 10-character locator allowing a positioning accuracy of around 13 metres. Even higher resolution use for future appli-cations should be defined by successively sub-dividing in the same way; alternating 10 and 24 subdivisions using numbers and letter pairs.

  • When the locator is specified at lower resolutions, e.g. 6 characters such as in VHF contests, the centre of the squaroid at that resolution should be taken. This is equivalent to, for example, appending the 6-digit location IO90IV with the mid-range char-acters 44LL to give IO90IV44LL for the purposes of calculation. A quoted locator of IO90IV58 would have “LL” appended for the calculation.

  • That the WGS84 worldwide spheroid be the standard mapping be used for all locator conversions.
  • IARU Locators should always be quoted and used employing all Upper Case (Capital) letters.
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