Why are the codes based on numbers, as opposed to using a wider range of alphanumeric characters?
The main reason that the codes are purely numeric (other from the colon separator character) is the desire to retain as simple a relationship as possible with numeric values of latitude and longitude, as encoded directly into all but the leading character of each “cycle” of the hierarchy. Having said that, a case could be made for using a letter instead of a number for the leading character (since these are in effect a “special digit” rather than a number), however the decision has been made to use two existing precedents (WMO squares, see above, plus “Blue Pages” subdivisions of these squares), rather than introduce a new notation convention. A separate advantage of a (basically) all-number system, as opposed to one incorporating other alphabetic characters, is that there is then a reduced chance of accidental transliteration of similar looking characters (e.g. O and 0, I and 1, lowercase “L” and 1), either during manual code entry, or OCR (optical character recognition) copying. Also
