You can actually use more or less than three digits. My GPS claims
that my office is at TQ 28866 64213, although the last digit is pure
noise and the next to last one is not very accurate because of selective
availability (it's also more than 1 metre square!). On the other hand, the
1:25000 map covering where I live is called TQ 08/18 as it contains the
two 10km squares, TQ 08 and TQ 18. There are maps down to about a scale
of 1:1250 in urban areas, but they tend to be used for legal purposes.
> What system is used in other countries? The OS system is a very good system
> indeed. An advantage of it is that if you want to find the distance between
> two points then you can use Pythagoras' theorem.
This only works in a compact country, where the aberration from the
earth's curvature is small. On the equator, latitude and longitude
give a square grid; for a country like the UK you can construct a fairly
square grid by using a tangent plane; but, for a country like the USA,
the curvature can't be ignored, so they also just use latitude and
longitude, with all its defects.
My GPS reckons there are also: Taiwanese, Swiss, Swedish, Irish and German
grids, together with the Maidenhead locators and something it calls
UTM/UPS. Sweden is the least obvious, as it is long and thin.
obSETI: For all except long baseline work, the 10km grid is probably