Following on from Krissel's Wikipedia link, here's
Ordnance Survey:
QUOTE
The roots of Great Britain's Ordnance Survey (OS) go back to 1747, when King George II commissioned a military survey of the Scottish highlands following the Jacobite revolt of 1745. William Roy was the engineer responsible for this pioneering work; one of the staff involved was noted artist Paul Sandby. It was not until 1790 that the Board of Ordnance (the predecessor of the Ministry of Defence) began a national military survey starting with the south coast of England in anticipation of a French invasion.
So it is "ordnance" because of its military origins...
How the heck did we get from wellington boots to maps...? 
Edit to add: I've just realised how the US came about - George II was obviously so concerned about finding a route to the Scottish rebels he didn't notice what was going on in North America between 1747 and 1776...
