I thought perhaps I was confusing two words, but I wasn't. You were :-)
Ordinance = a law Ordnance = explosives
The maps are called "Ordnance Survey" - so I guess you may be right about artillery range maps. In fact you are:
"The name Ordnance Survey hints at how it all began.
Britain’s mapping agency has its roots in military strategy: Mapping the Scottish Highlands following rebellion in 1745. Later, as the French Revolution rumbled on the other side of the English Channel, there were real fears the bloodshed may sweep across to our shores.
So the government ordered its defence ministry of the time – the Board of Ordnance – to begin a survey of England’s vulnerable southern coasts. Until then, maps had lacked the detail required for moving troops and planning campaigns.
It was an innovative young engineer called William Roy who was tasked with the initial small-scale military survey of Scotland.
Starting in 1747, it took eight years to complete what was known as the Great Map at a scale of 1:36 000 (1.75 inches to a mile). Roads, hills, rivers, types of land cover and settlements were recorded. William Roy described it as rather a ‘magnificent military sketch than a very accurate map of the country’.
Roy’s surveying parties of about eight relied on simple surveying compasses to measure the angles, and chains up to 50 feet long to measure distance between important features. Much of the rest was sketched in by eye.
Nevertheless, the map was a powerful tool as part of a broader strategy to open up access to the Highlands.
The fact that Roy was just 21 years old with no military commission when he started the survey makes his achievements even more extraordinary.
His work paved the way for modern surveying and he understood the strategic importance of accurate maps. At the time of his death in 1790 his vision of a national survey for Britain was almost within reach."