There are few sights more stirring to a map enthusiast than the tapered outline of a triangulation pillar emerging from the mist of a bleak mountain. There are, or were, 6,173 of these mute beacons - each hand-cast in concrete, 4 feet high and 2 feet square at its base - scattered the length and breadth of Britain.
Although hill-walkers have always used them as aids to navigation, you are just as likely to find a trig pillar at the corner of a Black Country copse, or marooned by reeds in the Fens.
Erected during one of the most extraordinary episodes in the history of the Ordnance Survey, these trapezial monuments gave rise to the maps that opened the door to the British countryside.
The Ordnance Survey story begins in the 1740s with William Roy's pioneering military survey of the rebellious Scottish Highlands, but the era that turned maps into an everyday tool for tourists dawned in the 1930s.
Before this time, Britain had been mapped in a piecemeal fashion, county by county. There was no consistency. A traveller in Yorkshire, or Lancashire, would be relying on less accurate maps than a traveller in Norfolk.
Britain needed measuring anew, and the man the OS put in charge was Captain Martin Hotine, head of the Trigonometrical and Levelling Division.
Hotine had served on the North-West Frontier during the First World War and then in Persia and Mesopotamia - now Iraq; he was decisive, ingenious and tough.
The survey technique then in use was a highly refined modification of a method that had been pioneered 400 years earlier in the Low Countries.
"Triangulation" required the measurement of a single base line, from which imaginary triangles would be extended until the whole country was covered.
By measuring the angles between the corners of the triangles, distances could be computed. It was a simple, painstaking procedure.
And Hotine realised that he could improve the accuracy of his survey if his teams took their theodolite readings from solid pillars rather than wobbly tripods.
He designed the pillar himself. Each was to be cast with great precision, so that the theodolite sat quarely upon a brass plate set into the flat top of the pillar. Every pillar had to be sited in open ground so that it was "intervisible" with its neighbours.
The construction of more than 6,000 of these precision edifices was an epic in itself. Many of the locations were remote, or required mountaineering skills to reach.
The pillar on top of Cadair Idris in Wales took 12 days to construct, and required a team of packhorses rented from a Dolgelly coal merchant to carry the 42 cwt of chippings, sand and cement to the 3,000ft summit of the mountain. In his report, the builder complained of "terrible" mists and heavy rain.
During the filming of Map Man, the series on cartography that I am presenting on BBC2, we experimentally lugged a sack of cement from Glasgow to the summit of an uninhabited island off the north-west coast of the Highlands.
Predictably, the effort was considerable ("Why can't we just fill it with Rice Krispies?" complained one of the crew).
Hotine's pillars bemused the public. Were they part of an aircraft guidance system? Or sundials perhaps? Some reasoned that they must be the mountings for anti-aircraft guns, or searchlights. For a while, it was thought that they were Jubilee memorials.
But building the pillars was only a start. The surveying itself demanded that small teams set up a theodolite and take a "round" of readings to adjacent pillars, which would be illuminated by lights. To reduce the distortion caused by heated air, the OS took readings at night.
On Ben Macdhui in the heart of the Cairngorms, the surveying team had to camp on the snowy summit for 10 days until the skies cleared, while the team on Ben Nevis spent 22 days in snow and sub-zero temperatures. On Fair Isle, the observer dislocated his shoulder fighting off Arctic skuas.
The great "retriangulation" took 17 years to complete and another 11 years to refine. By 1962, the Ordnance Survey had filled in the "primary" triangles with smaller triangles, and the organisation had taken observations of nearly 22,000 stations.
The 1960s was a decisive decade for the OS as it saw the transformation of the one-inch-to-the-mile series into a bestseller. The redesigned seventh series of the one-inch rocketed to sales of nearly one-and-a-half million a year. A cartographic classic, this was the map that turned a generation on to the great outdoors.
I'm sure I wasn't the only teenager who recognised the OS one-inch as an escape map. Here was every public footpath in Britain, every peak and stream and hamlet, fixed with all-seeing accuracy. Every map was covered with coded landscapes ripe for exploration.
When I was 16, my father and uncle abandoned my younger cousin Stephen and me in the centre of Dartmoor, to see whether we could find our way to the edge of the moor, in thick mist. We were equipped with two days' food, a tent, a compass and a one-inch-to-the-mile Ordnance Survey map.
To this day, I'm not sure whether this was a considered rite of passage, or a parental attempt to be rid of irksome teenagers.
But as we stepped tentatively into an opaque murk of featureless peat and impenetrable cloud, Stephen and I knew that we were entirely dependent on Sheet 187.
The events of the following two days - and a night - taught me that I could trust the Ordnance Survey to steer me unsighted across England's most notorious moor.
Metrification converted the old one-inch maps into the now-familiar 1:50,000 scale "Landranger" series. Today, these are the Ordnance Survey's bestselling product. Covering England, Scotland and Wales in 204 sheets, these have proved to be the most versatile scale for travel, tourism - and the military.
Every road is shown, and every public right of way, together with sightseeing "must-knows" such as park-and-ride locations, gardens viewpoints and picnic sites.
These are also the maps used by the Royal Air Force to plan its bombing runs. When we went to RAF Lossiemouth to film Tornado bombers for next week's Map Man programme, I was amazed to find navigators sprawled over the Scottish Landranger maps that I'd recently used for a family holiday.
"There's nothing better," said one of the flyers. "They're the best maps for anticipating what the ground will look like as you pass over it at 500 mph."
Walkers meanwhile are being encouraged by the Ordnance Survey to abandon the Landranger for the larger-scale Explorer series. Increased access to the countryside through the new right-to-roam legislation is obliging Britons to brush-up on their navigational skills.
If you're going to avoid a confrontation with an angry farmer, you need to know whether your public footpath runs to the right, or to the left, of the field wall. And field walls are not marked on Landranger maps.
Public footpaths and bridleways are much easier to follow on the 1:25,000 Explorer maps, which show the new "Access Lands" in a pale yellow wash surrounded by an orange border.
There is little that an Explorer map does not show. Sheet OL40 ("The Broads") even marks the wooden stake that I crashed into with a sailing boat last winter.
The largest objects to escape the universal eye of the Ordnance Survey are the features coyly described as "Sensitive Sites". Taking its cue from government departments, the OS will omit the insides of prisons and various other establishments that might compromise security.
But the proliferation of commercial mapmaking agencies, and the availability of aerial photography, has made it pointless to deny the existence of these sites, and on recent editions of maps, a number of nuclear establishments, early-warning stations and MOD complexes have appeared. If you can see it from the outside, you will see it on the map.
In the 250 years since William Roy tramped the Highlands with his teams of redcoat surveyors, every square metre of Britain has been mapped to an astonishing degree of accuracy.
Today, the Ordnance Survey's "MasterMap" is probably the world's largest spatial database; a virtual map containing more than 440 million features, each of them identified by a unique computer number.
MasterMap is online, infinitely expandable, and updated on a daily basis. But the same digital revolution has also condemned Hotine's triangulation pillars.
Global positioning satellites have rendered redundant all but a few of these cartographic icons. Some have been dragged with chains from the farmland they were obstructing.
Many have suffered the indignity of being left unrepaired after lightning strikes or the abuse of vandals. And I'm saddened to see that my own favourite pillar, on the 1,062-metre summit of Bidein a' Ghlas Thuill in the western Highlands, has developed an ominous split.
- Nicholas Crane presents the final episode of 'Map Man' in the current series, 'Hotine's Ordnance Survey 1935-1950', on BBC2 this Thursday, at 7.30pm.