You'd expect a valued traditional artefact to at least be solid. So the Giant's Ring, or the Harland & Wolff cranes, may have nothing much in common with each other, but they are both big and obvious. Neither of them could be demolished without Belfast suffering a real loss.
But what do you do when the big thing that everyone is familiar with, which everyone feels defines the character of their home place, is not made of metal, brick or stone, but is wholly intangible, like a beam of light?
You can put a preservation order on a structure, like a lighthouse, but can you put one on the beam? That is the question that is exercising people in south Co Down, where the whole look of the night sky is about to change.
The lighthouse at St John's Point is at the centre of this. This is the beautiful yellow and black pillar that commands a landscape that Van Morrison made famous in a spoken song, where he spent hours birdwatching, and near Coney Island, where he was "famished".
The lighthouse is not going to be dismantled; no one would stand for that. But the big, sweeping beam of light that scours the sky and brushes the hills and stretches out over the sea - that's going to change.
People who have got lost on country roads have found their bearings by that light, just as fishermen at sea have.
Currently, if you stand and look up at the lighthouse, even in daytime, you can see the huge lamp and its reflectors turning.
The casing looks like rings of gold, or burnished copper. They catch the sun. The glass looks like a giant diamond with concentric facets cut into it.
But who needs it? Who really needs it? The Commissioner of Irish Lights, who is responsible for it, is planning to replace the revolving lamp with a static LED (light-emitting diode) and local people are protesting - the night in Lecale would not be the same without the familiar beam traversing the sky and sweeping the hills and the sea.
The ecologically-minded are divided on this. The current light is warm and, though not actually natural, it seems closer to the colour of the sun. LED is harsher, more like the moon.
But LED is cheaper. That's the whole point. And that warm, golden lamp is sitting in a pool of toxic mercury. Irish Lights will remove that. Better to leave it where it is, say the defenders of the lighthouse.
Eileen Peters, a retired English teacher, says that people on Coney Island bring in the washing by the light of the beam from St John's Point, as it sweeps across the land and out to sea. She says it is visible from the Isle of Man, whereas the light of the LED replacement won't travel as far.
Ross Mulhall, who fishes these waters and is studying for his skipper's licence, says that the huge lamp turns through the day in its casing to keep its timing right.
"The light flashes twice every seven-and-a-half seconds and every light on the coast flashes to a different rhythm so that you don't mix them up."
Ross has been campaigning for the lighthouse through a Facebook page and has attracted significant support.
One of those who is adamant that the light should not be changed is the writer Dr Damian Smyth. He lives in Downpatrick and celebrates the area in his work.
He says: "It is a cultural identifier for people who live under its reach; it is a heritage asset - not the bricks and mortar, but the light itself. It reaches inland from the coast as well as outwards to the sea, and is regarded by people as contributing to the quality of life - belonging, identity, community."
William Mulhall, Ross's father, who owns a shop in Ardglass, is also supporting the campaign.
"That lighthouse is part of our history, it is a tourist attraction. People come in yachts, and cruisers and oil tankers go by and they all need their light."
But is it a practical necessity that it should turn? Ardglass skipper Simon Wills is more exercised by laws governing the size of fishing net meshes, or panels, which he says discriminate against the Northern Irish fisherman, and it is hard to get him off that subject.
"But, yes, you would see it and it would give you a point to head to. You would really want to keep it that way."
Retired skipper, Rodney Harding, fished out of Ardglass for 20 years.
He says: "The lighthouse was very important when there was thick fog, or if my radar was gone, or my electrical equipment was gone. And it was the law that we had to pay the lighthouse dues and people were being taken to court for not paying them.
"It could be a life-threatening situation if they changed that light to one with a shorter range."
The lighthouse dues are another thing that annoys Simon Wills. "The whole Northern Ireland fleet pays them - about £350 a year," he says.
Ross Mulhall, who fishes as far as 30 miles away from Ardglass, says that there are occasions on a boat when all the lights go out. You can have a power cut. Then you need the lighthouse.
"When you are out fishing, the electrics can go on the boat. You can have a blackout. That can happen quite a lot when you are out at sea. You can be in the dark, but if you can see the lighthouse you can get safely home."
And he says it is one thing to know where you are by consulting a map on a computer, but quite another experience to locate yourself by the light you see with your own eyes.
Dr Smyth insists that the practical reasons for keeping the light as it is are the most important.
"Principally, first and foremost, it remains a lifesaver, in spite of the myth of GPS and modern technology; even Irish Lights aren't suggesting abandoning the light function, or questioning its purpose," he says. "That remains a most treacherous coastline, from Kilkeel right up the coast to Portavogie, and the light still plays a critical role in navigation."
Yet, he says: "Unusually, people are united in identifying that this is an issue of light and its quality, its character, rather than simply whether or not it fulfils a narrowly-defined functional purpose."
That is certainly how Eileen Peters thinks of it. She dismisses the new LED lights on some lighthouses as "winking willies".
She says: "You would lose the whole character of the land. It's like saying, 'St Anne's church in Killough is no good; we'll put up a portacabin'. Or, 'We'll put a light on a stick'.
"It's just losing the whole essence of the light that was built in 1844 and that has been rotating since. The Titanic sailed out past it. It must have been one of the few comforting last sights the people on that ship ever saw."
Eileen adds: "The light in Dunmore East has changed and it's now a winking willie. The people there are very unhappy with it."
But surely the lighthouse simply has a job to do - and that is to guide ships towards port and home? Simon Wills isn't paying his £350 a year dues so that the people of Coney Island can bring their washing in on moonless nights.
Why should Irish Lights care if Dr Smyth regards the light as a memorial to those lost at sea? That's not what they put it there for.
They say it will still guide mariners, that it will be visible for 18 miles and that that is enough.
"The light will be lighting at St John's for many years to come and the long tradition of keeping the sea safe for the mariner will continue," says Irish Lights.
Further, the redevelopment of several lighthouses will create new tourist accommodation at sites in St John's Point, Co Down, the other St John's Point in Donegal, Rathlin Island, Black Head, Antrim and at Fanad Head.
Dr Smyth says: "This is Castle Ward; it's the embattled and shrunken fishing industry; it's a memorial to the many lives lost, even in very recent times; it's an important and valued daily player in the lives of the community and the district. It is also a tourist destination in Lecale - no visit to the area is complete without a visit to Saul, Strangford, Killough, Coney Island and the big light.
"The St John's Point Light is more important to us than the cranes are to Belfast - and they are important. It's home."
Eileen Peters knows that local feeling is strong. She has left petitions in many of the local shops and bars and restaurants and people are signing them.
And she is urging her neighbours to join her at a meeting with Irish Lights in the Old Spar at the harbour in Killough, where William Mulhall exhibits his hand-built boats and his paintings, on March 14 at 2pm.
St John’s Point lighthouse was commissioned in 1844 to a design by George Halpin.
The height of the original tower was tripled in 1893 and, until the early-1950s, the tower was painted white with black bands.
However, in 1954, Irish Lights took the decision to repaint the tower in its distinctive black and yellow colour scheme.
The lighthouse was formerly powered by whale oil and coal gas until, in 1981, it was converted to electricity.
In common with many Irish lighthouses, St John’s is now completely automated and thus no longer manned.
St John’s is famous — or infamous — for a number of reasons.
In 1846, James Hosken, the captain of Isambard Kingdom Brunel’s SS Great Britain (the world’s first ironclad ship), mistook St John’s for the Calf lighthouse on the Isle of Man as he set out on the vessel’s third crossing of the Atlantic.
The navigational error — believed to be the result of out of date charts — caused the Great Britain to run aground in Dundrum Bay, where it spent almost a year.
In August 1847, it was floated free at a cost of £34,000 and taken back to Liverpool, but the expense exhausted the shipping line’s remaining reserves.
It was not the only occasion that St John’s was mistaken for another lighthouse.
In 1855, the Fortune left Liverpool bound for Australia when it too ran aground in Dundrum Bay.
The captain, James McCarthy, mistakenly thought St John’s was the Kish lighthouse, about seven miles off the Co Dublin coast.
The Fortune ran aground near Tyrella and one passenger lost his life while abandoning ship.