Thomas Marks re-orientates his London bearings at the prompting of a not-so-discreet new skyscraper. 

A shard is a piece that’s been broken off something else and, as such, it tends to take on an accidental and often dangerous shape. So there’s an inevitable irony in pointing out that Renzo Piano’s vast London Bridge skyscraper, known as The Shard, has now been completed. The building is an extraordinary structural achievement that’s been made to resemble a colossal fragment. Piano has gone for stylised breakage: vast staggered planes of glass, a scalene tilt, a splintered steel spire.

But The Shard also looks like a breakaway because of its geography, its startling isolation south of the river – where the Thames-scape is predominantly low-rise. It’s as if it has cleaved off from the clustered towers of the City or Canary Wharf, cleaved then drifted, cleaved and drifted then snagged in the railway lines that trail into Kent from London Bridge Station.

In a sense, the structure’s great bulk ought to quash such flights of fancy. But for all that The Shard may primarily be a triumph of mechanical engineering, for all the commercial imperatives of the development, the building is also proving a remarkable source of urban surprise. It interrupts old vistas and arranges new ones. In doing so, it is changing how I map the city in my head as I move through its streets.

The south of the river has never been so sharply visible. Across the former goods yards behind King’s Cross, The Shard’s glinting proximity, its sheer glass bravado, comes as a shock. And I’m confused when it pierces the tree-crowns as I stroll east through Hyde Park: convinced at first it’s an optical illusion, that the building is not where it should be, it’s a while until I realise how this view charts the kinks in the river.

Elsewhere, strange juxtapositions between The Shard and existing landmarks are toying with the city’s skyline. From the canal basin behind City Road, it’s a skinny twin to one of the Barbican tower blocks. When I squint across the river from Embankment, it looks to have rocketed upwards, like a giant glass beanstalk, through the flat concrete roofs of the South Bank Centre. And from Parliament Hill, the Shard is an enormous blade poised to sliver the dome of St Paul’s with one clean cut.

This seems appropriate, since Piano’s lone skyscraper might supplement – and even supplant – some of the imaginative functions that Wren’s great dome has long performed for both the capital’s inhabitants and its visitors. Pip, in Great Expectations, uses it as a monumental compass to navigate away from the ‘blood and fat and foam’ of Smithfield Market: ‘I rubbed it off with all possible speed by turning into a street where I saw the great black dome of St Paul’s bulging at me’.

I don’t suppose The Shard will help us find moral as well as spatial bearings – as St Paul’s does for Pip. But, as the dome is for him, this jagged skyscraper is a bold orientation point in a city that sometimes seems as jumbled as its tube map. Its extraordinary visibility – its presence from such distance – lets us imagine the city whole even when we feel it is in pieces.

Photograph: Hayes Davidson/Nick Wood