The craft · the data

The data behind every map

Every Cityform plate is a real square kilometre of Britain, measured from the air. This is the survey data it is built from — and what it looks like before it becomes an object.

A companion to How it’s made, for anyone who wants to see the measurements themselves. Nothing here is an artist’s impression; every figure is drawn from a real Environment Agency survey of Sheffield city centre.

Light from the air

What real LIDAR looks like

The heights come from LIDAR — light detection and ranging. An aircraft flies a grid over the country firing a laser straight down, hundreds of thousands of pulses a second, and times each echo to fix the height of that point to within a few centimetres. Millions of returns are resampled onto a regular one-metre grid — a height for every square metre, like an image where the value is elevation rather than colour. Here is one real square kilometre of Sheffield city centre, straight from the survey.

A technical two-panel figure of real airborne LIDAR over Sheffield city centre. Panel A is a Digital Surface Model with every one-metre cell coloured by elevation, a colour bar from about 50 to 100 metres above sea level, a 200-metre scale bar and a north arrow. Panel B is a hillshade of the same heights lit from the north-west, with a shadow-to-lit illumination bar, showing buildings, streets and the railway in relief. — Real airborne LIDAR of one square kilometre of Sheffield city centre — an Environment Agency Digital Surface Model at 1 m. A: every cell coloured by height, in metres above sea level. B: the same grid as a north-west hillshade. About a million measured heights, before any of it becomes a model.

One pulse, more than one return

How does one flight measure both the rooftops and the ground beneath them? Because a single laser pulse can come back more than once. Fired over a tree, part of the beam reflects straight off the canopy while the rest slips through the leaves and bounces off the ground a moment later — two echoes, two heights, from one pulse. The earliest echo builds the surface; the last builds the terrain. Over a solid roof there is only one echo, so the two surfaces meet.

Why one flight gives both models: a pulse over a tree returns first from the canopy (surface) and last from the ground (terrain); over a solid roof there is a single return.

Three layers from one scan

Surface, terrain, and what stands on it

Those two returns give two surfaces. The DSM (digital surface model) keeps everything the laser saw first — roofs, trees, walls. The DTM (digital terrain model) strips those away to leave bare ground. Subtract one from the other and you are left with a third layer: the height of everything that stands on the ground. That difference — the nDSM — is the part Cityform raises into buildings.

Three colour-mapped panels of the same Sheffield square kilometre. A: the surface model with every one-metre cell coloured by height in metres above sea level. B: the bare terrain with buildings and trees removed. C: the surface minus the terrain, isolating each building and tree as a height above the ground, from zero to about thirty metres. — The same scan, three ways. A surface (DSM) and B bare terrain (DTM) share a scale in metres above sea level; subtract one from the other and C isolates every building and tree as a height above the ground.

Measured ground

A slice through the city

Cut a straight line across the kilometre and look at it side-on. The ground is never a flat slab: it climbs and falls, and the buildings stand on top of it wherever it happens to be. This is the difference between a Cityform plate and a model that drops extruded outlines onto a level base — the terrain is real, and the buildings sit on it.

Why measured ground matters, in one slice. The bare terrain (red) falls and rises across the kilometre; the buildings (black) sit on top of it, not on a flat slab. Cityform keeps both.

From grid to object

The height grid as a surface

It helps to picture the height grid the way the software does — not as a flat map but as a surface, every cell lifted to its measured height. Seen from an angle the kilometre stands up: the ground rolls, blocks rise to their roofs, and the valley falls away. This is the relief the model is built on. The remaining job is to give it walls and a floor, so it becomes one watertight solid rather than a single draped sheet — and then it is sliced into layers and printed at 1:11000.

A three-dimensional view of the Sheffield height grid, every one-metre cell lifted to its measured elevation and coloured by height, showing the city as a relief surface with taller buildings rising in yellow. — The same height grid lifted into three dimensions — the relief the printer builds from, before it is sealed into one watertight solid on its base.

Open data

Where the data comes from

All of this is public. Britain’s nations each release their own LIDAR under the Open Government Licence, and OpenStreetMap supplies the building, road and water outlines that tell the heights apart. That open data does not cover the country evenly, though. England is mapped in full by the Environment Agency; Scotland and Wales run their own surveys — the Scottish public-sector programme and Natural Resources Wales — which reach most of the ground but thin out toward the uplands and the coasts. Northern Ireland has no equivalent national release. So a plate can be built almost anywhere in Great Britain, with a few squares near the Welsh and Scottish edges that come back too sparse to use.

A map of the United Kingdom shaded by LIDAR coverage. England is full national coverage from the Environment Agency; Scotland and Wales have open programmes that thin toward their edges; Northern Ireland has no national programme. Sheffield is marked in England. — England, Scotland and Wales each release their own public LIDAR — Cityform builds anywhere that open data reaches. Coverage thins toward the Welsh and Scottish edges; Northern Ireland has no national programme.

Attribution

Heights from public-sector LIDAR — Environment Agency (England), Scottish Remote Sensing Portal (Scotland) and Natural Resources Wales (Wales) — licensed under the Open Government Licence v3.0. Outlines © OpenStreetMap contributors, licensed under the ODbL.