Model my Property › UK lidar explained
UK lidar explained: DTM, DSM and how it models your land
Lidar is why a model of your farm can show the bank behind the barn and the dip where the stream runs. Here is what it is, what the different files mean, where it covers, and what it is good and bad at.
What lidar is
Lidar (light detection and ranging) is measured from an aircraft that flies over in strips, firing hundreds of thousands of laser pulses a second at the ground. The time each pulse takes to bounce back, combined with the aircraft's exact position from GPS, gives the height of whatever it hit. The result is a dense cloud of points, often several per square metre, accurate to around 15 cm in height on open ground.
Some of each pulse reflects off the top of a tree or roof, and some carries on through gaps in the leaves to the ground beneath. That is what lets lidar separate the ground from what stands on it.
DTM and DSM: the two files you will meet
| DTM (digital terrain model) | DSM (digital surface model) | |
|---|---|---|
| Shows | The bare ground, with buildings and trees taken away | The top of everything: roofs, tree crowns, hedges |
| Good for | The shape of the land, drainage, banks and old earthworks | Building and tree heights, the height of a hedge |
| Looks like | A smooth landscape, as if everything had been lifted off | A bumpy surface with every wood and roof standing up |
Subtract the DTM from the DSM and you get the height of everything standing on the ground. Model my Property uses exactly this: the DTM for the ground of your model, and the difference for how tall your woods, hedges and buildings should be.
Resolution
Lidar is usually supplied as a grid, where each square holds one height. The UK national datasets mainly use 1 m squares, and some areas also have 50 cm or 25 cm grids. At 1 m, a ditch, a hedge bank or a farm track shows up clearly. On a model 200 mm wide of a 1 km farm, one millimetre already covers 5 m of land, so 1 m lidar holds more detail than any home printer can show.
Coverage in the UK
- England: the Environment Agency's National Lidar Programme has flown the whole of England, and its 1 m composite DTM and DSM fill most of the remaining gaps from older surveys. It is free under the Open Government Licence.
- Wales: the Welsh Government publishes national lidar through DataMapWales, also under the Open Government Licence.
- Scotland: lidar is published by the Scottish Remote Sensing Portal in phases. Much of the Central Belt, the cities and many rural areas are covered, but not yet everywhere.
- Northern Ireland: coverage is patchier, and Model my Property falls back to coarser elevation data there.
Outside lidar areas, the site uses Terrain Tiles, a worldwide blend of satellite and national elevation data. Hills and valleys still come through, but small features such as banks and ditches do not.
Things lidar gets wrong
- It is a snapshot. A survey shows the land on the day it was flown, which may be several years ago. A new barn may be missing, and a felled wood may still be standing.
- Water does not reflect well. Lakes and rivers often come back flat or with gaps, which are filled in by the data supplier.
- Dense woodland hides the ground. Under thick conifers few pulses reach the ground, so the DTM there is smoother than the real floor.
- Season matters. Surveys flown in winter see through bare hedges and trees more easily, and summer surveys record them fuller.
- Edges of surveys. Where two surveys flown in different years meet, there can be a small step in height.
Is it free to use?
The England, Wales and Scotland lidar is published as open data. You may use it for personal and commercial purposes, including printing and selling a model, as long as you acknowledge the source. Model my Property credits each source on the site and in the 3D viewer it makes, and the sources are listed on the about page.
Getting lidar yourself
You do not need to: Model my Property fetches it automatically for England, Wales, Scotland, the Netherlands and France. If you want the raw files for your own work, the Defra Data Services Platform (England), DataMapWales and the Scottish Remote Sensing Portal all offer downloads as GeoTIFF files. You can add your own GeoTIFF to the site too, and it is read on your computer and never uploaded.
What lidar reveals
Looking at the bare-earth DTM of your own land is often a surprise. Old ridge and furrow, filled-in ponds, the line of a lost hedge, a drained marsh or the banks of an Iron Age enclosure can all show up, even when nothing is visible on the ground. Many such sites have been discovered from lidar. The site's lidar view draws your files as shaded relief on the map, so look along the fields there, or turn the 3D model so the light falls low across it.
More guides: How it works · A 3D printed farm model · 3D print terrain from lidar · A 3D model of your house · Choosing the size and scale · Printing and painting a land model
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